.\" dhcpd.conf.5
.\"
-.\" Copyright (c) 2004-2011 by Internet Systems Consortium, Inc. ("ISC")
+.\" Copyright (c) 2004-2019 by Internet Systems Consortium, Inc. ("ISC")
.\" Copyright (c) 1996-2003 by Internet Software Consortium
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.\"
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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-.\" $Id: dhcpd.conf.5,v 1.111 2011/06/01 23:25:37 sar Exp $
+.\" $Id: dhcpd.conf.5,v 1.114 2012/04/02 22:47:35 sar Exp $
.\"
.TH dhcpd.conf 5
.SH NAME
.IR dhcpd,
the Internet Systems Consortium DHCP Server.
.PP
-The dhcpd.conf file is a free-form ASCII text file. It is parsed by
-the recursive-descent parser built into dhcpd. The file may contain
+The dhcpd.conf file is a free-form ASCII text file. It is parsed by
+the recursive-descent parser built into dhcpd. The file may contain
extra tabs and newlines for formatting purposes. Keywords in the file
-are case-insensitive. Comments may be placed anywhere within the
-file (except within quotes). Comments begin with the # character and
+are case-insensitive. Comments may be placed anywhere within the
+file (except within quotes). Comments begin with the # character and
end at the end of the line.
.PP
-The file essentially consists of a list of statements. Statements
+The file essentially consists of a list of statements. Statements
fall into two broad categories - parameters and declarations.
.PP
Parameter statements either say how to do something (e.g., how long a
Declarations are used to describe the topology of the
network, to describe clients on the network, to provide addresses that
can be assigned to clients, or to apply a group of parameters to a
-group of declarations. In any group of parameters and declarations,
+group of declarations. In any group of parameters and declarations,
all parameters must be specified before any declarations which depend
on those parameters may be specified.
.PP
Declarations about network topology include the \fIshared-network\fR
-and the \fIsubnet\fR declarations. If clients on a subnet are to be
+and the \fIsubnet\fR declarations. If clients on a subnet are to be
assigned addresses
dynamically, a \fIrange\fR declaration must appear within the
-\fIsubnet\fR declaration. For clients with statically assigned
+\fIsubnet\fR declaration. For clients with statically assigned
addresses, or for installations where only known clients will be
-served, each such client must have a \fIhost\fR declaration. If
+served, each such client must have a \fIhost\fR declaration. If
parameters are to be applied to a group of declarations which are not
related strictly on a per-subnet basis, the \fIgroup\fR declaration
can be used.
even if no addresses will be dynamically allocated on that subnet.
.PP
Some installations have physical networks on which more than one IP
-subnet operates. For example, if there is a site-wide requirement
+subnet operates. For example, if there is a site-wide requirement
that 8-bit subnet masks be used, but a department with a single
physical ethernet network expands to the point where it has more than
254 nodes, it may be necessary to run two 8-bit subnets on the same
-ethernet until such time as a new physical network can be added. In
+ethernet until such time as a new physical network can be added. In
this case, the \fIsubnet\fR declarations for these two networks must be
enclosed in a \fIshared-network\fR declaration.
.PP
Some sites may have departments which have clients on more than one
subnet, but it may be desirable to offer those clients a uniform set
of parameters which are different than what would be offered to
-clients from other departments on the same subnet. For clients which
+clients from other departments on the same subnet. For clients which
will be declared explicitly with \fIhost\fR declarations, these
declarations can be enclosed in a \fIgroup\fR declaration along with
-the parameters which are common to that department. For clients
+the parameters which are common to that department. For clients
whose addresses will be dynamically assigned, class declarations and
conditional declarations may be used to group parameter assignments
based on information the client sends.
consulting that client's \fIhost\fR declaration (if any), and then
consulting any \fIclass\fR declarations matching the client,
followed by the \fIpool\fR, \fIsubnet\fR and \fIshared-network\fR
-declarations for the IP address assigned to the client. Each of
+declarations for the IP address assigned to the client. Each of
these declarations itself appears within a lexical scope, and all
declarations at less specific lexical scopes are also consulted for
-client option declarations. Scopes are never considered
+client option declarations. Scopes are never considered
twice, and if parameters are declared in more than one scope, the
parameter declared in the most specific scope is the one that is
used.
When dhcpd tries to find a \fIhost\fR declaration for a client, it
first looks for a \fIhost\fR declaration which has a
\fIfixed-address\fR declaration that lists an IP address that is valid
-for the subnet or shared network on which the client is booting. If
+for the subnet or shared network on which the client is booting. If
it doesn't find any such entry, it tries to find an entry which has
no \fIfixed-address\fR declaration.
.SH EXAMPLES
.fi
.PP
Notice that at the beginning of the file, there's a place
-for global parameters. These might be things like the organization's
+for global parameters. These might be things like the organization's
domain name, the addresses of the name servers (if they are common to
-the entire organization), and so on. So, for example:
+the entire organization), and so on. So, for example:
.nf
option domain-name "isc.org";
.PP
The most obvious reason for having subnet-specific parameters as
shown in Figure 1 is that each subnet, of necessity, has its own
-router. So for the first subnet, for example, there should be
+router. So for the first subnet, for example, there should be
something like:
.nf
option routers 204.254.239.1;
.fi
.PP
-Note that the address here is specified numerically. This is not
+Note that the address here is specified numerically. This is not
required - if you have a different domain name for each interface on
your router, it's perfectly legitimate to use the domain name for that
-interface instead of the numeric address. However, in many cases
+interface instead of the numeric address. However, in many cases
there may be only one domain name for all of a router's IP addresses, and
it would not be appropriate to use that name here.
.PP
.fi
.PP
You may have noticed that while some parameters start with the
-\fIoption\fR keyword, some do not. Parameters starting with the
+\fIoption\fR keyword, some do not. Parameters starting with the
\fIoption\fR keyword correspond to actual DHCP options, while
parameters that do not start with the option keyword either control
the behavior of the DHCP server (e.g., how long a lease dhcpd will
give out), or specify client parameters that are not optional in the
DHCP protocol (for example, server-name and filename).
.PP
-In Figure 1, each host had \fIhost-specific parameters\fR. These
+In Figure 1, each host had \fIhost-specific parameters\fR. These
could include such things as the \fIhostname\fR option, the name of a
file to upload (the \fIfilename\fR parameter) and the address of the
server from which to upload the file (the \fInext-server\fR
-parameter). In general, any parameter can appear anywhere that
+parameter). In general, any parameter can appear anywhere that
parameters are allowed, and will be applied according to the scope in
which the parameter appears.
.PP
-Imagine that you have a site with a lot of NCD X-Terminals. These
+Imagine that you have a site with a lot of NCD X-Terminals. These
terminals come in a variety of models, and you want to specify the
-boot files for each model. One way to do this would be to have host
+boot files for each model. One way to do this would be to have host
declarations for each server and group them by model:
.nf
.SH ADDRESS POOLS
.PP
The
-.B pool
-declaration can be used to specify a pool of addresses that will be
+\fBpool\fR and \fBpool6\fR
+declarations can be used to specify a pool of addresses that will be
treated differently than another pool of addresses, even on the same
-network segment or subnet. For example, you may want to provide a
+network segment or subnet. For example, you may want to provide a
large set of addresses that can be assigned to DHCP clients that are
registered to your DHCP server, while providing a smaller set of
addresses, possibly with short lease times, that are available for
-unknown clients. If you have a firewall, you may be able to arrange
+unknown clients. If you have a firewall, you may be able to arrange
for addresses from one pool to be allowed access to the Internet,
while addresses in another pool are not, thus encouraging users to
-register their DHCP clients. To do this, you would set up a pair of
+register their DHCP clients. To do this, you would set up a pair of
pool declarations:
.PP
.nf
that control which clients are allowed access to the pool and which
aren't. Each entry in a pool's permit list is introduced with the
.I allow
-or \fIdeny\fR keyword. If a pool has a permit list, then only those
+or \fIdeny\fR keyword. If a pool has a permit list, then only those
clients that match specific entries on the permit list will be
-eligible to be assigned addresses from the pool. If a pool has a
+eligible to be assigned addresses from the pool. If a pool has a
deny list, then only those clients that do not match any entries on
-the deny list will be eligible. If both permit and deny lists exist
+the deny list will be eligible. If both permit and deny lists exist
for a pool, then only clients that match the permit list and do not
match the deny list will be allowed access.
+.PP
+The \fBpool6\fR declaration is similar to the \fBpool\fR declaration.
+Currently it is only allowed within a \fBsubnet6\fR declaration, and
+may not be included directly in a shared network declaration.
+In addition to the \fBrange6\fR statement it allows the \fBprefix6\fR
+statement to be included. You may include \fBrange6\fR statements
+for both NA and TA and \fBprefix6\fR statements in a single
+\fBpool6\fR statement.
.SH DYNAMIC ADDRESS ALLOCATION
Address allocation is actually only done when a client is in the INIT
state and has sent a DHCPDISCOVER message. If the client thinks it
about the address.
.PP
There may be a host declaration matching the client's identification.
-If that host declaration contains a fixed-address declaration that
+If that host declaration contains a fixed-address declaration that
lists an IP address that is valid for the network segment to which the
-client is connected. In this case, the DHCP server will never do
-dynamic address allocation. In this case, the client is \fIrequired\fR
-to take the address specified in the host declaration. If the
-client sends a DHCPREQUEST for some other address, the server will respond
-with a DHCPNAK.
+client is connected, the DHCP server will never do dynamic address allocation.
+In this case, the client is \fIrequired\fR to take the address specified
+in the host declaration. If the client sends a DHCPREQUEST for some other
+address, the server will respond with a DHCPNAK.
.PP
When the DHCP server allocates a new address for a client (remember,
this only happens if the client has sent a DHCPDISCOVER), it first
receive the existing lease, then the server will look in the list of
address pools for the network segment to which the client is attached
for a lease that is not in use and that the client is permitted to
-have. It looks through each pool declaration in sequence (all
+have. It looks through each pool declaration in sequence (all
.I range
declarations that appear outside of pool declarations are grouped into
-a single pool with no permit list). If the permit list for the pool
+a single pool with no permit list). If the permit list for the pool
allows the client to be allocated an address from that pool, the pool
-is examined to see if there is an address available. If so, then the
-client is tentatively assigned that address. Otherwise, the next
-pool is tested. If no addresses are found that can be assigned to
+is examined to see if there is an address available. If so, then the
+client is tentatively assigned that address. Otherwise, the next
+pool is tested. If no addresses are found that can be assigned to
the client, no response is sent to the client.
.PP
If an address is found that the client is permitted to have, and that
has never been assigned to any client before, the address is
-immediately allocated to the client. If the address is available for
+immediately allocated to the client. If the address is available for
allocation but has been previously assigned to a different client, the
server will keep looking in hopes of finding an address that has never
before been assigned to a client.
.PP
The DHCP server generates the list of available IP addresses from a
-hash table. This means that the addresses are not sorted in any
+hash table. This means that the addresses are not sorted in any
particular order, and so it is not possible to predict the order in
-which the DHCP server will allocate IP addresses. Users of previous
+which the DHCP server will allocate IP addresses. Users of previous
versions of the ISC DHCP server may have become accustomed to the DHCP
server allocating IP addresses in ascending order, but this is no
longer possible, and there is no way to configure this behavior with
version 3 of the ISC DHCP server.
.SH IP ADDRESS CONFLICT PREVENTION
The DHCP server checks IP addresses to see if they are in use before
-allocating them to clients. It does this by sending an ICMP Echo
-request message to the IP address being allocated. If no ICMP Echo
+allocating them to clients. It does this by sending an ICMP Echo
+request message to the IP address being allocated. If no ICMP Echo
reply is received within a second, the address is assumed to be free.
This is only done for leases that have been specified in range
statements, and only when the lease is thought by the DHCP server to
.PP
If a response is received to an ICMP Echo request, the DHCP server
assumes that there is a configuration error - the IP address is in use
-by some host on the network that is not a DHCP client. It marks the
-address as abandoned, and will not assign it to clients.
+by some host on the network that is not a DHCP client. It marks the
+address as abandoned, and will not assign it to clients. The lease will
+remain abandoned for a minimum of abandon-lease-time seconds.
.PP
If a DHCP client tries to get an IP address, but none are available,
but there are abandoned IP addresses, then the DHCP server will
-attempt to reclaim an abandoned IP address. It marks one IP address
+attempt to reclaim an abandoned IP address. It marks one IP address
as free, and then does the same ICMP Echo request check described
-previously. If there is no answer to the ICMP Echo request, the
+previously. If there is no answer to the ICMP Echo request, the
address is assigned to the client.
.PP
The DHCP server does not cycle through abandoned IP addresses if the
-first IP address it tries to reclaim is free. Rather, when the next
+first IP address it tries to reclaim is free. Rather, when the next
DHCPDISCOVER comes in from the client, it will attempt a new
allocation using the same method described here, and will typically
try a new IP address.
.SH DHCP FAILOVER
This version of the ISC DHCP server supports the DHCP failover
-protocol as documented in draft-ietf-dhc-failover-12.txt. This is
+protocol as documented in draft-ietf-dhc-failover-12.txt. This is
not a final protocol document, and we have not done interoperability
testing with other vendors' implementations of this protocol, so you
must not assume that this implementation conforms to the standard.
peers are running the same version of the ISC DHCP server.
.PP
The failover protocol allows two DHCP servers (and no more than two)
-to share a common address pool. Each server will have about half of
+to share a common address pool. Each server will have about half of
the available IP addresses in the pool at any given time for
-allocation. If one server fails, the other server will continue to
+allocation. If one server fails, the other server will continue to
renew leases out of the pool, and will allocate new addresses out of
the roughly half of available addresses that it had when
communications with the other server were lost.
It is possible during a prolonged failure to tell the remaining server
that the other server is down, in which case the remaining server will
(over time) reclaim all the addresses the other server had available
-for allocation, and begin to reuse them. This is called putting the
+for allocation, and begin to reuse them. This is called putting the
server into the PARTNER-DOWN state.
.PP
You can put the server into the PARTNER-DOWN state either by using the
.B omshell (1)
command or by stopping the server, editing the last failover state
-declaration in the lease file, and restarting the server. If you use
+declaration in the lease file, and restarting the server. If you use
this last method, change the "my state" line to:
.PP
.nf
.B failover peer "\fIname\fB" state {
-.B my state partner-down;
+.B my state partner-down;.
.B peer state \fIstate\fB at \fIdate\fB;
.B }
.fi
and the other server comes back up, the other server will not know
that the first server was in the PARTNER-DOWN state, and may issue
addresses previously issued by the other server to different clients,
-resulting in IP address conflicts. Before putting a server into
+resulting in IP address conflicts. Before putting a server into
PARTNER-DOWN state, therefore, make
.I sure
that the other server will not restart automatically.
.PP
The failover protocol defines a primary server role and a secondary
-server role. There are some differences in how primaries and
+server role. There are some differences in how primaries and
secondaries act, but most of the differences simply have to do with
providing a way for each peer to behave in the opposite way from the
-other. So one server must be configured as primary, and the other
+other. So one server must be configured as primary, and the other
must be configured as secondary, and it doesn't matter too much which
one is which.
.SH FAILOVER STARTUP
When a server starts that has not previously communicated with its
failover peer, it must establish communications with its failover peer
-and synchronize with it before it can serve clients. This can happen
+and synchronize with it before it can serve clients. This can happen
either because you have just configured your DHCP servers to perform
failover for the first time, or because one of your failover servers
has failed catastrophically and lost its database.
.PP
In the case where both servers detect that they have never before
communicated with their partner, they both come up in this recovery
-state and follow the procedure we have just described. In this case,
+state and follow the procedure we have just described. In this case,
no service will be provided to DHCP clients until MCLT has expired.
.SH CONFIGURING FAILOVER
In order to configure failover, you need to write a peer declaration
that configures the failover protocol, and you need to write peer
references in each pool declaration for which you want to do
-failover. You do not have to do failover for all pools on a given
-network segment. You must not tell one server it's doing failover
-on a particular address pool and tell the other it is not. You must
+failover. You do not have to do failover for all pools on a given
+network segment. You must not tell one server it's doing failover
+on a particular address pool and tell the other it is not. You must
not have any common address pools on which you are not doing
failover. A pool declaration that utilizes failover would look like this:
.PP
.nf
failover peer "foo" {
primary;
- address anthrax.rc.vix.com;
+ address anthrax.rc.example.com;
port 519;
- peer address trantor.rc.vix.com;
+ peer address trantor.rc.example.com;
peer port 520;
max-response-delay 60;
max-unacked-updates 10;
.PP
The statements in the peer declaration are as follows:
.PP
-The
+The
.I primary
and
.I secondary
described earlier under DHCP FAILOVER.
.RE
.PP
-The
+The
.I address
statement
.RS 0.25i
value is used as an identifier, it may not be omitted.
.RE
.PP
-The
+The
.I peer address
statement
.RS 0.25i
messages.
.RE
.PP
-The
+The
.I port
statement
.RS 0.25i
used by default.
.RE
.PP
-The
+The
.I peer port
statement
.RS 0.25i
.PP
The \fBmax-response-delay\fR statement tells the DHCP server how
many seconds may pass without receiving a message from its failover
-peer before it assumes that connection has failed. This number
+peer before it assumes that connection has failed. This number
should be small enough that a transient network failure that breaks
the connection will not result in the servers being out of
communication for a long time, but large enough that the server isn't
-constantly making and breaking connections. This parameter must be
+constantly making and breaking connections. This parameter must be
specified.
.RE
.PP
.PP
The \fBmax-unacked-updates\fR statement tells the remote DHCP server how
many BNDUPD messages it can send before it receives a BNDACK
-from the local system. We don't have enough operational experience
-to say what a good value for this is, but 10 seems to work. This
+from the local system. We don't have enough operational experience
+to say what a good value for this is, but 10 seems to work. This
parameter must be specified.
.RE
.PP
-The
+The
.I mclt
statement
.RS 0.25i
.PP
.B mclt \fIseconds\fR\fB;\fR
.PP
-The \fBmclt\fR statement defines the Maximum Client Lead Time. It
+The \fBmclt\fR statement defines the Maximum Client Lead Time. It
must be specified on the primary, and may not be specified on the
-secondary. This is the length of time for which a lease may be
-renewed by either failover peer without contacting the other. The
+secondary. This is the length of time for which a lease may be
+renewed by either failover peer without contacting the other. The
longer you set this, the longer it will take for the running server to
-recover IP addresses after moving into PARTNER-DOWN state. The
+recover IP addresses after moving into PARTNER-DOWN state. The
shorter you set it, the more load your servers will experience when
-they are not communicating. A value of something like 3600 is
+they are not communicating. A value of something like 3600 is
probably reasonable, but again bear in mind that we have no real
operational experience with this.
.RE
.PP
-The
+The
.I split
statement
.RS 0.25i
.PP
-.B split \fIindex\fR\fB;\fR
+.B split \fIbits\fR\fB;\fR
.PP
The split statement specifies the split between the primary and
-secondary for the purposes of load balancing. Whenever a client
+secondary for the purposes of load balancing. Whenever a client
makes a DHCP request, the DHCP server runs a hash on the client
identification, resulting in value from 0 to 255. This is used as
an index into a 256 bit field. If the bit at that index is set,
how many of the leading bits are set to one. So, in practice, higher
split values will cause the primary to serve more clients than the
secondary. Lower split values, the converse. Legal values are between
-0 and 255, of which the most reasonable is 128.
+0 and 256 inclusive, of which the most reasonable is 128. Note that
+a value of 0 makes the secondary responsible for all clients and a value
+of 256 makes the primary responsible for all clients.
.RE
.PP
-The
+The
.I hba
statement
.RS 0.25i
.PP
The hba statement specifies the split between the primary and
secondary as a bitmap rather than a cutoff, which theoretically allows
-for finer-grained control. In practice, there is probably no need
-for such fine-grained control, however. An example hba statement:
+for finer-grained control. In practice, there is probably no need
+for such fine-grained control, however. An example hba statement:
.PP
.nf
hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:
.fi
.PP
This is equivalent to a \fBsplit 128;\fR statement, and identical. The
-following two examples are also equivalent to a \fBsplit\fR of 128, but
+following two examples are also equivalent to a \fBsplit\fR of 128, but
are not identical:
.PP
.nf
\fBsplit\fR should be used.
.RE
.PP
-The
+The
.I load balance max seconds
statement
.RS 0.25i
messages but not responding to some client requests, the other
failover peer will take over its client load automatically as the
clients retry.
+.PP
+It is possible to disable load balancing between peers by setting this
+value to 0 on both peers. Bear in mind that this means both peers will
+respond to all DHCPDISCOVERs or DHCPREQUESTs.
.RE
.PP
The
This version of the DHCP Server evaluates pool balance on a schedule,
rather than on demand as leases are allocated. The latter approach
proved to be slightly klunky when pool misbalanced reach total
-saturation...when any server ran out of leases to assign, it also lost
+saturation \(em when any server ran out of leases to assign, it also lost
its ability to notice it had run dry.
.PP
In order to understand pool balance, some elements of its operation
.RE
.SH CLIENT CLASSING
Clients can be separated into classes, and treated differently
-depending on what class they are in. This separation can be done
+depending on what class they are in. This separation can be done
either with a conditional statement, or with a match statement within
-the class declaration. It is possible to specify a limit on the
+the class declaration. It is possible to specify a limit on the
total number of clients within a particular class or subclass that may
hold leases at one time, and it is possible to specify automatic
subclassing based on the contents of the client packet.
.PP
+Classing support for DHCPv6 clients was added in 4.3.0. It follows
+the same rules as for DHCPv4 except that support for billing classes
+has not been added yet.
+.PP
To add clients to classes based on conditional evaluation, you can
specify a matching expression in the class statement:
.PP
}
.fi
.PP
+Please note that the values used in match expressions may only come from
+data or options that are part of the client packet. It is not possible to
+use values constructed through one or more executable statements. This
+stems from the fact that client classification occurs before any statements
+are executed. Attempting to do so will yield indeterminate results.
+.PP
Note that whether you use matching expressions or add statements (or
both) to classify clients, you must always write a class declaration
-for any class that you use. If there will be no match statement and
+for any class that you use. If there will be no match statement and
no in-scope statements for a class, the declaration should look like
this:
.PP
.fi
.SH SUBCLASSES
.PP
-In addition to classes, it is possible to declare subclasses. A
+In addition to classes, it is possible to declare subclasses. A
subclass is a class with the same name as a regular class, but with a
specific submatch expression which is hashed for quick matching.
This is essentially a speed hack - the main difference between five
classes with match expressions and one class with five subclasses is
-that it will be quicker to find the subclasses. Subclasses work as
+that it will be quicker to find the subclasses. Subclasses work as
follows:
.PP
.nf
The data following the class name in the subclass declaration is a
constant value to use in matching the match expression for the class.
When class matching is done, the server will evaluate the match
-expression and then look the result up in the hash table. If it
+expression and then look the result up in the hash table. If it
finds a match, the client is considered a member of both the class and
the subclass.
.PP
-Subclasses can be declared with or without scope. In the above
+Subclasses can be declared with or without scope. In the above
example, the sole purpose of the subclass is to allow some clients
access to one address pool, while other clients are given access to
-the other pool, so these subclasses are declared without scopes. If
+the other pool, so these subclasses are declared without scopes. If
part of the purpose of the subclass were to define different parameter
values for some clients, you might want to declare some subclasses
with scopes.
.SH PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION
.PP
You may specify a limit to the number of clients in a class that can
-be assigned leases. The effect of this will be to make it difficult
-for a new client in a class to get an address. Once a class with
+be assigned leases. The effect of this will be to make it difficult
+for a new client in a class to get an address. Once a class with
such a limit has reached its limit, the only way a new client in that
class can get a lease is for an existing client to relinquish its
lease, either by letting it expire, or by sending a DHCPRELEASE
-packet. Classes with lease limits are specified as follows:
+packet. Classes with lease limits are specified as follows:
.PP
.nf
class "limited-1" {
It is possible to declare a
.I spawning class\fR.
A spawning class is a class that automatically produces subclasses
-based on what the client sends. The reason that spawning classes
+based on what the client sends. The reason that spawning classes
were created was to make it possible to create lease-limited classes
-on the fly. The envisioned application is a cable-modem environment
+on the fly. The envisioned application is a cable-modem environment
where the ISP wishes to provide clients at a particular site with more
than one IP address, but does not wish to provide such clients with
their own subnet, nor give them an unlimited number of IP addresses
.PP
Many cable modem head-end systems can be configured to add a Relay
Agent Information option to DHCP packets when relaying them to the
-DHCP server. These systems typically add a circuit ID or remote ID
-option that uniquely identifies the customer site. To take advantage
+DHCP server. These systems typically add a circuit ID or remote ID
+option that uniquely identifies the customer site. To take advantage
of this, you can write a class declaration as follows:
.PP
.nf
.fi
.PP
Now whenever a request comes in from a customer site, the circuit ID
-option will be checked against the class's hash table. If a subclass
+option will be checked against the class\'s hash table. If a subclass
is found that matches the circuit ID, the client will be classified in
-that subclass and treated accordingly. If no subclass is found
+that subclass and treated accordingly. If no subclass is found
matching the circuit ID, a new one will be created and logged in the
.B dhcpd.leases
-file, and the client will be classified in this new class. Once the
+file, and the client will be classified in this new class. Once the
client has been classified, it will be treated according to the rules
of the class, including, in this case, being subject to the per-site
limit of four leases.
.PP
In some cases, it may be useful to use one expression to assign a
client to a particular class, and a second expression to put it into a
-subclass of that class. This can be done by combining the \fBmatch
+subclass of that class. This can be done by combining the \fBmatch
if\fR and \fBspawn with\fR statements, or the \fBmatch if\fR and
-\fBmatch\fR statements. For example:
+\fBmatch\fR statements. For example:
.PP
.nf
class "jr-cable-modems" {
compliant so any DNS server supporting RFC 2136 should be able to
accept updates from the DHCP server.
.PP
-Two DNS update schemes are currently implemented, and another is
-planned. The two that are currently implemented are the ad-hoc DNS
-update mode and the interim DHCP-DNS interaction draft update mode.
-In the future we plan to add a third mode which will be the standard
-DNS update method based on the RFCS for DHCP-DNS interaction and DHCID
-The DHCP server must be configured to use one of the two
-currently-supported methods, or not to do dns updates.
-This can be done with the
+There are two DNS schemes implemented. The interim option is
+based on draft revisions of the DDNS documents while the standard
+option is based on the RFCs for DHCP-DNS interaction and DHCIDs.
+A third option, ad-hoc, was deprecated and has now been removed
+from the code base. The DHCP server must be configured to use
+one of the two currently-supported methods, or not to do DNS updates.
+.PP
+New installations should use the standard option. Older
+installations may want to continue using the interim option for
+backwards compatibility with the DNS database until the database
+can be updated. This can be done with the
.I ddns-update-style
configuration parameter.
-.SH THE AD-HOC DNS UPDATE SCHEME
-The ad-hoc Dynamic DNS update scheme is
-.B now deprecated
-and
-.B
-does not work.
-In future releases of the ISC DHCP server, this scheme will not likely be
-available. The interim scheme works, allows for failover, and should now be
-used. The following description is left here for informational purposes
-only.
-.PP
-The ad-hoc Dynamic DNS update scheme implemented in this version of
-the ISC DHCP server is a prototype design, which does not
-have much to do with the standard update method that is being
-standardized in the IETF DHC working group, but rather implements some
-very basic, yet useful, update capabilities. This mode
-.B does not work
-with the
-.I failover protocol
-because it does not account for the possibility of two different DHCP
-servers updating the same set of DNS records.
-.PP
-For the ad-hoc DNS update method, the client's FQDN is derived in two
-parts. First, the hostname is determined. Then, the domain name is
-determined, and appended to the hostname.
-.PP
-The DHCP server determines the client's hostname by first looking for
-a \fIddns-hostname\fR configuration option, and using that if it is
-present. If no such option is present, the server looks for a
-valid hostname in the FQDN option sent by the client. If one is
-found, it is used; otherwise, if the client sent a host-name option,
-that is used. Otherwise, if there is a host declaration that applies
-to the client, the name from that declaration will be used. If none
-of these applies, the server will not have a hostname for the client,
-and will not be able to do a DNS update.
-.PP
-The domain name is determined from the
-.I ddns-domainname
-configuration option. The default configuration for this option is:
-.nf
-.sp 1
- option server.ddns-domainname = config-option domain-name;
-
-.fi
-So if this configuration option is not configured to a different
-value (over-riding the above default), or if a domain-name option
-has not been configured for the client's scope, then the server will
-not attempt to perform a DNS update.
-.PP
-The client's fully-qualified domain name, derived as we have
-described, is used as the name on which an "A" record will be stored.
-The A record will contain the IP address that the client was assigned
-in its lease. If there is already an A record with the same name in
-the DNS server, no update of either the A or PTR records will occur -
-this prevents a client from claiming that its hostname is the name of
-some network server. For example, if you have a fileserver called
-"fs.sneedville.edu", and the client claims its hostname is "fs", no
-DNS update will be done for that client, and an error message will be
-logged.
-.PP
-If the A record update succeeds, a PTR record update for the assigned
-IP address will be done, pointing to the A record. This update is
-unconditional - it will be done even if another PTR record of the same
-name exists. Since the IP address has been assigned to the DHCP
-server, this should be safe.
-.PP
-Please note that the current implementation assumes clients only have
-a single network interface. A client with two network interfaces
-will see unpredictable behavior. This is considered a bug, and will
-be fixed in a later release. It may be helpful to enable the
-.I one-lease-per-client
-parameter so that roaming clients do not trigger this same behavior.
-.PP
-The DHCP protocol normally involves a four-packet exchange - first the
-client sends a DHCPDISCOVER message, then the server sends a
-DHCPOFFER, then the client sends a DHCPREQUEST, then the server sends
-a DHCPACK. In the current version of the server, the server will do
-a DNS update after it has received the DHCPREQUEST, and before it has
-sent the DHCPACK. It only sends the DNS update if it has not sent
-one for the client's address before, in order to minimize the impact
-on the DHCP server.
-.PP
-When the client's lease expires, the DHCP server (if it is operating
-at the time, or when next it operates) will remove the client's A and
-PTR records from the DNS database. If the client releases its lease
-by sending a DHCPRELEASE message, the server will likewise remove the
-A and PTR records.
-.SH THE INTERIM DNS UPDATE SCHEME
-The interim DNS update scheme operates mostly according to several
-drafts considered by the IETF. While the drafts have since become
-RFCs the code was written before they were finalized and there are
-some differences between our code and the final RFCs. We plan to
-update our code, probably adding a standard DNS update option, at
-some time. The basic framework is similar with the main material
-difference being that a DHCID RR was assigned in the RFCs whereas
-our code continues to use an experimental TXT record. The format
-of the TXT record bears a resemblance to the DHCID RR but it is not
-equivalent (MD5 vs SHA1, field length differences etc).
-The standard RFCs are:
+.SH THE DNS UPDATE SCHEME
+the interim and standard DNS update schemes operate mostly according
+to work from the IETF. The interim version was based on the drafts
+in progress at the time while the standard is based on the completed
+RFCs. The standard RFCs are:
.PP
.nf
.ce 3
draft-ietf-dhc-ddns-resolution-??.txt
.fi
.PP
-Because our implementation is slightly different than the standard, we
-will briefly document the operation of this update style here.
+The basic framework for the two schemes is similar with the main
+material difference being that a DHCID RR is used in the standard
+version while the interim versions uses a TXT RR. The format
+of the TXT record bears a resemblance to the DHCID RR but it is not
+equivalent (MD5 vs SHA2, field length differences etc).
.PP
-The first point to understand about this style of DNS update is that
-unlike the ad-hoc style, the DHCP server does not necessarily
-always update both the A and the PTR records. The FQDN option
+In these two schemes the DHCP server does not necessarily
+always update both the A and the PTR records. The FQDN option
includes a flag which, when sent by the client, indicates that the
-client wishes to update its own A record. In that case, the server
-can be configured either to honor the client's intentions or ignore
-them. This is done with the statement \fIallow client-updates;\fR or
-the statement \fIignore client-updates;\fR. By default, client
+client wishes to update its own A record. In that case, the server
+can be configured either to honor the client\'s intentions or ignore
+them. This is done with the statement \fIallow client-updates;\fR or
+the statement \fIignore client-updates;\fR. By default, client
updates are allowed.
.PP
If the server is configured to allow client updates, then if the
client sends a fully-qualified domain name in the FQDN option, the
server will use that name the client sent in the FQDN option to update
-the PTR record. For example, let us say that the client is a visitor
-from the "radish.org" domain, whose hostname is "jschmoe". The
-server is for the "example.org" domain. The DHCP client indicates in
-the FQDN option that its FQDN is "jschmoe.radish.org.". It also
-indicates that it wants to update its own A record. The DHCP server
+the PTR record. For example, let us say that the client is a visitor
+from the "radish.org" domain, whose hostname is "jschmoe". The
+server is for the "example.org" domain. The DHCP client indicates in
+the FQDN option that its FQDN is "jschmoe.radish.org.". It also
+indicates that it wants to update its own A record. The DHCP server
therefore does not attempt to set up an A record for the client, but
does set up a PTR record for the IP address that it assigns the
-client, pointing at jschmoe.radish.org. Once the DHCP client has an
+client, pointing at jschmoe.radish.org. Once the DHCP client has an
IP address, it can update its own A record, assuming that the
"radish.org" DNS server will allow it to do so.
.PP
If the server is configured not to allow client updates, or if the
-client doesn't want to do its own update, the server will simply
-choose a name for the client from either the fqdn option (if present)
-or the hostname option (if present). It will use its own
-domain name for the client, just as in the ad-hoc update scheme.
-It will then update both the A and PTR record, using the name that it
-chose for the client. If the client sends a fully-qualified domain
-name in the fqdn option, the server uses only the leftmost part of the
-domain name - in the example above, "jschmoe" instead of
-"jschmoe.radish.org".
+client doesn\'t want to do its own update, the server will simply
+choose a name for the client. By default, the server will choose
+from the following three values:
+.PP
+ 1. \fBfqdn\fR option (if present)
+ 2. hostname option (if present)
+ 3. Configured hostname option (if defined).
+.PP
+If these defaults for choosing the host name are not appropriate
+you can write your own statement to set the ddns-hostname variable
+as you wish. If none of the above are found the server will use
+the host declaration name (if one) and use-host-decl-names is on.
+.PP
+It will use its own domain name for the client. It will then update
+both the A and PTR record, using the name that it chose for the client.
+If the client sends a fully-qualified domain name in the \fBfqdn\fR option,
+the server uses only the leftmost part of the domain name - in the example
+above, "jschmoe" instead of "jschmoe.radish.org".
.PP
Further, if the \fIignore client-updates;\fR directive is used, then
the server will in addition send a response in the DHCP packet, using
own updates if it chooses to do so. With \fIdeny client-updates;\fR, a
response is sent which indicates the client may not perform updates.
.PP
-Also, if the
-.I use-host-decl-names
-configuration option is enabled, then the host declaration's
-.I hostname
-will be used in place of the
-.I hostname
-option, and the same rules will apply as described above.
-.PP
-The other difference between the ad-hoc scheme and the interim
-scheme is that with the interim scheme, a method is used that
-allows more than one DHCP server to update the DNS database without
-accidentally deleting A records that shouldn't be deleted nor failing
-to add A records that should be added. The scheme works as follows:
+Both the standard and interim options also include a method to
+allow more than one DHCP server to update the DNS database without
+accidentally deleting A records that shouldn\'t be deleted nor failing
+to add A records that should be added. For the standard option the
+method works as follows:
.PP
When the DHCP server issues a client a new lease, it creates a text
-string that is an MD5 hash over the DHCP client's identification (see
-draft-ietf-dnsext-dhcid-rr-??.txt for details). The update adds an A
-record with the name the server chose and a TXT record containing the
-hashed identifier string (hashid). If this update succeeds, the
+string that is an SHA hash over the DHCP client\'s identification (see
+RFCs 4701 & 4702 for details). The update attempts to add an A
+record with the name the server chose and a DHCID record containing the
+hashed identifier string (hashid). If this update succeeds, the
server is done.
.PP
If the update fails because the A record already exists, then the DHCP
server attempts to add the A record with the prerequisite that there
-must be a TXT record in the same name as the new A record, and that
-TXT record's contents must be equal to hashid. If this update
-succeeds, then the client has its A record and PTR record. If it
+must be a DHCID record in the same name as the new A record, and that
+DHCID record\'s contents must be equal to hashid. If this update
+succeeds, then the client has its A record and PTR record. If it
fails, then the name the client has been assigned (or requested) is in
-use, and can't be used by the client. At this point the DHCP server
+use, and can\'t be used by the client. At this point the DHCP server
gives up trying to do a DNS update for the client until the client
chooses a new name.
.PP
-The interim DNS update scheme is called interim for two reasons.
-First, it does not quite follow the RFCs. The RFCs call for a
-new DHCID RRtype while he interim DNS update scheme uses a TXT record.
-The ddns-resolution draft called for the DHCP server to put a DHCID RR
-on the PTR record, but the \fIinterim\fR update method does not do this.
-In the final RFC this requirement was relaxed such that a server may
-add a DHCID RR to the PTR record.
-.PP
-In addition to these differences, the server also does not update very
-aggressively. Because each DNS update involves a round trip to the
-DNS server, there is a cost associated with doing updates even if they
-do not actually modify the DNS database. So the DHCP server tracks
-whether or not it has updated the record in the past (this information
-is stored on the lease) and does not attempt to update records that it
+The server also does not update very aggressively. Because each
+DNS update involves a round trip to the DNS server, there is a cost
+associated with doing updates even if they do not actually modify
+the DNS database. So the DHCP server tracks whether or not it has
+updated the record in the past (this information is stored on the
+lease) and does not attempt to update records that it
thinks it has already updated.
.PP
This can lead to cases where the DHCP server adds a record, and then
the record is deleted through some other mechanism, but the server
never again updates the DNS because it thinks the data is already
-there. In this case the data can be removed from the lease through
+there. In this case the data can be removed from the lease through
operator intervention, and once this has been done, the DNS will be
updated the next time the client renews.
+.PP
+The interim DNS update scheme was written before the RFCs were finalized
+and does not quite follow them. The RFCs call for a new DHCID RRtype
+while the interim DNS update scheme uses a TXT record. In addition
+the ddns-resolution draft called for the DHCP server to put a DHCID RR
+on the PTR record, but the \fIinterim\fR update method does not do this.
+In the final RFC this requirement was relaxed such that a server may
+add a DHCID RR to the PTR record.
+.PP
+.SH DDNS IN DUAL STACK ENVIRONMENTS
+As described in RFC 4703, section 5.2, in order to perform DDNS in dual
+stack environments, both IPv4 and IPv6 servers would need to be configured
+to use the standard update style and participating IPv4 clients MUST
+convey DUIDs as described in RFC 4361, section 6.1., in their
+dhcp-client-identifiers.
+.PP
+In a nutshell, this mechanism is intended to use globally unique DUIDs
+to idenfity both IPv4 and IPv6 clients, and where a device has both
+IPv4 and IPv6 leases it is identified by the same DUID. This allows
+a dual stack client to use the same FQDN for both mappings, while
+being protected from updates for other clients by the rules of conflict
+detection.
+.PP
+However, not all IPv4 clients implement this behavior which makes
+supporting them dual stack environments problematic. In order to
+address this issue ISC DHCP (as of 4.4.0) supports a new mode of
+DDNS conflict resolution referred to as Dual Stack Mixed Mode (DSMM).
+.PP
+The concept behind DSMM is relatively simple. All dhcp servers of one
+protocol (IPv4 or v6) use one ddns-update-style (interim or standard)
+while all servers of the "other" protocol will use the "other"
+ddns-udpate-style. In this way, all servers of a given protocol are
+using the same record type (TXT or DHCID) for their DHCID RR entries.
+This allows conflict detection to be enforced within each protocol
+without interferring with the other's entries.
+.PP
+DSMM modifications now ensure that IPv4 DSMM servers only ever modify
+A records, their associated PTR records and DHCID records, while DSMM
+IPv6 severs only modify AAAA records, their associated PTR records,
+and DHCID records.
+.PP
+Note that DSMM is not a perfect solution, it is a compromise that can
+work well provided all participating DNS updaters play by DSMM rules.
+As with anything else in life, it only works as well as those who
+particpate behave.
+.PP
+While conflict detection is enabled by default, DSMM is not. To enable
+DSMM, both update-conflict-detection and ddns-dual-stack-mixed-mode must
+be true.
+.PP
+.SH PROTECTING DNS ENTRIES FOR STATIC CLIENTS
+Built into conflict resolution is the protection of manually made entries
+for static clients. Per the rules of conflict resolution, a DNS updater
+may not alter forward DNS entries unless there is a DHCID RR which matches
+for whom the update is being made. Therefore, any forward DNS entries
+without a corresponding DHCID RR cannot be altered by such an updater.
+.PP
+In some environments, it may be desirable to use only this aspect of conflict
+resolution and allow DNS updaters to overwrite entries for dynamic clients
+regardless of what client owns them. In other words, the presence or lack
+of a DHCID RR is used to determine whether entries may or may not be
+overwritten. Whether or not the client matches the data value of the DHCID
+RR is irrelevant. This behavior, off by default, can be configured through
+the parameter, ddns-guard-id-must-match. As with DSMM, this behavior is
+can only be enabled if conflict resolution is enabled. This behavior should
+be considered carefully before electing to use it.
+.PP
+There is an additional parameter that can be used with DSMM
+ddns-other-guard-is-dynamic. When enabled along with DSMM, a server will
+regard the presence of a DHCID RR of the other style type as indicating that
+the forward DNS entries for that FQDN should be dynamic and may be overwritten.
+For example, such a server using interim style could overwrite the DNS entries
+for an FQDN if there is only a DHDID type DHDID RR for the FQDN. Essentially,
+if there are dynamic entries for one protocol, that is enough to overcome the
+static protection of entries for the other protocol. This behavior warrants
+careful consideration before electing to use it.
+.PP
.SH DYNAMIC DNS UPDATE SECURITY
.PP
When you set your DNS server up to allow updates from the DHCP server,
you may be exposing it to unauthorized updates. To avoid this, you
should use TSIG signatures - a method of cryptographically signing
-updates using a shared secret key. As long as you protect the
-secrecy of this key, your updates should also be secure. Note,
+updates using a shared secret key. As long as you protect the
+secrecy of this key, your updates should also be secure. Note,
however, that the DHCP protocol itself provides no security, and that
clients can therefore provide information to the DHCP server which the
DHCP server will then use in its updates, with the constraints
.fi
.PP
You will also have to configure your DHCP server to do updates to
-these zones. To do so, you need to add something like this to your
+these zones. To do so, you need to add something like this to your
dhcpd.conf file:
.PP
.nf
.fi
.PP
The \fIprimary\fR statement specifies the IP address of the name
-server whose zone information is to be updated.
+server whose zone information is to be updated. In addition to
+the \fIprimary\fR statement there are also the \fIprimary6\fR ,
+\fIsecondary\fR and \fIsecondary6\fR statements. The \fIprimary6\fR
+statement specifies an IPv6 address for the name server. The
+secondaries provide for additional addresses for name servers
+to be used if the primary does not respond. The number of name
+servers the DDNS code will attempt to use before giving up
+is limited and is currently set to three.
.PP
Note that the zone declarations have to correspond to authority
records in your name server - in the above example, there must be an
-SOA record for "example.org." and for "17.10.10.in-addr.arpa.". For
+SOA record for "example.org." and for "17.10.10.in-addr.arpa.". For
example, if there were a subdomain "foo.example.org" with no separate
-SOA, you could not write a zone declaration for "foo.example.org."
+SOA, you could not write a zone declaration for "foo.example.org."
Also keep in mind that zone names in your DHCP configuration should end in a
"."; this is the preferred syntax. If you do not end your zone name in a
".", the DHCP server will figure it out. Also note that in the DHCP
configuration, zone names are not encapsulated in quotes where there are in
the DNS configuration.
.PP
-You should choose your own secret key, of course. The ISC BIND 8 and
-9 distributions come with a program for generating secret keys called
-dnssec-keygen. The version that comes with BIND 9 is likely to produce a
-substantially more random key, so we recommend you use that one even
-if you are not using BIND 9 as your DNS server. If you are using BIND 9's
+You should choose your own secret key, of course. The ISC BIND 9
+distribution comes with a program for generating secret keys called
+dnssec-keygen. If you are using BIND 9\'s
dnssec-keygen, the above key would be created as follows:
.PP
.nf
dnssec-keygen -a HMAC-MD5 -b 128 -n USER DHCP_UPDATER
.fi
.PP
-If you are using the BIND 8 dnskeygen program, the following command will
-generate a key as seen above:
-.PP
+The key name, algorithm, and secret must match that being used by the DNS
+server. The DHCP server currently supports the following algorithms:
.nf
- dnskeygen -H 128 -u -c -n DHCP_UPDATER
+
+ HMAC-MD5
+ HMAC-SHA1
+ HMAC-SHA224
+ HMAC-SHA256
+ HMAC-SHA384
+ HMAC-SHA512
.fi
.PP
You may wish to enable logging of DNS updates on your DNS server.
.fi
.PP
You must create the /var/log/named-auth.info and
-/var/log/update-debug.log files before starting the name server. For
+/var/log/update-debug.log files before starting the name server. For
more information on configuring ISC BIND, consult the documentation
that accompanies it.
.SH REFERENCE: EVENTS
.PP
There are three kinds of events that can happen regarding a lease, and
it is possible to declare statements that occur when any of these
-events happen. These events are the commit event, when the server
+events happen. These events are the commit event, when the server
has made a commitment of a certain lease to a client, the release
event, when the client has released the server from its commitment,
and the expiry event, when the commitment expires.
To declare a set of statements to execute when an event happens, you
must use the \fBon\fR statement, followed by the name of the event,
followed by a series of statements to execute when the event happens,
-enclosed in braces. Events are used to implement DNS
-updates, so you should not define your own event handlers if you are
-using the built-in DNS update mechanism.
-.PP
-The built-in version of the DNS update mechanism is in a text
-string towards the top of server/dhcpd.c. If you want to use events
-for things other than DNS updates, and you also want DNS updates, you
-will have to start out by copying this code into your dhcpd.conf file
-and modifying it.
+enclosed in braces.
.SH REFERENCE: DECLARATIONS
.PP
.B The
the contents of that file as though it were entered in place of the
include statement.
.PP
-.B The
+.B The
.I shared-network
.B statement
.PP
client should boot.
.PP
.I Name
-should be the name of the shared network. This name is used when
+should be the name of the shared network. This name is used when
printing debugging messages, so it should be descriptive for the
-shared network. The name may have the syntax of a valid domain name
+shared network. The name may have the syntax of a valid domain name
(although it will never be used as such), or it may be any arbitrary
name, enclosed in quotes.
.PP
-.B The
+.B The
.I subnet
.B statement
.PP
information to tell whether or not an IP address is on that subnet.
It may also be used to provide subnet-specific parameters and to
specify what addresses may be dynamically allocated to clients booting
-on that subnet. Such addresses are specified using the \fIrange\fR
+on that subnet. Such addresses are specified using the \fIrange\fR
declaration.
.PP
The
.I subnet-number
should be an IP address or domain name which resolves to the subnet
-number of the subnet being described. The
+number of the subnet being described. The
.I netmask
should be an IP address or domain name which resolves to the subnet mask
-of the subnet being described. The subnet number, together with the
+of the subnet being described. The subnet number, together with the
netmask, are sufficient to determine whether any given IP address is
on the specified subnet.
.PP
the desired subnet mask, since any subnet-mask option statement will
override the subnet mask declared in the subnet statement.
.PP
-.B The
+.B The
.I subnet6
.B statement
.PP
information to tell whether or not an IPv6 address is on that subnet6.
It may also be used to provide subnet-specific parameters and to
specify what addresses may be dynamically allocated to clients booting
-on that subnet.
+on that subnet.
.PP
The
.I subnet6-number
.fi
.PP
For any subnet on which addresses will be assigned dynamically, there
-must be at least one \fIrange\fR statement. The range statement
-gives the lowest and highest IP addresses in a range. All IP
+must be at least one \fIrange\fR statement. The range statement
+gives the lowest and highest IP addresses in a range. All IP
addresses in the range should be in the subnet in which the
-\fIrange\fR statement is declared. The \fIdynamic-bootp\fR flag may
+\fIrange\fR statement is declared. The \fIdynamic-bootp\fR flag may
be specified if addresses in the specified range may be dynamically
-assigned to BOOTP clients as well as DHCP clients. When specifying a
+assigned to BOOTP clients as well as DHCP clients. When specifying a
single address, \fIhigh-address\fR can be omitted.
.PP
.B The
.PP
For any IPv6 subnet6 on which addresses will be assigned dynamically, there
must be at least one \fIrange6\fR statement. The \fIrange6\fR statement
-can either be the lowest and highest IPv6 addresses in a \fIrange6\fR, or
-use CIDR notation, specified as ip6-address/bits. All IP addresses
+can either be the lowest and highest IPv6 addresses in a \fIrange6\fR, or
+use CIDR notation, specified as ip6-address/bits. All IP addresses
in the \fIrange6\fR should be in the subnet6 in which the
\fIrange6\fR statement is declared.
.PP
network is computed at each request with an IA_TA option. Release and Confirm
ignores temporary addresses.
.PP
-Any IPv6 addresses given to hosts with \fIfixed-address6\fR are excluded
+Any IPv6 addresses given to hosts with \fIfixed-address6\fR are excluded
from the \fIrange6\fR, as are IPv6 addresses on the server itself.
.PP
.PP
.PP
The
.B host
-declaration provides a scope in which to provide configuration information about
-a specific client, and also provides a way to assign a client a fixed address.
-The host declaration provides a way for the DHCP server to identify a DHCP or
-BOOTP client, and also a way to assign the client a static IP address.
+declaration provides a way for the DHCP server to identify a DHCP or
+BOOTP client. This allows the server to provide configuration
+information including fixed addresses or, in DHCPv6, fixed prefixes
+for a specific client.
.PP
If it is desirable to be able to boot a DHCP or BOOTP client on more than one
-subnet with fixed addresses, more than one address may be specified in the
+subnet with fixed v4 addresses, more than one address may be specified in the
.I fixed-address
declaration, or more than one
.B host
statement may be specified matching the same client.
.PP
+The
+.I fixed-address6
+declaration is used for v6 addresses. At this time it only works with a single
+address. For multiple addresses specify multiple
+.B host
+statements.
+.PP
If client-specific boot parameters must change based on the network
-to which the client is attached, then multiple
+to which the client is attached, then multiple
.B host
declarations should be used. The
.B host
\fIhost\fR declaration or the client does not provide a
\fRdhcp-client-identifier\fR option, by matching the \fIhardware\fR
parameter in the \fIhost\fR declaration to the network hardware
-address supplied by the client. BOOTP clients do not normally
+address supplied by the client. BOOTP clients do not normally
provide a \fIdhcp-client-identifier\fR, so the hardware address must
be used for all clients that may boot using the BOOTP protocol.
.PP
.B only
the \fIdhcp-client-identifier\fR option and the hardware address can be
used to match a host declaration, or the \fIhost-identifier option\fR
-parameter for DHCPv6 servers. For example, it is not possible to
-match a host declaration to a \fIhost-name\fR option. This is
+parameter for DHCPv6 servers. For example, it is not possible to
+match a host declaration to a \fIhost-name\fR option. This is
because the host-name option cannot be guaranteed to be unique for any
given client, whereas both the hardware address and
\fIdhcp-client-identifier\fR option are at least theoretically
.fi
.PP
The group statement is used simply to apply one or more parameters to
-a group of declarations. It can be used to group hosts, shared
+a group of declarations. It can be used to group hosts, shared
networks, subnets, or even other groups.
.SH REFERENCE: ALLOW AND DENY
The
different meanings depending on the context. In a pool context, these
keywords can be used to set up access lists for address allocation
pools. In other contexts, the keywords simply control general server
-behavior with respect to clients based on scope. In a non-pool
+behavior with respect to clients based on scope. In a non-pool
context, the
.I ignore
keyword can be used in place of the
\fBignore unknown-clients;\fR
.PP
The \fBunknown-clients\fR flag is used to tell dhcpd whether
-or not to dynamically assign addresses to unknown clients. Dynamic
+or not to dynamically assign addresses to unknown clients. Dynamic
address assignment to unknown clients is \fBallow\fRed by default.
An unknown client is simply a client that has no host declaration.
.PP
or not to respond to bootp queries. Bootp queries are \fBallow\fRed
by default.
.PP
-This option does not satisfy the requirement of failover peers for denying
-dynamic bootp clients. The \fBdeny dynamic bootp clients;\fR option should
-be used instead. See the ALLOW AND DENY WITHIN POOL DECLARATIONS section
-of this man page for more details.
-.PP
.B The
.I booting
.B keyword
.PP
The \fBbooting\fR flag is used to tell dhcpd whether or not to respond
to queries from a particular client. This keyword only has meaning
-when it appears in a host declaration. By default, booting is
+when it appears in a host declaration. By default, booting is
\fBallow\fRed, but if it is disabled for a particular client, then
that client will not be able to get an address from the DHCP server.
.PP
.PP
Host declarations can match client messages based on the DHCP Client
Identifier option or based on the client's network hardware type and
-MAC address. If the MAC address is used, the host declaration will
+MAC address. If the MAC address is used, the host declaration will
match any client with that MAC address - even clients with different
-client identifiers. This doesn't normally happen, but is possible
+client identifiers. This doesn't normally happen, but is possible
when one computer has more than one operating system installed on it -
for example, Microsoft Windows and NetBSD or Linux.
.PP
The \fBduplicates\fR flag tells the DHCP server that if a request is
received from a client that matches the MAC address of a host
declaration, any other leases matching that MAC address should be
-discarded by the server, even if the UID is not the same. This is a
+discarded by the server, even if the UID is not the same. This is a
violation of the DHCP protocol, but can prevent clients whose client
identifiers change regularly from holding many leases at the same time.
By default, duplicates are \fBallow\fRed.
\fBignore declines;\fR
.PP
The DHCPDECLINE message is used by DHCP clients to indicate that the
-lease the server has offered is not valid. When the server receives
+lease the server has offered is not valid. When the server receives
a DHCPDECLINE for a particular address, it normally abandons that
address, assuming that some unauthorized system is using it.
Unfortunately, a malicious or buggy client can, using DHCPDECLINE
-messages, completely exhaust the DHCP server's allocation pool. The
-server will reclaim these leases, but while the client is running
-through the pool, it may cause serious thrashing in the DNS, and it
-will also cause the DHCP server to forget old DHCP client address
+messages, completely exhaust the DHCP server's allocation pool. The
+server will eventually reclaim these leases, but not while the client
+is running through the pool. This may cause serious thrashing in the DNS,
+and it will also cause the DHCP server to forget old DHCP client address
allocations.
.PP
The \fBdeclines\fR flag tells the DHCP server whether or not to honor
-DHCPDECLINE messages. If it is set to \fBdeny\fR or \fBignore\fR in
+DHCPDECLINE messages. If it is set to \fBdeny\fR or \fBignore\fR in
a particular scope, the DHCP server will not respond to DHCPDECLINE
messages.
.PP
+The \fBdeclines\fR flag is only supported by DHCPv4 servers. Given the large
+IPv6 address space and the internal limits imposed by the server's
+address generation mechanism we don't think it is necessary for DHCPv6
+servers at this time.
+.PP
+Currently, abandoned IPv6 addresses are reclaimed in one of two ways:
+ a) Client renews a specific address:
+ If a client using a given DUID submits a DHCP REQUEST containing
+ the last address abandoned by that DUID, the address will be
+ reassigned to that client.
+
+ b) Upon the second restart following an address abandonment. When
+ an address is abandoned it is both recorded as such in the lease
+ file and retained as abandoned in server memory until the server
+ is restarted. Upon restart, the server will process the lease file
+ and all addresses whose last known state is abandoned will be
+ retained as such in memory but not rewritten to the lease file.
+ This means that a subsequent restart of the server will not see the
+ abandoned addresses in the lease file and therefore have no record
+ of them as abandoned in memory and as such perceive them as free
+ for assignment.
+.PP
+The total number addresses in a pool, available for a given DUID value,
+is internally limited by the server's address generation mechanism. If
+through mistaken configuration, multiple clients are using the same
+DUID they will competing for the same addresses causing the server to reach
+this internal limit rather quickly. The internal limit isolates this type
+of activity such that address range is not exhausted for other DUID values.
+The appearance of the following error log, can be an indication of this
+condition:
+
+ "Best match for DUID <XX> is an abandoned address, This may be a
+ result of multiple clients attempting to use this DUID"
+
+ where <XX> is an actual DUID value depicted as colon separated
+ string of bytes in hexadecimal values.
+.PP
.B The
.I client-updates
.B keyword
\fBdeny client-updates;\fR
.PP
The \fBclient-updates\fR flag tells the DHCP server whether or not to
-honor the client's intention to do its own update of its A record.
-This is only relevant when doing \fIinterim\fR DNS updates. See the
-documentation under the heading THE INTERIM DNS UPDATE SCHEME for
-details.
+honor the client's intention to do its own update of its A record. See
+the documentation under the heading THE DNS UPDATE SCHEME for details.
.PP
.B The
.I leasequery
.PP
The \fBleasequery\fR flag tells the DHCP server whether or not to
answer DHCPLEASEQUERY packets. The answer to a DHCPLEASEQUERY packet
-includes information about a specific lease, such as when it was
-issued and when it will expire. By default, the server will not
+includes information about a specific lease, such as when it was
+issued and when it will expire. By default, the server will not
respond to these packets.
.SH ALLOW AND DENY WITHIN POOL DECLARATIONS
.PP
DHCPDISCOVER or a DHCPREQUEST message - an address will be allocated
to the client (either the old address it's requesting, or a new
address) and then that address will be tested to see if it's okay to
-let the client have it. If the client requested it, and it's not
-okay, the server will send a DHCPNAK message. Otherwise, the server
-will simply not respond to the client. If it is okay to give the
+let the client have it. If the client requested it, and it's not
+okay, the server will send a DHCPNAK message. Otherwise, the server
+will simply not respond to the client. If it is okay to give the
address to the client, the server will send a DHCPACK message.
.PP
The primary motivation behind pool declarations is to have address
-allocation pools whose allocation policies are different. A client
+allocation pools whose allocation policies are different. A client
may be denied access to one pool, but allowed access to another pool
-on the same network segment. In order for this to work, access
+on the same network segment. In order for this to work, access
control has to be done during address allocation, not after address
allocation is done.
.PP
.PP
If specified, this statement either allows or prevents allocation from
this pool to any client that has been authenticated using the DHCP
-authentication protocol. This is not yet supported.
+authentication protocol. This is not yet supported.
.PP
\fBunauthenticated clients;\fR
.PP
If specified, this statement either allows or prevents allocation from
this pool to any client that has not been authenticated using the DHCP
-authentication protocol. This is not yet supported.
+authentication protocol. This is not yet supported.
.PP
\fBall clients;\fR
.PP
If specified, this statement either allows or prevents allocation from
-this pool to all clients. This can be used when you want to write a
+this pool to all clients. This can be used when you want to write a
pool declaration for some reason, but hold it in reserve, or when you
want to renumber your network quickly, and thus want the server to
force all clients that have been allocated addresses from this pool to
e.g. 4 2007/08/24 11:14:32 -7200
.SH REFERENCE: PARAMETERS
The
+.I abandon-lease-time
+statement
+.RS 0.25i
+.PP
+.B abandon-lease-time \fItime\fR\fB;\fR
+.PP
+.I Time
+should be the maximum amount of time (in seconds) that an abandoned IPv4 lease
+remains unavailable for assignment to a client. Abandoned leases will only be
+offered to clients if there are no free leases. If not defined, the default
+abandon lease time is 86400 seconds (24 hours). Note the abandoned lease time
+for a given lease is preserved across server restarts. The parameter may only
+be set at the global scope and is evaluated only once during server startup.
+.PP
+Values less than sixty seconds are not recommended as this is below the ping
+check threshold and can cause leases once abandoned but since returned to the
+free state to not be pinged before being offered. If the requested time is
+larger than 0x7FFFFFFF - 1 or the sum of the current time plus the abandoned time isgreater than 0x7FFFFFFF it is treated as infinite.
+.RE
+.PP
+The
.I adaptive-lease-time-threshold
statement
.RS 0.25i
The DHCP and BOOTP protocols both require DHCP and BOOTP clients to
set the broadcast bit in the flags field of the BOOTP message header.
Unfortunately, some DHCP and BOOTP clients do not do this, and
-therefore may not receive responses from the DHCP server. The DHCP
+therefore may not receive responses from the DHCP server. The DHCP
server can be made to always broadcast its responses to clients by
setting this flag to \'on\' for the relevant scope; relevant scopes would be
inside a conditional statement, as a parameter for a class, or as a parameter
-for a host declaration. To avoid creating excess broadcast traffic on your
+for a host declaration. To avoid creating excess broadcast traffic on your
network, we recommend that you restrict the use of this option to as few
-clients as possible. For example, the Microsoft DHCP client is known not
+clients as possible. For example, the Microsoft DHCP client is known not
to have this problem, as are the OpenTransport and ISC DHCP clients.
.RE
.PP
.B always-reply-rfc1048 \fIflag\fR\fB;\fR
.PP
Some BOOTP clients expect RFC1048-style responses, but do not follow
-RFC1048 when sending their requests. You can tell that a client is
+RFC1048 when sending their requests. You can tell that a client is
having this problem if it is not getting the options you have
configured for it and if you see in the server log the message
"(non-rfc1048)" printed with each BOOTREQUEST that is logged.
If you want to send rfc1048 options to such a client, you can set the
.B always-reply-rfc1048
option in that client's host declaration, and the DHCP server will
-respond with an RFC-1048-style vendor options field. This flag can
+respond with an RFC-1048-style vendor options field. This flag can
be set in any scope, and will affect all clients covered by that
scope.
.RE
Network administrators setting up authoritative DHCP servers for their
networks should always write \fBauthoritative;\fR at the top of their
configuration file to indicate that the DHCP server \fIshould\fR send
-DHCPNAK messages to misconfigured clients. If this is not done,
+DHCPNAK messages to misconfigured clients. If this is not done,
clients will be unable to get a correct IP address after changing
subnets until their old lease has expired, which could take quite a
long time.
.PP
Usually, writing \fBauthoritative;\fR at the top level of the file
-should be sufficient. However, if a DHCP server is to be set up so
+should be sufficient. However, if a DHCP server is to be set up so
that it is aware of some networks for which it is authoritative and
some networks for which it is not, it may be more appropriate to
declare authority on a per-network-segment basis.
network, but not authoritative for others, nor is it meaningful to
specify that the server is authoritative for some host declarations
and not others.
+.PP
+In order for DHCPINFORMs to be responded to by the server,
+they must match to subnets over which the server has authority;
+otherwise they will be ignored and logged. To minimize the
+impact on logging volume, only the first and every subsequent 100th
+occurrence of an ignored DHCPINFORM is logged.
.RE
.PP
The \fIboot-unknown-clients\fR statement
If the \fIboot-unknown-clients\fR statement is present and has a value
of \fIfalse\fR or \fIoff\fR, then clients for which there is no
.I host
-declaration will not be allowed to obtain IP addresses. If this
+declaration will not be allowed to obtain IP addresses. If this
statement is not present or has a value of \fItrue\fR or \fIon\fR,
then clients without host declarations will be allowed to obtain IP
addresses, as long as those addresses are not restricted by
and \fIdeny\fR statements within their \fIpool\fR declarations.
.RE
.PP
+The \fIcheck-secs-byte-order\fR statement
+.RS 0.25i
+.PP
+.B check-secs-byte-order \fIflag\fB;\fR
+.PP
+When \fIcheck-secs-byte-order\fR is enabled, the server will check for DHCPv4
+clients that do the byte ordering on the secs field incorrectly. This field
+should be in network byte order but some clients get it wrong. When this
+parameter is enabled the server will examine the secs field and if it looks
+wrong (high byte non zero and low byte zero) swap the bytes. The default
+is disabled. This parameter is only useful when doing load balancing within
+failover. (Formerly, this behavior had to be enabled during compilation
+configuration via --enable-secs-byteorder).
+.PP
The \fIdb-time-format\fR statement
.RS 0.25i
.PP
.B ddns-hostname \fIname\fB;\fR
.PP
The \fIname\fR parameter should be the hostname that will be used in
-setting up the client's A and PTR records. If no ddns-hostname is
+setting up the client's A and PTR records. If no \fIddns-hostname\fR is
specified in scope, then the server will derive the hostname
automatically, using an algorithm that varies for each of the
different update methods.
domain-name (FQDN).
.RE
.PP
+The \fIddns-dual-stack-mixed-mode\fR statement
+.RS 0.25i
+.PP
+.B ddns-dual-stack-mixed-mode \fIflag\fB;\fR
+.PP
+The \fIddns-dual-stack-mixed-mode\fR parameter controls whether or not the
+server applies Dual Stack Mixed Mode rules during DDNS conflict resolution.
+This parameter is off by default, has no effect unless
+update-conflict-detection is enabled, and may only be specified at the
+global scope.
+.RE
+.PP
+The \fIddns-guard-id-must-match\fR statement
+.RS 0.25i
+.PP
+.B ddns-guard-id-must-match \fIflag\fB;\fR
+.PP
+The \fIddns-guard-id-must-match\fR parameter controls whether or not a
+the client id within a DHCID RR must match that of the DNS update's client
+to permit DNS entries associated with that DHCID RR to be ovewritten.
+Proper conflict resolution requires ID matching and should only be disabled
+after careful consideration. When disabled, it is allows any DNS updater to
+replace DNS entries that have an associated DHCID RR, regardless of client
+identity. This parameter is on by default, has no effect unless
+update-conflict-detection is enabled, and may only be specified at the global
+scope.
+.RE
+.PP
+The \fddns-local-address4\fR and \fddns-local-address6\fR statements
+.RS 0.25i
+.PP
+.B ddns-local-address4 \fIaddress\fB;\fR
+.PP
+.B ddns-local-address6 \fIaddress\fB;\fR
+.PP
+The \fIaddress\fR parameter should be the local IPv4 or IPv6 address
+the server should use as the from address when sending DDNS update
+requests.
+.RE
+.PP
+The \fIddns-other-guard-is-dynamic\fR statement
+.RS 0.25i
+.PP
+.B ddns-other-guard-is-dynamic \fIflag\fB;\fR
+.PP
+The \fIddns-other-guard-is-dynamic\fR parameter controls whether or not a
+a server running DSMM will consider the presence of the other update style
+DHCID RR as an indcation that a DNS entries may be overwritten. It should
+only be enabled after careful study as it allows DNS entries that would
+otherwise be protected as static, to be overwritten in certain cases. This
+paramater is off by default, has no effect unless ddns-dual-stack-mixed-mode
+is enabled, and may only be specified at the global scope.
+.RE
+.PP
The \fIddns-rev-domainname\fR statement
.RS 0.25i
.PP
.B ddns-rev-domainname \fIname\fB;\fR
+.PP
The \fIname\fR parameter should be the domain name that will be
appended to the client's reversed IP address to produce a name for use
-in the client's PTR record. By default, this is "in-addr.arpa.", but
+in the client's PTR record. By default, this is "in-addr.arpa.", but
the default can be overridden here.
.PP
The reversed IP address to which this domain name is appended is
always the IP address of the client, in dotted quad notation, reversed
- for example, if the IP address assigned to the client is
-10.17.92.74, then the reversed IP address is 74.92.17.10. So a
+10.17.92.74, then the reversed IP address is 74.92.17.10. So a
client with that IP address would, by default, be given a PTR record
of 10.17.92.74.in-addr.arpa.
.RE
.PP
The
.I style
-parameter must be one of \fBad-hoc\fR, \fBinterim\fR or \fBnone\fR.
+parameter must be one of \fBstandard\fR, \fBinterim\fR or \fBnone\fR.
The \fIddns-update-style\fR statement is only meaningful in the outer
scope - it is evaluated once after reading the dhcpd.conf file, rather
than each time a client is assigned an IP address, so there is no way
is \fBnone\fR.
.RE
.PP
-.B The
+.B The
.I ddns-updates
.B statement
.RS 0.25i
\fBddns-updates \fIflag\fR\fB;\fR
.PP
The \fIddns-updates\fR parameter controls whether or not the server will
-attempt to do a DNS update when a lease is confirmed. Set this to \fIoff\fR
+attempt to do a DNS update when a lease is confirmed. Set this to \fIoff\fR
if the server should not attempt to do updates within a certain scope.
-The \fIddns-updates\fR parameter is on by default. To disable DNS
+The \fIddns-updates\fR parameter is on by default. To disable DNS
updates in all scopes, it is preferable to use the
\fIddns-update-style\fR statement, setting the style to \fInone\fR.
.RE
.RS 0.25i
.PP
.B delayed-ack \fIcount\fR\fB;\fR
+.PP
.B max-ack-delay \fImicroseconds\fR\fB;\fR
.PP
.I Count
-should be an integer value from zero to 2^16-1, and defaults to 28. The
-count represents how many DHCPv4 replies maximum will be queued pending
-transmission until after a database commit event. If this number is
-reached, a database commit event (commonly resulting in fsync() and
-representing a performance penalty) will be made, and the reply packets
-will be transmitted in a batch afterwards. This preserves the RFC2131
-direction that "stable storage" be updated prior to replying to clients.
-Should the DHCPv4 sockets "go dry" (select() returns immediately with no
-read sockets), the commit is made and any queued packets are transmitted.
+should be an integer value from zero to 2^16-1 and defaults to 0, which means
+that the feature is disabled. Otherwise, 28 may be a sensible starting point
+for many configurations (SO_SNDBUF size / 576 bytes.) The count represents how
+many DHCPv4 replies maximum will be queued pending transmission until after a
+database commit event. If this number is reached, a database commit event
+(commonly resulting in fsync() and representing a performance penalty) will be
+made, and the reply packets will be transmitted in a batch afterwards. This
+preserves the RFC2131 direction that "stable storage" be updated prior to
+replying to clients. Should the DHCPv4 sockets "go dry" (select() returns
+immediately with no read sockets), the commit is made and any queued packets
+are transmitted.
.PP
Similarly, \fImicroseconds\fR indicates how many microseconds are permitted
to pass inbetween queuing a packet pending an fsync, and performing the
fsync. Valid values range from 0 to 2^32-1, and defaults to 250,000 (1/4 of
a second).
.PP
-Please note that as delayed-ack is currently experimental, the delayed-ack
-feature is not compiled in by default, but must be enabled at compile time
-with \'./configure --enable-delayed-ack\'.
+The delayed-ack feature is compiled in by default, but can be disabled
+at compile time with \'./configure --disable-delayed-ack\'. Please note
+that the delayed-ack feature is not currently compatible with support for
+DHPCv4-over-DHCPv6 so when a 4to6 port ommand line argument enables this
+in the server the delayed-ack value is reset to 0.
.RE
.PP
-The
-.I dhcp-cache-threshold
+The
+.I dhcp-cache-threshold
statement
.RS 0.25i
.PP
the lease time). This parameter expresses the percentage of the total
lease time, measured from the beginning, during which a
client's attempt to renew its lease will result in getting
-the already assigned lease, rather than an extended lease.
+the already assigned lease, rather than an extended lease. This feature
+is supported for both IPv4 and IPv6 and down to the pool level and for
+IPv6 all three pool types: NA, TA and PD.
.PP
Clients that attempt renewal frequently can cause the server to
update and write the database frequently resulting in a performance
impact on the server. The \fIdhcp-cache-threshold\fR
statement instructs the DHCP server to avoid updating leases too
-frequently thus avoiding this behavior. Instead the server assigns the
-same lease with no modifications except for CLTT (Client Last
-Transmission Time) which does not require disk operations. This
-feature applies to IPv4 only.
+frequently thus avoiding this behavior. Instead the server replies with the
+same lease (i.e. reuses it) with no modifications except for CLTT (Client Last
+Transmission Time) and for IPv4:
+
+ the lease time sent to the client is shortened by the age of
+ the lease
+
+while for IPv6:
+
+ the preferred and valid lifetimes sent to the client are
+ shortened by the age of the lease.
+
+None of these changes require writing the lease to disk.
+
+.PP
+When an existing lease is matched to a renewing client, it will be reused
+if all of the following conditions are true:
+.nf
+ 1. The dhcp-cache-threshold is larger than zero
+ 2. The current lease is active
+ 3. The percentage of the lease time that has elapsed is less than
+ dhcp-cache-threshold
+ 4. The client information provided in the renewal does not alter
+ any of the following:
+ a. DNS information and DNS updates are enabled
+ b. Billing class to which the lease is associated (IPv4 only)
+ c. The host declaration associated with the lease (IPv4 only)
+ d. The client id - this may happen if a client boots without
+ a client id and then starts using one in subsequent
+ requests. (IPv4 only)
+.fi
+.PP
+While lease data is not written to disk when a lease is reused, the server
+will still execute any on-commit statements.
+.PP
+Note that the lease can be reused if the options the client or relay agent
+sends are changed. These changes will not be recorded in the in-memory or
+on-disk databases until the client renews after the threshold time is reached.
.RE
.PP
The
.B do-forward-updates \fIflag\fB;\fR
.PP
The \fIdo-forward-updates\fR statement instructs the DHCP server as
-to whether it should attempt to update a DHCP client's A record
-when the client acquires or renews a lease. This statement has no
-effect unless DNS updates are enabled and \fBddns-update-style\fR is
-set to \fBinterim\fR. Forward updates are enabled by default. If
-this statement is used to disable forward updates, the DHCP server
-will never attempt to update the client's A record, and will only ever
-attempt to update the client's PTR record if the client supplies an
-FQDN that should be placed in the PTR record using the fqdn option.
-If forward updates are enabled, the DHCP server will still honor the
-setting of the \fBclient-updates\fR flag.
+to whether it should attempt to update a DHCP client\'s A record
+when the client acquires or renews a lease. This statement has no
+effect unless DNS updates are enabled. Forward updates are enabled
+by default. If this statement is used to disable forward updates,
+the DHCP server will never attempt to update the client\'s A record,
+and will only ever attempt to update the client\'s PTR record if the
+client supplies an FQDN that should be placed in the PTR record using
+the \fBfqdn\fR option. If forward updates are enabled, the DHCP server
+will still honor the setting of the \fBclient-updates\fR flag.
+.RE
+.PP
+The
+.I dont-use-fsync
+statement
+.RS 0.25i
+.PP
+.B dont-use-fsync \fIflag\fB;\fR
+.PP
+The \fIdont-use-fsync\fR statement instructs the DHCP server if
+it should call fsync() when writing leases to the lease file. By
+default and if the flag is set to false the server \fBwill\fR call
+fsync(). Suppressing the call to fsync() may increase the performance
+of the server but it also adds a risk that a lease will not be
+properly written to the disk after it has been issued to a client
+and before the server stops. This can lead to duplicate leases
+being issued to different clients. Using this option is \fBnot
+recommended\FR.
.RE
.PP
The
.B dynamic-bootp-lease-length\fR \fIlength\fR\fB;\fR
.PP
The \fIdynamic-bootp-lease-length\fR statement is used to set the
-length of leases dynamically assigned to BOOTP clients. At some
+length of leases dynamically assigned to BOOTP clients. At some
sites, it may be possible to assume that a lease is no longer in
use if its holder has not used BOOTP or DHCP to get its address within
-a certain time period. The period is specified in \fIlength\fR as a
-number of seconds. If a client reboots using BOOTP during the
+a certain time period. The period is specified in \fIlength\fR as a
+number of seconds. If a client reboots using BOOTP during the
timeout period, the lease duration is reset to \fIlength\fR, so a
BOOTP client that boots frequently enough will never lose its lease.
Needless to say, this parameter should be adjusted with extreme
.RE
.PP
The
+.I echo-client-id
+statement
+.RS 0.25i
+.PP
+.B echo-client-id\fR \fIflag\fR\fB;\fR
+.PP
+The \fIecho-client-id\fR statement is used to enable or disable RFC 6842
+compliant behavior. If the echo-client-id statement is present and has a
+value of true or on, and a DHCP DISCOVER or REQUEST is received which contains
+the client identifier option (Option code 61), the server will copy the option
+into its response (DHCP ACK or NAK) per RFC 6842. In other words if the
+client sends the option it will receive it back. By default, this flag is off
+and client identifiers will not echoed back to the client.
+.RE
+.PP
+The
.I filename
statement
.RS 0.25i
.RE
.PP
The
+.I fixed-prefix6
+declaration
+.RS 0.25i
+.PP
+.B fixed-prefix6\fR \fIlow-address\fR \fB/\fR \fIbits\fR\fB;\fR
+.PP
+The \fIfixed-prefix6\fR declaration is used to assign a fixed
+IPv6 prefix to a client. It should only appear in a \fIhost\fR
+declaration, but multiple \fIfixed-prefix6\fR statements may appear
+in a single \fIhost\fR declaration.
+.PP
+The \fIlow-address\fR specifies the start of the prefix and the \fIbits\fR
+specifies the size of the prefix in bits.
+.PP
+If there are multiple prefixes for a given host entry the server will
+choose one that matches the requested prefix size or, if none match,
+the first one.
+.PP
+If there are multiple \fIhost\fR declarations the server will try to
+choose a declaration where the \fIfixed-address6\fR matches the client's
+subnet. If none match it will choose one that doesn't have a \fIfixed-address6\fR
+statement.
+.PP
+Note Well: Unlike the fixed address the fixed prefix does not need to match
+a subnet in order to be served. This allows you to provide a prefix to
+a client that is outside of the subnet on which the client makes the request
+to the the server.
+.RE
+.PP
+The
.I get-lease-hostnames
statement
.RS 0.25i
or not to look up the domain name corresponding to the IP address of
each address in the lease pool and use that address for the DHCP
\fIhostname\fR option. If \fIflag\fR is true, then this lookup is
-done for all addresses in the current scope. By default, or if
+done for all addresses in the current scope. By default, or if
\fIflag\fR is false, no lookups are done.
.RE
.PP
-The
+The
.I hardware
statement
.RS 0.25i
.I host
statement.
.I hardware-type
-must be the name of a physical hardware interface type. Currently,
+must be the name of a physical hardware interface type. Currently,
only the
.B ethernet
and
The
.I hardware-address
should be a set of hexadecimal octets (numbers from 0 through ff)
-separated by colons. The \fIhardware\fR statement may also be used
+separated by colons. The \fIhardware\fR statement may also be used
for DHCP clients.
.RE
.PP
-The
+The
.I host-identifier option
statement
.RS 0.25i
.PP
.B host-identifier option \fIoption-name option-data\fB;\fR
.PP
+or
+.PP
+.B host-identifier v6relopt \fInumber option-name option-data\fB;\fR
+.PP
This identifies a DHCPv6 client in a
.I host
statement.
.I option-name
-is any option, and
+is any option, and
.I option-data
-is the value for the option that the client will send. The
+is the value for the option that the client will send. The
.I option-data
-must be a constant value.
+must be a constant value. In the v6relopts case the additional number
+is the relay to examine for the specified option name and value. The
+values are the same as for the v6relay option. 0 is a no-op, 1 is the
+relay closest to the client, 2 the next one in and so on. Values that
+are larger than the maximum number of relays (currently 32) indicate the
+relay closest to the server independent of number.
+.RE
+.PP
+The
+.I ignore-client-uids
+statement
+.RS 0.25i
+.PP
+.B ignore-client-uids \fIflag\fB;\fR
+.PP
+If the \fIignore-client-uids\fR statement is present and has a value of
+\fItrue\fR or \fIon\fR, the UID for clients will not be recorded.
+If this statement is not present or has a value of \fIfalse\fR or
+\fIoff\fR, then client UIDs will be recorded.
.RE
.PP
The
.B lease-file-name \fIname\fB;\fR
.PP
.I Name
-should be the name of the DHCP server's lease file. By default, this
-is DBDIR/dhcpd.leases. This statement \fBmust\fR appear in the outer
-scope of the configuration file - if it appears in some other scope,
-it will have no effect. Furthermore, it has no effect if overridden
-by the
-.B -lf
-flag or the
-.B PATH_DHCPD_DB
-environment variable.
+Where \fIname\fR is the name of the DHCP server's lease file. By default,
+this is DBDIR/dhcpd.leases. This statement \fBmust\fR appear in the outer
+scope of the configuration file - if it appears in some other scope, it will
+have no effect. The value must be the absolute path of the file to use.
+The order of precedence the server uses for the lease file name
+is:
+.PP
+ 1. \fBlease-file-name\fR configuration file statement.
+ 2. \fB-lf\fR command line flag.
+ 3. \fBPATH_DHCPD_DB\fR environment variable.
+.RE
+.PP
+The
+.I dhcpv6-lease-file-name
+statement
+.RS 0.25i
+.PP
+.B dhcpv6-lease-file-name \fIname\fB;\fR
+.PP
+Where \fIname\fR is the name of the DHCP server's lease file when the server
+is running DHCPv6. By default, this is DBDIR/dhcpd6.leases. This statement
+\fBmust\fR appear in the outer scope of the configuration file - if it appears
+in some other scope, it will have no effect. The value must be the absolute
+path of the file to use. The order of precedence the server uses
+for the lease file name is:
+.PP
+ 1. \fBdhcpv6-lease-file-name\fR configuration file statement.
+ 2. \fB-lf\fR command line flag.
+ 3. \fBPATH_DHCPD6_DB\fR environment variable.
+.RE
+.PP
+The
+.I lease-id-format
+parameter
+.RS 0.25i
+.PP
+.B lease-id-format \fIformat\fB;\fR
+.PP
+The \fIformat\fR parameter must be either \fBoctal\fR or \fBhex\fR.
+This parameter governs the format used to write certain values to lease
+files. With the default format, octal, values are written as quoted strings in
+which non-printable characters are represented as octal escapes -
+a backslash character followed by three octal digits. When the hex format
+is specified, values are written as an unquoted series of pairs of
+hexadecimal digits, separated by colons.
+
+Currently, the values written out based on lease-id-format are the server-duid,
+the uid (DHCPv4 leases), and the IAID_DUID (DHCPv6 leases). Note the server
+automatically reads the values in either format.
.RE
.PP
The
.RE
.PP
The
-.I dhcpv6-lease-file-name
-statement
-.RS 0.25i
-.PP
-.B dhcpv6-lease-file-name \fIname\fB;\fR
-.PP
-.I Name
-is the name of the lease file to use if and only if the server is running
-in DHCPv6 mode. By default, this is DBDIR/dhcpd6.leases. This statement,
-like
-.I lease-file-name,
-\fBmust\fR appear in the outer scope of the configuration file. It
-has no effect if overridden by the
-.B -lf
-flag or the
-.B PATH_DHCPD6_DB
-environment variable. If
-.I dhcpv6-lease-file-name
-is not specified, but
-.I lease-file-name
-is, the latter value will be used.
-.RE
-.PP
-The
.I local-port
statement
.RS 0.25i
to the specified \fIaddress\fR, rather than requests sent to all addresses.
Since serving directly attached DHCP clients implies that the server must
respond to requests sent to the all-ones IP address, this option cannot be
-used if clients are on directly attached networks...it is only realistically
+used if clients are on directly attached networks; it is only realistically
useful for a server whose only clients are reached via unicasts, such as via
DHCP relay agents.
.PP
.RE
.PP
The
+.I local-address6
+and
+.I bind-local-address6
+statements
+.RS 0.25i
+.PP
+.B local-address6 \fIaddress\fB;\fR
+.PP
+.B bind-local-address6 \fIflag\fB;\fR
+.PP
+The \fIlocal-address6\fR statement causes the DHCP server to send IPv6
+packets as originating from the specified IPv6 \fIaddress\fR, rather than
+leaving the kernel to fill in the source address field.
+.PP
+When \fIbind-local-address6\fR is present and has a value of true or on,
+service sockets are bound to \fIaddress\fR too.
+.PP
+By default \fIaddress\fR is the undefined address and the
+\fIbind-local-address6\fR is disabled, both may only be set at the global
+scope.
+.RE
+.PP
+The
.I log-facility
statement
.RS 0.25i
.B log-facility \fIfacility\fB;\fR
.PP
This statement causes the DHCP server to do all of its logging on the
-specified log facility once the dhcpd.conf file has been read. By
-default the DHCP server logs to the daemon facility. Possible log
+specified log facility once the dhcpd.conf file has been read. By
+default the DHCP server logs to the daemon facility. Possible log
facilities include auth, authpriv, cron, daemon, ftp, kern, lpr, mail,
mark, news, ntp, security, syslog, user, uucp, and local0 through
-local7. Not all of these facilities are available on all systems,
+local7. Not all of these facilities are available on all systems,
and there may be other facilities available on other systems.
.PP
In addition to setting this value, you may need to modify your
.I syslog.conf
-file to configure logging of the DHCP server. For example, you might
+file to configure logging of the DHCP server. For example, you might
add a line like this:
.PP
.nf
file, log messages printed while parsing the dhcpd.conf file or before
parsing it are logged to the default log facility. To prevent this,
see the README file included with this distribution, which describes
+BUG: where is that mentioned in README?
how to change the default log facility. When this parameter is used,
the DHCP server prints its startup message a second time after parsing
the configuration file, so that the log will be as complete as
.RE
.PP
The
+.I log-threshold-high
+and
+.I log-threshold-low
+statements
+.RS 0.25i
+.PP
+.B log-threshold-high \fIpercentage\fB;\fR
+.PP
+.B log-threshold-low \fIpercentage\fB;\fR
+.PP
+The \fIlog-threshold-low\fR and \fIlog-threshold-high\fR statements
+are used to control when a message is output about pool usage. The
+value for both of them is the percentage of the pool in use. If the
+high threshold is 0 or has not been specified, no messages will be
+produced. If a high threshold is given, a message is output once the
+pool usage passes that level. After that, no more messages will be
+output until the pool usage falls below the low threshold. If the low
+threshold is not given, it default to a value of zero.
+.PP
+A special case occurs when the low threshold is set to be higer than
+the high threshold. In this case, a message will be generated each time
+a lease is acknowledged when the pool usage is above the high threshold.
+.PP
+Note that threshold logging will be automatically disabled for shared
+subnets whose total number of addresses is larger than (2^64)-1. The server
+will emit a log statement at startup when threshold logging is disabled as
+shown below:
+
+ "Threshold logging disabled for shared subnet of ranges: <addresses>"
+
+This is likely to have no practical runtime effect as CPUs are unlikely
+to support a server actually reaching such a large number of leases.
+.RE
+.PP
+The
.I max-lease-time
statement
.RS 0.25i
.I Time
should be the minimum length in seconds that will be assigned to a
lease.
-The default is the minimum of 300 seconds or
+The default is the minimum of 300 seconds or
\fBmax-lease-time\fR.
.RE
.PP
should be the minimum number of seconds since a client began trying to
acquire a new lease before the DHCP server will respond to its request.
The number of seconds is based on what the client reports, and the maximum
-value that the client can report is 255 seconds. Generally, setting this
+value that the client can report is 255 seconds. Generally, setting this
to one will result in the DHCP server not responding to the client's first
request, but always responding to its second request.
.PP
This can be used
to set up a secondary DHCP server which never offers an address to a client
-until the primary server has been given a chance to do so. If the primary
+until the primary server has been given a chance to do so. If the primary
server is down, the client will bind to the secondary server, but otherwise
-clients should always bind to the primary. Note that this does not, by
+clients should always bind to the primary. Note that this does not, by
itself, permit a primary server and a secondary server to share a pool of
dynamically-allocatable addresses.
.RE
.PP
The \fInext-server\fR statement is used to specify the host address of
the server from which the initial boot file (specified in the
-\fIfilename\fR statement) is to be loaded. \fIServer-name\fR should
+\fIfilename\fR statement) is to be loaded. \fIServer-name\fR should
be a numeric IP address or a domain name.
.RE
.PP
.B omapi-port\fR \fIport\fR\fB;\fR
.PP
The \fIomapi-port\fR statement causes the DHCP server to listen for
-OMAPI connections on the specified port. This statement is required
+OMAPI connections on the specified port. This statement is required
to enable the OMAPI protocol, which is used to examine and modify the
state of the DHCP server as it is running.
.RE
.PP
If this flag is enabled, whenever a client sends a DHCPREQUEST for a
particular lease, the server will automatically free any other leases
-the client holds. This presumes that when the client sends a
+the client holds. This presumes that when the client sends a
DHCPREQUEST, it has forgotten any lease not mentioned in the
DHCPREQUEST - i.e., the client has only a single network interface
.I and
it does not remember leases it's holding on networks to which it is
-not currently attached. Neither of these assumptions are guaranteed
+not currently attached. Neither of these assumptions are guaranteed
or provable, so we urge caution in the use of this statement.
.RE
.PP
The
+.I persist-eui-64-leases
+statement
+.RS 0.25i
+.PP
+.B persist-eui-64-leases \fIflag\fR\fB;\fR
+.PP
+When this flag is enabled, the server will write EUI-64 based leases to the
+leases file. Since such leases can only, ever be valid for a single DUID value
+it can be argued that writing them to the leases file isn't essential and not
+doing so may have perfomance advantages. See \fIuse-eui-64\fR statement for
+more details on EUI-64 based address allocation. The flag is enabled by
+default and may only be set at the global scope.
+.RE
+.PP
+The
.I pid-file-name
statement
.RS 0.25i
.I name\fR\fB;\fR
.PP
.I Name
-should be the name of the DHCP server's process ID file. This is the
+should be the name of the DHCP server's process ID file. This is the
file in which the DHCP server's process ID is stored when the server
-starts. By default, this is RUNDIR/dhcpd.pid. Like the
-.I lease-file-name
-statement, this statement must appear in the outer scope
-of the configuration file. It has no effect if overridden by the
-.B -pf
-flag or the
-.B PATH_DHCPD_PID
-environment variable.
+starts. By default, this is RUNDIR/dhcpd.pid. Like the \fIlease-file-name\fR
+statement, this statement must appear in the outer scope of the configuration
+file. The order of precedence used by the server is:
+.PP
+ 1. \fBpid-file-name\fR configuration file statement.
+ 2. \fB-lf\fR command line flag.
+ 3. \fBPATH_DHCPD_PID\fR environment variable.
.PP
The
.I dhcpv6-pid-file-name
.I Name
is the name of the pid file to use if and only if the server is running
in DHCPv6 mode. By default, this is DBDIR/dhcpd6.pid. This statement,
-like
-.I pid-file-name,
-\fBmust\fR appear in the outer scope of the configuration file. It
-has no effect if overridden by the
-.B -pf
-flag or the
-.B PATH_DHCPD6_PID
-environment variable. If
-.I dhcpv6-pid-file-name
-is not specified, but
-.I pid-file-name
-is, the latter value will be used.
+like \fIpid-file-name\fr, \fBmust\fR appear in the outer scope of the
+configuration file. The order of precedence used by the server is:
+.PP
+ 1. \fBdhcpv6-pid-file-name\fR configuration file statement.
+ 2. \fB-lf\fR command line flag.
+ 3. \fBPATH_DHCPD6_PID\fR environment variable.
+.PP
.RE
.PP
The
.PP
When the DHCP server is considering dynamically allocating an IP
address to a client, it first sends an ICMP Echo request (a \fIping\fR)
-to the address being assigned. It waits for a second, and if no
-ICMP Echo response has been heard, it assigns the address. If a
+to the address being assigned. It waits for a second, and if no
+ICMP Echo response has been heard, it assigns the address. If a
response \fIis\fR heard, the lease is abandoned, and the server does
-not respond to the client.
+not respond to the client. The lease will remain abandoned for a minimum
+of abandon-lease-time seconds.
+.PP
+If a there are no free addressses but there are abandoned IP addresses, the
+DHCP server will attempt to reclaim an abandoned IP address regardless of the
+value of abandon-lease-time.
.PP
This \fIping check\fR introduces a default one-second delay in responding
-to DHCPDISCOVER messages, which can be a problem for some clients. The
+to DHCPDISCOVER messages, which can be a problem for some clients. The
default delay of one second may be configured using the ping-timeout
parameter. The ping-check configuration parameter can be used to control
checking - if its value is false, no ping check is done.
.RE
.PP
The
+.I ping-cltt-secs
+statement
+.RS 0.25i
+.PP
+.B ping-cltt-secs
+.I seconds\fR\fB;\fR
+.PP
+The server will conduct a ping check if all the following conditions are true:
+.PP
+1. Ping checking is enabled.
+.PP
+2. The server is responding to a DISCOVER.
+.PP
+3. The lease to be offered is neither static nor active (i.e. still a valid
+lease).
+.PP
+4. And any of the following are true:
+ a. This will be the first offer of this lease (CLTT is 0).
+ b. The lease is being offered to a client other than its previous owner
+ c. The lease is being offered to its previous owner and more than
+ \fBping-cltt-secs\fR have elapsed since CLTT of the original lease.
+ d. The lease was abandoned and the server is attempting to reclaim it.
+
+.PP
+The \fBping-cltt-secs\fR statement allows the user to specify the amount of
+time that must elaspe after CLTT before a ping check will be conducted.
+The default value is sixty seconds.
+.RE
+.PP
+The
.I ping-timeout
statement
.RS 0.25i
ICMP Echo response to be heard, if no ICMP Echo response has been received
before the timeout expires, it assigns the address. If a response \fIis\fR
heard, the lease is abandoned, and the server does not respond to the client.
-If no value is set, ping-timeout defaults to 1 second.
+If no value is set, ping-timeout defaults to 1 second. (See also ping-timeout-ms
+below)
+.RE
+.PP
+The
+.I ping-timeout-ms
+statement
+.RS 0.25i
+.PP
+.B ping-timeout-ms
+.I milliseconds\fR\fB;\fR
+.PP
+Allows you to specify the ping timeout in milliseconds rather than
+seconds. If this value is greater than zero, the server will use it
+in place of ping-timeout. The default value is zero.
.RE
.PP
The
applications move off of the address and onto currently valid addresses
(should there still be any open TCP sockets or similar).
.PP
-The preferred lifetime defaults to the renew+rebind timers, or 3/4 the
-default lease time if none were specified.
+The preferred lifetime defaults to 5/8 the default lease time.
+.RE
+.PP
+The
+.I prefix-length-mode
+statement
+.RS 0.25i
+.PP
+.B prefix-length-mode
+.I mode\fR\fB;\fR
+.PP
+According to RFC 3633, DHCPv6 clients may specify preferences when soliciting
+prefixes by including an IA_PD Prefix option within the IA_PD option. Among
+the preferences that may be conveyed is the "prefix-length". When non-zero it
+indicates a client's desired length for offered prefixes. The RFC states that
+servers "MAY choose to use the information...to select prefix(es)" but does
+not specify any particular rules for doing so. The prefix-length-mode statement
+can be used to set the prefix selection rules employed by the server,
+when clients send a non-zero prefix-length value. The mode parameter must
+be one of \fBignore\fR, \fBprefer\fR, \fBexact\fR, \fBminimum\fR, or
+\fBmaximum\fR where:
+.PP
+1. ignore - The requested length is ignored. The server will offer the first
+available prefix.
+.PP
+2. prefer - The server will offer the first available prefix with the same
+length as the requested length. If none are found then it will offer the
+first available prefix of any length. This is the default behavior.
+.PP
+3. exact - The server will offer the first available prefix with the same
+length as the requested length. If none are found, it will return a status
+indicating no prefixes available.
+.PP
+4. minimum - The server will offer the first available prefix with the same
+length as the requested length. If none are found, it will return the first
+available prefix whose length is greater than (e.g. longer than), the
+requested value. If none of those are found, it will return a status
+indicating no prefixes available. For example, if client requests a length
+of /60, and the server has available prefixes of lengths /56 and /64, it will
+offer prefix of length /64.
+.PP
+5. maximum - The server will offer the first available prefix with the same
+length as the requested length. If none are found, it will return the first
+available prefix whose length is less than (e.g. shorter than), the
+requested value. If none of those are found, it will return a status
+indicating no prefixes available. For example, if client requests a length
+of /60, and the server has available prefixes of lengths /56 and /64, it will
+offer a prefix of length /56.
+.PP
+In general "first available" is determined by the order in which pools are
+defined in the server's configuration. For example, if a subnet is defined
+with three prefix pools A,B, and C:
+.PP
+.nf
+subnet 3000::/64 {
+ # pool A
+ pool6 {
+ :
+ }
+ # pool B
+ pool6 {
+ :
+ }
+ # pool C
+ pool6 {
+ :
+ }
+}
+.fi
+.PP
+then the pools will be checked in the order A, B, C. For modes \fBprefer\fR,
+\fBminimum\fR, and \fBmaximum\fR this may mean checking the pools in that order
+twice. A first pass through is made looking for an available prefix of exactly
+the preferred length. If none are found, then a second pass is performed
+starting with pool A but with appropriately adjusted length criteria.
+.RE
+.PP
+The
+.I release-on-roam
+statement
+.RS 0.25i
+.PP
+.B release-on-roam \fIflag\fB;\fR
+.PP
+When enabled and the dhcpd server detects that a DHCPv6 client (IAID+DUID)
+has roamed to a new network, it will release the pre-existing leases on the
+old network and emit a log statement similiar to the following:
+
+ "Client: <id> roamed to new network, releasing lease: <address>"
+
+The server will carry out all of the same steps that would normally occur
+when a client explicitly releases a lease. When release-on-roam is disabled
+(the default) the server makes such leases unavailable until they expire or
+the server is restarted. Clients that need leases in multiple networks must
+supply a unique IAID in each IA. This parameter may only be specified at
+the global level.
.RE
.PP
The
.B server-identifier \fIhostname\fR\fB;\fR
.PP
The server-identifier statement can be used to define the value that
-is sent in the DHCP Server Identifier option for a given scope. The
+is sent in the DHCP Server Identifier option for a given scope. The
value specified \fBmust\fR be an IP address for the DHCP server, and
must be reachable by all clients served by a particular scope.
.PP
The use of the server-identifier statement is not recommended - the only
reason to use it is to force a value other than the default value to be
-sent on occasions where the default value would be incorrect. The default
+sent on occasions where the default value would be incorrect. The default
value is the first IP address associated with the physical network interface
on which the request arrived.
.PP
.RE
.PP
The
+.I server-id-check
+statement
+.RS 0.25i
+.PP
+.B server-id-check \fIflag\fR\fB;\fR
+.PP
+The server-id-check statement is used to control whether or not a server,
+participating in failover, verifies that the value of the
+dhcp-server-identifier option in received DHCP REQUESTs match the server's
+id before processing the request. Server id checking is disabled by default.
+Setting this flag enables id checking and thereafter the server will only
+process requests that match. Note the flag setting should be consistent
+between failover partners.
+.PP
+Unless overridden by use of the server-identifier statement, the value the
+server uses as its id will be the first IP address associated with the
+physical network interface on which the request arrived.
+.PP
+In order to reduce runtime overhead the server only checks for a server id
+option in the global and subnet scopes. Complicated configurations
+may result in different server ids for this check and when the server id for
+a reply packet is determined, which would prohibit the server from responding.
+.PP
+The primary use for this option is when a client broadcasts a request
+but requires that the response come from a specific failover peer.
+An example of this would be when a client reboots while its lease is still
+active - in this case both servers will normally respond. Most of the
+time the client won't check the server id and can use either of the responses.
+However if the client does check the server id it may reject the response
+if it came from the wrong peer. If the timing is such that the "wrong"
+peer responds first most of the time the client may not get an address for
+some time.
+.PP
+Care should be taken before enabling this option.
+.PP
+.RE
+.PP
+The
.I server-duid
statement
.RS 0.25i
If you choose EN, you must include the enterprise number and the
enterprise-identifier.
.PP
+If there is a server-duid statement in the lease file it will take precedence
+over the server-duid statement from the config file and a
+dhcp6.server-id option in the config file will override both.
+.PP
The default server-duid type is LLT.
.RE
.PP
.B server-name "\fIname\fB";\fR
.PP
The \fIserver-name\fR statement can be used to inform the client of
-the name of the server from which it is booting. \fIName\fR should
+the name of the server from which it is booting. \fIName\fR should
be the name that will be provided to the client.
.RE
.PP
The
+.I dhcpv6-set-tee-times
+statement
+.RS 0.25i
+.PP
+.B dhcpv6-set-tee-times\fR \fIflag\fR\fB;\fR
+.PP
+The \fIdhcpv6-set-tee-times\fR statement enables setting T1 and T2 to the
+values recommended in RFC 3315 (Section 22.4). When setting T1 and T2, the
+server will use dhcp-renewal-time and dhcp-rebinding-time, respectively.
+A value of zero tells the client it may choose its own value.
+
+When those options are not defined then values will be set to zero unless the
+global \fIdhcpv6-set-tee-times\fR is enabled. When this option is enabled the
+times are calculated as recommended by RFC 3315, Section 22.4:
+
+ T1 will be set to 0.5 times the shortest preferred lifetime
+ in the reply. If the "shortest" preferred lifetime is
+ 0xFFFFFFFF, T1 will set to 0xFFFFFFFF.
+
+ T2 will be set to 0.8 times the shortest preferred lifetime
+ in the reply. If the "shortest" preferred lifetime is
+ 0xFFFFFFFF, T2 will set to 0xFFFFFFFF.
+
+Keep in mind that given sufficiently small lease lifetimes, the above
+calculations will result in the two values being equal. For example, a 9 second
+lease lifetime would yield T1 = T2 = 4 seconds, which would cause clients to
+issue rebinds only. In such a case it would likely be better to explicitly
+define the values.
+
+Note that dhcpv6-set-tee-times is intended to be transitional and will likely
+be removed in a future release. Once removed the behavior will be to use
+the configured values when present or calculate them per the RFC. If you want
+zeros, define them as zeros.
+.RE
+.PP
+The
.I site-option-space
statement
.RS 0.25i
.B site-option-space "\fIname\fB";\fR
.PP
The \fIsite-option-space\fR statement can be used to determine from
-what option space site-local options will be taken. This can be used
+what option space site-local options will be taken. This can be used
in much the same way as the \fIvendor-option-space\fR statement.
Site-local options in DHCP are those options whose numeric codes are
-greater than 224. These options are intended for site-specific
+greater than 224. These options are intended for site-specific
uses, but are frequently used by vendors of embedded hardware that
-contains DHCP clients. Because site-specific options are allocated
+contains DHCP clients. Because site-specific options are allocated
on an ad hoc basis, it is quite possible that one vendor's DHCP client
might use the same option code that another vendor's client uses, for
-different purposes. The \fIsite-option-space\fR option can be used
+different purposes. The \fIsite-option-space\fR option can be used
to assign a different set of site-specific options for each such
vendor, using conditional evaluation (see \fBdhcp-eval (5)\fR for
details).
the server will record the relay agent information options sent during
the client's initial DHCPREQUEST message when the client was in the
SELECTING state and behave as if those options are included in all
-subsequent DHCPREQUEST messages sent in the RENEWING state. This
+subsequent DHCPREQUEST messages sent in the RENEWING state. This
works around a problem with relay agent information options, which is
that they usually not appear in DHCPREQUEST messages sent by the
client in the RENEWING state, because such messages are unicast
perform standard DHCID multiple-client, one-name conflict detection. If
the parameter has been set false, the server will skip this check and
instead simply tear down any previous bindings to install the new
-binding without question. The default is true.
+binding without question. The default is true and this parameter may only
+be specified at the global scope.
.RE
.PP
The
If the \fIupdate-optimization\fR parameter is false for a given client,
the server will attempt a DNS update for that client each time the
client renews its lease, rather than only attempting an update when it
-appears to be necessary. This will allow the DNS to heal from
+appears to be necessary. This will allow the DNS to heal from
database inconsistencies more easily, but the cost is that the DHCP
-server must do many more DNS updates. We recommend leaving this option
-enabled, which is the default. This option only affects the behavior of
-the interim DNS update scheme, and has no effect on the ad-hoc DNS update
-scheme. If this parameter is not specified, or is true, the DHCP server
+server must do many more DNS updates. We recommend leaving this option
+enabled, which is the default. If this parameter is not specified,
+or is true, the DHCP server
will only update when the client information changes, the client gets a
different lease, or the client's lease expires.
.RE
.PP
The \fIupdate-static-leases\fR flag, if enabled, causes the DHCP
server to do DNS updates for clients even if those clients are being
-assigned their IP address using a \fIfixed-address\fR statement - that
-is, the client is being given a static assignment. This can only
-work with the \fIinterim\fR DNS update scheme. It is not
-recommended because the DHCP server has no way to tell that the update
-has been done, and therefore will not delete the record when it is not
-in use. Also, the server must attempt the update each time the
-client renews its lease, which could have a significant performance
-impact in environments that place heavy demands on the DHCP server.
+assigned their IP address using a \fIfixed-address\fR or
+\fIfixed-address6\fR statement - that is, the client is being given a
+static assignment. It is not recommended because the DHCP server has
+no way to tell that the update has been done, and therefore will not
+delete the record when it is not in use. Also, the server must attempt
+the update each time the client renews its lease, which could have a
+significant performance impact in environments that place heavy demands
+on the DHCP server. This feature is supported for both DHCPv4 and DHCPv6,
+and update modes standard or interim. It is disabled by default.
+.RE
+.PP
+The
+.I use-eui-64
+statement
+.RS 0.25i
+.PP
+.B use-eui-64 \fIflag\fB;\fR
+.PP
+
+(Support for this must be enabled at compile time, see EUI_64 in
+ includes/site.h)
+
+The \fIuse-eui-64\fR flag, if enabled, instructs the server to construct an
+address using the client's EUI-64 DUID (Type 3, HW Type EUI-64), rather than
+creating an address using the dynamic algorithm. This means that a given DUID
+will always generate the same address for a given pool and further that the
+address is guaranteed to be unique to that DUID. The IPv6 address will be
+calculated from the EUI-64 link layer address, conforming to RFC 2373, unless
+there is a host declaration for the client-id.
+
+The range6 statement for EUI-64 must define full /64 bit ranges. Invalid ranges
+will be flagged during configuration parsing as errors. See the following
+example:
+
+ subnet6 fc00:e4::/64 {
+ use-eui-64 true;
+ range6 fc00:e4::/64;
+ }
+
+The statement may be specified down to the pool level, allowing a mixture of
+dynamic and EUI-64 based pools.
+
+During lease file parsing, any leases which map to an EUI-64 pool, that have a
+non-EUI-64 DUID or for which the lease address is not the EUI-64 address for
+that DUID in that pool, will be discarded.
+
+If a host declaration exists for the DUID, the server grants the address
+(fixed-prefix6, fixed-address6) according to the host declaration, regardless
+of the DUID type of the client (even for EUI-64 DUIDs).
+
+If a client request's an EUI-64 lease for a given network, and the resultant
+address conflicts with a fixed address reservation, the server will send the
+client a "no addresses available" response.
+
+Any client with a non-conforming DUID (not type 3 or not hw type EUI-64) that
+is not linked to a host declaration, which requests an address from an EUI-64
+enabled pool will be ignored and the event will be logged.
+
+Pools that are configured for EUI-64 will be skipped for dynamic allocation.
+If there are no pools in the shared network from which to allocate, the client
+will get back a no addresses available status.
+
+On an EUI-64 enabled pool, any client with a DUID 3, HW Type EUI-64, requesting
+a solicit/renew and including IA_NA that do not match the EUI-64 policy, they
+will be treated as though they are "outside" the subnet for a given client
+message:
+
+ Solicit - Server will advertise with EUI-64 ia suboption, but with rapid
+ commit off
+ Request - Server will send "an address not on link status", and no ia
+ suboption Renew/Rebind - Server will send the requested address ia
+ suboption with lifetimes of 0, plus an EUI-64 ia
+
+Whether or not EUI-64 based leases are written out to the lease database
+may be controlled by \fIpersist-eui-64-leases\fR statement.
.RE
.PP
The
If the \fIuse-host-decl-names\fR parameter is true in a given scope,
then for every host declaration within that scope, the name provided
for the host declaration will be supplied to the client as its
-hostname. So, for example,
+hostname. So, for example,
.PP
.nf
group {
host joe {
hardware ethernet 08:00:2b:4c:29:32;
- fixed-address joe.fugue.com;
+ fixed-address joe.example.com;
}
}
host joe {
hardware ethernet 08:00:2b:4c:29:32;
- fixed-address joe.fugue.com;
+ fixed-address joe.example.com;
option host-name "joe";
}
.fi
.PP
+Additionally, enabling use-host-decl-names instructs the server to use
+the host declaration name in the the forward DNS name, if no other values
+are available. This value selection process is discussed in more detail
+under DNS updates.
+.PP
An \fIoption host-name\fR statement within a host declaration will
override the use of the name in the host declaration.
.PP
It should be noted here that most DHCP clients completely ignore the
host-name option sent by the DHCP server, and there is no way to
-configure them not to do this. So you generally have a choice of
+configure them not to do this. So you generally have a choice of
either not having any hostname to client IP address mapping that the
-client will recognize, or doing DNS updates. It is beyond
+client will recognize, or doing DNS updates. It is beyond
the scope of this document to describe how to make this
determination.
.RE
If the \fIuse-lease-addr-for-default-route\fR parameter is true in a
given scope, then instead of sending the value specified in the
routers option (or sending no value at all), the IP address of the
-lease being assigned is sent to the client. This supposedly causes
+lease being assigned is sent to the client. This supposedly causes
Win95 machines to ARP for all IP addresses, which can be helpful if
-your router is configured for proxy ARP. The use of this feature is
+your router is configured for proxy ARP. The use of this feature is
not recommended, because it won't work for many DHCP clients.
.RE
.PP
.B vendor-option-space \fIstring\fR\fB;\fR
.PP
The \fIvendor-option-space\fR parameter determines from what option
-space vendor options are taken. The use of this configuration
+space vendor options are taken. The use of this configuration
parameter is illustrated in the \fBdhcp-options(5)\fR manual page, in
the \fIVENDOR ENCAPSULATED OPTIONS\fR section.
.RE
.SH SETTING PARAMETER VALUES USING EXPRESSIONS
Sometimes it's helpful to be able to set the value of a DHCP server
-parameter based on some value that the client has sent. To do this,
-you can use expression evaluation. The
+parameter based on some value that the client has sent. To do this,
+you can use expression evaluation. The
.B dhcp-eval(5)
-manual page describes how to write expressions. To assign the result
+manual page describes how to write expressions. To assign the result
of an evaluation to an option, define the option as follows:
.nf
.sp 1
It's often useful to allocate a single address to a single client, in
approximate perpetuity. Host statements with \fBfixed-address\fR clauses
exist to a certain extent to serve this purpose, but because host statements
-are intended to approximate \'static configuration\', they suffer from not
+are intended to approximate \'static configuration\', they suffer from not
being referenced in a littany of other Server Services, such as dynamic DNS,
failover, \'on events\' and so forth.
.PP
dhcpd(8), dhcpd.leases(5), dhcp-options(5), dhcp-eval(5), RFC2132, RFC2131.
.SH AUTHOR
.B dhcpd.conf(5)
-was written by Ted Lemon
-under a contract with Vixie Labs. Funding
-for this project was provided by Internet Systems Consortium.
+is maintained by ISC.
Information about Internet Systems Consortium can be found at
.B https://www.isc.org.
projects / thirdparty / dhcp.git / blobdiff