client/dhclient-script.8

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  27 .\" $Id: dhclient-script.8,v 1.14 2010/07/02 23:09:14 sar Exp $
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  29 .TH dhclient-script 8
  30 .SH NAME
  31 dhclient-script - DHCP client network configuration script
  32 .SH DESCRIPTION
  33 The DHCP client network configuration script is invoked from time to
  34 time by \fBdhclient(8)\fR.  This script is used by the dhcp client to
  35 set each interface's initial configuration prior to requesting an
  36 address, to test the address once it has been offered, and to set the
  37 interface's final configuration once a lease has been acquired.  If no
  38 lease is acquired, the script is used to test predefined leases, if
  39 any, and also called once if no valid lease can be identified.
  40 .PP
  41 This script is not meant to be customized by the end user.  If local
  42 customizations are needed, they should be possible using the enter and
  43 exit hooks provided (see HOOKS for details).   These hooks will allow the
  44 user to override the default behaviour of the client in creating a
  45 .B /etc/resolv.conf
  46 file.
  47 .PP
  48 No standard client script exists for some operating systems, even though
  49 the actual client may work, so a pioneering user may well need to create
  50 a new script or modify an existing one.  In general, customizations specific
  51 to a particular computer should be done in the
  52 .B ETCDIR/dhclient.conf
  53 file.   If you find that you can't make such a customization without
  54 customizing
  55 .B ETCDIR/dhclient.conf
  56 or using the enter and exit hooks, please submit a bug report.
  57 .SH HOOKS
  58 When it starts, the client script first defines a shell function,
  59 .B make_resolv_conf ,
  60 which is later used to create the
  61 .B /etc/resolv.conf
  62 file.   To override the default behaviour, redefine this function in
  63 the enter hook script.
  64 .PP
  65 On after defining the make_resolv_conf function, the client script checks
  66 for the presence of an executable
  67 .B ETCDIR/dhclient-enter-hooks
  68 script, and if present, it invokes the script inline, using the Bourne
  69 shell \'.\' command.   The entire environment documented under OPERATION
  70 is available to this script, which may modify the environment if needed
  71 to change the behaviour of the script.   If an error occurs during the
  72 execution of the script, it can set the exit_status variable to a nonzero
  73 value, and
  74 .B CLIENTBINDIR/dhclient-script
  75 will exit with that error code immediately after the client script exits.
  76 .PP
  77 After all processing has completed,
  78 .B CLIENTBINDIR/dhclient-script
  79 checks for the presence of an executable
  80 .B ETCDIR/dhclient-exit-hooks
  81 script, which if present is invoked using the \'.\' command.  The exit
  82 status of dhclient-script will be passed to dhclient-exit-hooks in the
  83 exit_status shell variable, and will always be zero if the script
  84 succeeded at the task for which it was invoked.   The rest of the
  85 environment as described previously for dhclient-enter-hooks is also
  86 present.   The
  87 .B ETCDIR/dhclient-exit-hooks
  88 script can modify the valid of exit_status to change the exit status
  89 of dhclient-script.
  90 .SH OPERATION
  91 When dhclient needs to invoke the client configuration script, it
  92 defines a set of variables in the environment, and then invokes
  93 .B CLIENTBINDIR/dhclient-script.
  94 In all cases, $reason is set to the name of the reason why the script
  95 has been invoked.   The following reasons are currently defined:
  96 MEDIUM, PREINIT, BOUND, RENEW, REBIND, REBOOT, EXPIRE, FAIL, STOP, RELEASE,
  97 NBI and TIMEOUT.
  98 .PP
  99 .SH MEDIUM
 100 The DHCP client is requesting that an interface's media type
 101 be set.  The interface name is passed in $interface, and the media
 102 type is passed in $medium.
 103 .SH PREINIT
 104 The DHCP client is requesting that an interface be configured as
 105 required in order to send packets prior to receiving an actual
 106 address.   For clients which use the BSD socket library, this means
 107 configuring the interface with an IP address of 0.0.0.0 and a
 108 broadcast address of 255.255.255.255.   For other clients, it may be
 109 possible to simply configure the interface up without actually giving
 110 it an IP address at all.   The interface name is passed in $interface,
 111 and the media type in $medium.
 112 .PP
 113 If an IP alias has been declared in dhclient.conf, its address will be
 114 passed in $alias_ip_address, and that ip alias should be deleted from
 115 the interface, along with any routes to it.
 116 .SH BOUND
 117 The DHCP client has done an initial binding to a new address.   The
 118 new ip address is passed in $new_ip_address, and the interface name is
 119 passed in $interface.   The media type is passed in $medium.   Any
 120 options acquired from the server are passed using the option name
 121 described in \fBdhcp-options\fR, except that dashes (\'-\') are replaced
 122 by underscores (\'_\') in order to make valid shell variables, and the
 123 variable names start with new_.  So for example, the new subnet mask
 124 would be passed in $new_subnet_mask.  Options from a non-default
 125 universe will have the universe name prepended to the option name, for
 126 example $new_dhcp6_server_id.  The options that the client
 127 explicitly requested via a PRL or ORO option are passed with the same
 128 option name as above but prepended with requested_ and with a value of 1,
 129 for example requested_subnet_mask=1.  No such variable is defined for
 130 options not requested by the client or options that don't require a
 131 request option, such as the ip address (*_ip_address) or expiration
 132 time (*_expiry).
 133 .PP
 134 Before actually configuring the address, dhclient-script should
 135 somehow ARP for it and exit with a nonzero status if it receives a
 136 reply.   In this case, the client will send a DHCPDECLINE message to
 137 the server and acquire a different address.   This may also be done in
 138 the RENEW, REBIND, or REBOOT states, but is not required, and indeed
 139 may not be desirable.
 140 .PP
 141 When a binding has been completed, a lot of network parameters are
 142 likely to need to be set up.   A new /etc/resolv.conf needs to be
 143 created, using the values of $new_domain_name and
 144 $new_domain_name_servers (which may list more than one server,
 145 separated by spaces).   A default route should be set using
 146 $new_routers, and static routes may need to be set up using
 147 $new_static_routes.
 148 .PP
 149 If an IP alias has been declared, it must be set up here.   The alias
 150 IP address will be written as $alias_ip_address, and other DHCP
 151 options that are set for the alias (e.g., subnet mask) will be passed
 152 in variables named as described previously except starting with
 153 $alias_ instead of $new_.   Care should be taken that the alias IP
 154 address not be used if it is identical to the bound IP address
 155 ($new_ip_address), since the other alias parameters may be incorrect
 156 in this case.
 157 .SH RENEW
 158 When a binding has been renewed, the script is called as in BOUND,
 159 except that in addition to all the variables starting with $new_, and
 160 $requested_ there is another set of variables starting with $old_.
 161 Persistent settings that may have changed need to be deleted - for
 162 example, if a local route to the bound address is being configured,
 163 the old local route should be deleted.  If the default route has changed,
 164 the old default route should be deleted.  If the static routes have changed,
 165 the old ones should be deleted.  Otherwise, processing can be done as with
 166 BOUND.
 167 .SH REBIND
 168 The DHCP client has rebound to a new DHCP server.  This can be handled
 169 as with RENEW, except that if the IP address has changed, the ARP
 170 table should be cleared.
 171 .SH REBOOT
 172 The DHCP client has successfully reacquired its old address after a
 173 reboot.   This can be processed as with BOUND.
 174 .SH EXPIRE
 175 The DHCP client has failed to renew its lease or acquire a new one,
 176 and the lease has expired.   The IP address must be relinquished, and
 177 all related parameters should be deleted, as in RENEW and REBIND.
 178 .SH FAIL
 179 The DHCP client has been unable to contact any DHCP servers, and any
 180 leases that have been tested have not proved to be valid.   The
 181 parameters from the last lease tested should be deconfigured.   This
 182 can be handled in the same way as EXPIRE.
 183 .SH STOP
 184 The dhclient has been informed to shut down gracefully, the
 185 dhclient-script should unconfigure or shutdown the interface as
 186 appropriate.
 187 .SH RELEASE
 188 The dhclient has been executed using the -r flag, indicating that the
 189 administrator wishes it to release its lease(s).  dhclient-script should
 190 unconfigure or shutdown the interface.
 191 .SH NBI
 192 No-Broadcast-Interfaces...dhclient was unable to find any interfaces
 193 upon which it believed it should commence DHCP.  What dhclient-script
 194 should do in this situation is entirely up to the implementor.
 195 .SH TIMEOUT
 196 The DHCP client has been unable to contact any DHCP servers.
 197 However, an old lease has been identified, and its parameters have
 198 been passed in as with BOUND.   The client configuration script should
 199 test these parameters and, if it has reason to believe they are valid,
 200 should exit with a value of zero.   If not, it should exit with a
 201 nonzero value.
 202 .SH V6ONLY
 203 The DHCPv4 client has received a requested valid v6-only-preferred
 204 option. The system should disable IPv4 on the interface. On its side
 205 the dhclient waits for V6ONLY_WAIT seconds (the timer is carried by
 206 the option with a minimum of MIN_V6ONLY_WAIT) before returning in
 207 the INIT state.
 208 .PP
 209 The usual way to test a lease is to set up the network as with REBIND
 210 (since this may be called to test more than one lease) and then ping
 211 the first router defined in $routers.  If a response is received, the
 212 lease must be valid for the network to which the interface is
 213 currently connected.   It would be more complete to try to ping all of
 214 the routers listed in $new_routers, as well as those listed in
 215 $new_static_routes, but current scripts do not do this.
 216 .SH FILES
 217 Each operating system should generally have its own script file,
 218 although the script files for similar operating systems may be similar
 219 or even identical.   The script files included in Internet
 220 Systems Consortium DHCP distribution appear in the distribution tree
 221 under client/scripts, and bear the names of the operating systems on
 222 which they are intended to work.
 223 .SH BUGS
 224 If more than one interface is being used, there's no obvious way to
 225 avoid clashes between server-supplied configuration parameters - for
 226 example, the stock dhclient-script rewrites /etc/resolv.conf.   If
 227 more than one interface is being configured, /etc/resolv.conf will be
 228 repeatedly initialized to the values provided by one server, and then
 229 the other.   Assuming the information provided by both servers is
 230 valid, this shouldn't cause any real problems, but it could be
 231 confusing.
 232 .SH SEE ALSO
 233 dhclient(8), dhcpd(8), dhcrelay(8), dhclient.conf(5) and
 234 dhclient.leases(5).
 235 .SH AUTHOR
 236 .B dhclient-script(8)
 237 To learn more about Internet Systems Consortium,
 238 see
 239 .B https://www.isc.org.