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.\"	$Id: dhcp-eval.5,v 1.33 2012/05/17 15:50:14 sar Exp $
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.TH dhcp-eval 5
.SH NAME
dhcp-eval - ISC DHCP conditional evaluation
.SH DESCRIPTION
The Internet Systems Consortium DHCP client and server both provide
the ability to perform conditional behavior depending on the contents
of packets they receive.  The syntax for specifying this conditional
behaviour is documented here.
.SH REFERENCE: CONDITIONAL BEHAVIOUR
Conditional behaviour may be  specified using the if statement and the else
or elsif statements or the switch and case statements.
A conditional statement can appear anywhere
that a regular statement (e.g., an option statement) can appear, and
can enclose one or more such statements.
.PP
.B CONDITIONAL BEHAVIOUR: IF
.PP
A typical conditional if statement in a server might be:
.PP
.nf
if option dhcp-user-class = "accounting" {
  max-lease-time 17600;
  option domain-name "accounting.example.org";
  option domain-name-servers ns1.accounting.example.org,
			     ns2.accounting.example.org;
} elsif option dhcp-user-class = "sales" {
  max-lease-time 17600;
  option domain-name "sales.example.org";
  option domain-name-servers ns1.sales.example.org,
			     ns2.sales.example.org;
} elsif option dhcp-user-class = "engineering" {
  max-lease-time 17600;
  option domain-name "engineering.example.org";
  option domain-name-servers ns1.engineering.example.org,
			     ns2.engineering.example.org;
} else {
  max-lease-time 600;
  option domain-name "misc.example.org";
  option domain-name-servers ns1.misc.example.org,
			     ns2.misc.example.org;
}
.fi
.PP
On the client side, an example of conditional evaluation might be:
.PP
.nf
# example.org filters DNS at its firewall, so we have to use their DNS
# servers when we connect to their network.  If we are not at
# example.org, prefer our own DNS server.
if not option domain-name = "example.org" {
  prepend domain-name-servers 127.0.0.1;
}
.fi
.PP
The
.B if
statement and the
.B elsif
continuation statement both take boolean expressions as their
arguments.  That is, they take expressions that, when evaluated,
produce a boolean result.  If the expression evaluates to true, then
the statements enclosed in braces following the
.B if
statement are executed, and all subsequent
.B elsif
and
.B else
clauses are skipped.  Otherwise, each subsequent
.B elsif
clause's expression is checked, until an elsif clause is encountered
whose test evaluates to true.  If such a clause is found, the
statements in braces following it are executed, and then any
subsequent
.B elsif
and
.B else
clauses are skipped.  If all the
.B if
and
.B elsif
clauses are checked but none
of their expressions evaluate true, then if there is an
.B else
clause, the statements enclosed in braces following the
.B else
are evaluated.  Boolean expressions that evaluate to null are
treated as false in conditionals.
.PP
.B CONDITIONAL BEHAVIOUR: SWITCH
.PP
The above example can be rewritten using a switch construct as well.
.PP
.nf
switch (option dhcp-user-class) {
  case "accounting":
    max-lease-time 17600;
    option domain-name "accounting.example.org";
    option domain-name-servers ns1.accounting.example.org,
			       ns2.accounting.example.org;
  case "sales":
    max-lease-time 17600;
    option domain-name "sales.example.org";
    option domain-name-servers ns1.sales.example.org,
			       ns2.sales.example.org;
    break;
  case "engineering":
    max-lease-time 17600;
    option domain-name "engineering.example.org";
    option domain-name-servers ns1.engineering.example.org,
			       ns2.engineering.example.org;
    break;
  default:
    max-lease-time 600;
    option domain-name "misc.example.org";
    option domain-name-servers ns1.misc.example.org,
			       ns2.misc.example.org;
    break;
}
.fi
.PP
The
.B switch
statement and the
.B case
statements can both be data expressions or numeric expressions.  Within
a switch statement they all must be the same type.  The server
evaluates the expression from the switch statement and then it evaluates
the expressions from the case statements until it finds a match.
.PP
If it finds a match it starts executing statements from that case
until the next break statement.  If it doesn't find a match it
starts from the default statement and again proceeds to the next
break statement.  If there is no match and no default it does nothing.
.PP
.SH BOOLEAN EXPRESSIONS
The following is the current list of boolean expressions that are
supported by the DHCP distribution.
.PP
.I data-expression-1 \fB=\fR \fIdata-expression-2\fR
.RS 0.25i
.PP
The \fB=\fR operator compares the values of two data expressions,
returning true if they are the same, false if they are not.  If
either the left-hand side or the right-hand side are null, the
result is also null.
.RE
.PP
.I data-expression-1 \fB~=\fR \fIdata-expression-2\fR
.I data-expression-1 \fB~~\fR \fIdata-expression-2\fR
.RS 0.25i
.PP
The \fB~=\fR and \fB~~\fR operators (not available on all systems) perform
extended regex(7) matching of the values of two data expressions, returning
true if \fIdata-expression-1\fR matches against the regular expression
evaluated by \fIdata-expression-2\fR, or false if it does not match or
encounters some error.  If either the left-hand side or the right-hand side
are null or empty strings, the result is also false.  The \fB~~\fR operator
differs from the \fB~=\fR operator in that it is case-insensitive.
.RE
.PP
.I boolean-expression-1 \fBand\fR \fIboolean-expression-2\fR
.PP
.RS 0.25i
The \fBand\fR operator evaluates to true if the boolean expression on
the left-hand side and the boolean expression on the right-hand side
both evaluate to true.  Otherwise, it evaluates to false.  If either
the expression on the left-hand side or the expression on the
right-hand side are null, the result is null.
.RE
.PP
.I boolean-expression-1 \fBor\fR \fIboolean-expression-2\fR
.PP
.RS 0.25i
The \fBor\fR operator evaluates to true if either the boolean
expression on the left-hand side or the boolean expression on the
right-hand side evaluate to true.  Otherwise, it evaluates to false.
If either the expression on the left-hand side or the expression on
the right-hand side are null, the result is null.
.RE
.PP
.B not \fIboolean-expression
.PP
.RS 0.25i
The \fBnot\fR operator evaluates to true if \fIboolean-expression\fR
evaluates to false, and returns false if \fIboolean-expression\fR evaluates
to true.  If \fIboolean-expression\fR evaluates to null, the result
is also null.
.RE
.PP
.B exists \fIoption-name\fR
.PP
.RS 0.25i
The \fBexists\fR expression returns true if the specified option
exists in the incoming DHCP packet being processed.
.RE
.B known
.PP
.RS 0.25i
The \fBknown\fR expression returns true if the client whose request is
currently being processed is known - that is, if there's a host
declaration for it.
.RE
.B static
.PP
.RS 0.25i
The \fBstatic\fR expression returns true if the lease assigned to the
client whose request is currently being processed is derived from a static
address assignment.
.RE
.SH DATA EXPRESSIONS
Several of the boolean expressions above depend on the results of
evaluating data expressions.  A list of these expressions is provided
here.
.PP
.B substring (\fIdata-expr\fB, \fIoffset\fB, \fIlength\fB)\fR
.PP
.RS 0.25i
The \fBsubstring\fR operator evaluates the data expression and returns
the substring of the result of that evaluation that starts
\fIoffset\fR bytes from the beginning, continuing for \fIlength\fR
bytes.  \fIOffset\fR and \fIlength\fR are both numeric expressions.
If \fIdata-expr\fR, \fIoffset\fR or \fIlength\fR evaluate to null,
then the result is also null.  If \fIoffset\fR is greater than or
equal to the length of the evaluated data, then a zero-length data
string is returned.  If \fIlength\fI is greater then the remaining
length of the evaluated data after \fIoffset\fR, then a data string
containing all data from \fIoffset\fR to the end of the evaluated data
is returned.
.RE
.PP
.B suffix (\fIdata-expr\fB, \fIlength\fB)\fR
.PP
.RS 0.25i
The \fBsuffix\fR operator evaluates \fIdata-expr\fR and returns the
last \fIlength\fR bytes of the result of that evaluation.  \fILength\fR
is a numeric expression.  If \fIdata-expr\fR or \fIlength\fR evaluate
to null, then the result is also null.  If \fIsuffix\fR evaluates to a
number greater than the length of the evaluated data, then the
evaluated data is returned.
.RE
.PP
.B lcase (\fIdata-expr\fB)\fR
.PP
.RS 0.25i
The \fBlcase\fR function returns the result of evaluating
\fIdata-expr\fR converted to lower case.  If \fIdata-expr\fR evaluates
to null, then the result is also null.
.RE
.PP
.B ucase (\fIdata-expr\fB)\fR
.PP
.RS 0.25i
The \fBucase\fR function returns the result of evaluating
\fIdata-expr\fR converted to upper case.  If \fIdata-expr\fR evaluates
to null, then the result is also null.
.RE
.PP
.B option \fIoption-name\fR
.PP
.RS 0.25i
The \fBoption\fR operator returns the contents of the specified option in
the packet to which the server is responding.
.RE
.PP
.B config-option \fIoption-name\fR
.PP
.RS 0.25i
The \fBconfig-option\fR operator returns the value for the specified option
that the DHCP client or server has been configured to send.
.RE
.PP
.B gethostname()
.PP
.RS 0.25i
The \fBgethostname()\fR function returns a data string whose contents are a
character string, the results of calling gethostname() on the local
system with a size limit of 255 bytes (not including NULL terminator).  This
can be used for example to configure dhclient to send the local hostname
without knowing the local hostname at the time dhclient.conf is written.
.RE
.PP
.B hardware
.PP
.RS 0.25i
The \fBhardware\fR operator returns a data string whose first element
is the type of network interface indicated in packet being considered,
and whose subsequent elements are client's link-layer address.  If
there is no packet, or if the RFC2131 \fIhlen\fR field is invalid,
then the result is null.  Hardware types include ethernet (1),
token-ring (6), and fddi (8).  Hardware types are specified by the
IETF, and details on how the type numbers are defined can be found in
RFC2131 (in the ISC DHCP distribution, this is included in the doc/
subdirectory).
.RE
.PP
.B packet (\fIoffset\fB, \fIlength\fB)\fR
.PP
.RS 0.25i
The \fBpacket\fR operator returns the specified portion of the packet
being considered, or null in contexts where no packet is being
considered.  \fIOffset\fR and \fIlength\fR are applied to the
contents packet as in the \fBsubstring\fR operator.
.RE
.PP
.I string
.PP
.RS 0.25i
A string, enclosed in quotes, may be specified as a data expression,
and returns the text between the quotes, encoded in ASCII.  The
backslash ('\\') character is treated specially, as in C programming: '\\t'
means TAB, '\\r' means carriage return, '\\n' means newline, and '\\b' means
bell.  Any octal value can be specified with '\\nnn', where nnn is any
positive octal number less than 0400.  Any hexadecimal value can be
specified with '\\xnn', where nn is any positive hexadecimal number less
than or equal to 0xff.
.RE
.PP
.I colon-separated hexadecimal list
.PP
.RS 0.25i
A list of hexadecimal octet values, separated by colons, may be
specified as a data expression.
.RE
.PP
.B concat (\fIdata-expr1\fB, ..., \fIdata-exprN\fB)\fR
.RS 0.25i
The expressions are evaluated, and the results of each evaluation are
concatenated in the sequence that the subexpressions are listed.  If
any subexpression evaluates to null, the result of the concatenation
is null.
.RE
.PP
.B reverse (\fInumeric-expr1\fB, \fIdata-expr2\fB)\fR
.RS 0.25i
The two expressions are evaluated, and then the result of evaluating
the data expression is reversed in place, using hunks of the size
specified in the numeric expression.  For example, if the numeric
expression evaluates to four, and the data expression evaluates to
twelve bytes of data, then the reverse expression will evaluate to
twelve bytes of data, consisting of the last four bytes of the
input data, followed by the middle four bytes, followed by the first
four bytes.
.RE
.PP
.B leased-address
.RS 0.25i
In any context where the client whose request is being processed has
been assigned an IP address, this data expression returns that IP
address.  In any context where the client whose request is being
processed has not been assigned an ip address, if this data expression
is found in executable statements executed on that client's behalf,
a log message indicating "there is no lease associated with this client"
is syslogged to the debug level (this is considered dhcpd.conf debugging
information).
.RE
.PP
.B binary-to-ascii (\fInumeric-expr1\fB, \fInumeric-expr2\fB,
.B \fIdata-expr1\fB,\fR \fIdata-expr2\fB)\fR
.RS 0.25i
Converts the result of evaluating data-expr2 into a text string
containing one number for each element of the result of evaluating
data-expr2.  Each number is separated from the other by the result of
evaluating data-expr1.  The result of evaluating numeric-expr1
specifies the base (2 through 16) into which the numbers should be
converted.  The result of evaluating numeric-expr2 specifies the
width in bits of each number, which may be either 8, 16 or 32.
.PP
As an example of the preceding three types of expressions, to produce
the name of a PTR record for the IP address being assigned to a
client, one could write the following expression:
.RE
.PP
.nf
        concat (binary-to-ascii (10, 8, ".",
                                 reverse (1, leased-address)),
                ".in-addr.arpa.");

.fi
.RE
.PP
.B encode-int (\fInumeric-expr\fB, \fIwidth\fB)\fR
.RS 0.25i
Numeric-expr is evaluated and encoded as a data string of the
specified width, in network byte order (most significant byte first).
If the numeric expression evaluates to the null value, the result is
also null.
.RE
.PP
.B pick-first-value (\fIdata-expr1\fR [ ... \fIexpr\fRn ] \fB)\fR
.RS 0.25i
The pick-first-value function takes any number of data expressions as
its arguments.  Each expression is evaluated, starting with the first
in the list, until an expression is found that does not evaluate to a
null value.  That expression is returned, and none of the subsequent
expressions are evaluated.  If all expressions evaluate to a null
value, the null value is returned.
.RE
.PP
.B host-decl-name
.RS 0.25i
The host-decl-name function returns the name of the host declaration
that matched the client whose request is currently being processed, if
any.  If no host declaration matched, the result is the null value.
.RE
.SH NUMERIC EXPRESSIONS
Numeric expressions are expressions that evaluate to an integer.  In
general, the maximum size of such an integer should not be assumed to
be representable in fewer than 32 bits, but the precision of such
integers may be more than 32 bits.
.PP
In addition to the following operators several standard math functions
are available.  They are:
.nf
operation    symbol
add            \fB+\fR
subtract       \fB-\fR
divide         \fB/\fR
multiply       \fB*\fR
modulus        \fB%\fR
bitwise and    \fB&\fR
bitwise or     \fB|\fR
bitwise xor    \fB^\fR
.fi
.PP
.B extract-int (\fIdata-expr\fB, \fIwidth\fB)\fR
.PP
.RS 0.25i
The \fBextract-int\fR operator extracts an integer value in network
byte order from the result of evaluating the specified data
expression.  Width is the width in bits of the integer to extract.
Currently, the only supported widths are 8, 16 and 32.  If the
evaluation of the data expression doesn't provide sufficient bits to
extract an integer of the specified size, the null value is returned.
.RE
.PP
.B lease-time
.PP
.RS 0.25i
The duration of the current lease - that is, the difference between
the current time and the time that the lease expires.
.RE
.PP
.I number
.PP
.RS 0.25i
Any number between zero and the maximum representable size may be
specified as a numeric expression.
.RE
.PP
.B client-state
.PP
.RS 0.25i
The current state of the client instance being processed.  This is
only useful in DHCP client configuration files.  Possible values are:
.TP 2
.I \(bu
Booting - DHCP client is in the INIT state, and does not yet have an
IP address.  The next message transmitted will be a DHCPDISCOVER,
which will be broadcast.
.TP
.I \(bu
Reboot - DHCP client is in the INIT-REBOOT state.  It has an IP
address, but is not yet using it.  The next message to be transmitted
will be a DHCPREQUEST, which will be broadcast.  If no response is
heard, the client will bind to its address and move to the BOUND state.
.TP
.I \(bu
Select - DHCP client is in the SELECTING state - it has received at
least one DHCPOFFER message, but is waiting to see if it may receive
other DHCPOFFER messages from other servers.  No messages are sent in
the SELECTING state.
.TP
.I \(bu
Request - DHCP client is in the REQUESTING state - it has received at
least one DHCPOFFER message, and has chosen which one it will
request.  The next message to be sent will be a DHCPREQUEST message,
which will be broadcast.
.TP
.I \(bu
Bound - DHCP client is in the BOUND state - it has an IP address.  No
messages are transmitted in this state.
.TP
.I \(bu
Renew - DHCP client is in the RENEWING state - it has an IP address,
and is trying to contact the server to renew it.  The next message to
be sent will be a DHCPREQUEST message, which will be unicast directly
to the server.
.TP
.I \(bu
Rebind - DHCP client is in the REBINDING state - it has an IP address,
and is trying to contact any server to renew it.  The next message to
be sent will be a DHCPREQUEST, which will be broadcast.
.RE
.SH REFERENCE: ACTION EXPRESSIONS
.PP
.B log (\fIpriority\fB, \fIdata-expr\fB)\fR
.RS 0.25i
.PP
Logging statements may be used to send information to the standard logging
channels.  A logging statement includes an optional priority (\fBfatal\fR,
\fBerror\fR, \fBinfo\fR, or \fBdebug\fR), and a data expression.
.PP
Logging statements take only a single data expression argument, so if you
want to output multiple data values, you will need to use the \fBconcat\fR
operator to concatenate them.
.RE
.PP
.B execute (\fIcommand-path\fB [, \fIdata-expr1\fB, ... \fIdata-exprN\fB]);\fR
.RS 0.25i
.PP
The \fBexecute\fR statement runs an external command.  The first argument
is a string literal containing the name or path of the command to run.
The other arguments, if present, are either string literals or data-
expressions which evaluate to text strings, to be passed as command-line
arguments to the command.
.PP
\fBexecute\fR is synchronous; the program will block until the external
command being run has finished.  Please note that lengthy program
execution (for example, in an "on commit" in dhcpd.conf) may result in
bad performance and timeouts.  Only external applications with very short
execution times are suitable for use.
.PP
Passing user-supplied data to an external application might be dangerous.
Make sure the external application checks input buffers for validity.
Non-printable ASCII characters will be converted into dhcpd.conf language
octal escapes ("\\nnn"), make sure your external command handles them as
such.
.PP
It is possible to use the execute statement in any context, not only
on events.  If you put it in a regular scope in the configuration file
you will execute that command every time a scope is evaluated.
.RE
.PP
.B parse-vendor-option;\fR
.RS 0.25i
.PP
The \fBparse-vendor-option\fR statement attempts to parse a vendor
option (code 43).  It is only useful while processing a packet on the server
and requires that the administrator has already used the
\fBvendor-option-space\fR statement to select a valid vendor space.
.PP
This functionality may be used if the server needs to take different
actions depending on the values the client placed in the vendor option
and the sub-options are not at fixed locations.  It is handled as an
action to allow an administrator to examine the incoming options and
choose the correct vendor space.
.RE
.SH REFERENCE: DYNAMIC DNS UPDATES
.PP
See the dhcpd.conf and dhclient.conf man pages for more information
about DDNS.
.SH SEE ALSO
dhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-options(5), dhcpd(8),
dhclient(8), RFC2132, RFC2131.
.SH AUTHOR
Information about Internet Systems Consortium can be found at
.B https://www.isc.org.