1.3. dhclient-script --- DHCP client network configuration script¶
1.3.1. Description¶
The DHCP client network configuration script is invoked from time to time by dhclient(8). This script is used by the DHCP client to set each interface's initial configuration prior to requesting an address, to test the address once it has been offered, and to set the interface's final configuration once a lease has been acquired. If no lease is acquired, the script is used to test predefined leases, if any, and is also called once if no valid lease can be identified.
This script is not meant to be customized by the end user. If local
customizations are needed, they should be possible using the enter and
exit hooks provided (see the section on
Hooks for details). These hooks will
allow the user to override the default behaviour of the client in
creating a /etc/resolv.conf file.
1.3.2. Hooks¶
When the client script starts, it first defines a shell function,
make_resolv_conf, which is later used to create the
/etc/resolv.conf file. To override the default behaviour,
redefine this function in the enter hook script.
On after defining the make_resolv_conf function, the client script
checks for the presence of an executable
/etc/lease/dhclient-enter-hooks script, and if present,
it invokes the script inline, using bash(1)'s .
command. The entire environment documented in the section on
Operation is available to this script,
which may modify the environment if needed to change the behaviour of
the script. If an error occurs during the execution of the script, it
can set the exit_status variable to a nonzero value, and
dhclient-script will exit with that error code immediately
after the client script exits.
After all processing has completed, dhclient-script checks
for the presence of an executable
/etc/lease/dhclient-exit-hooks script, which if present
is invoked using bash(1)'s . command. The exit status of
dhclient-script will be passed to
dhclient-exit-hooks in the exit_status shell variable,
and will always be 0 if the script succeeded at the task for which it
was invoked. The rest of the environment as described previously for
dhclient-enter-hooks is also present. The
dhclient-exit-hooks script can modify the value of
exit_status to change the exit status of dhclient-script.
1.3.3. Operation¶
When dhclient(8) needs to invoke the client configuration
script, it defines a set of variables in the environment, and then
invokes dhclient-script. In all cases, $reason is set to
the name of the reason why the script has been invoked. The following
reasons are currently defined: MEDIUM, PREINIT, BOUND,
RENEW, REBIND, REBOOT, EXPIRE, FAIL, STOP,
RELEASE, NBI, and TIMEOUT.
1.3.3.1. MEDIUM¶
The DHCP client is requesting that an interface's media type be set. The
interface name is passed in the $interface, and the media type is
passed in $medium.
1.3.3.2. PREINIT¶
The DHCP client is requesting that an interface be configured as
required in order to send packets prior to receiving an actual address.
For clients which use the BSD socket library, this means configuring the
interface with an IP address of 0.0.0.0 and a broadcast address of
255.255.255.255. For other clients, it may be possible to simply
configure the interface up without actually giving it an IP address at
all. The interface name is passed in $interface, and the media type in
$medium.
If an IP alias has been declared in dhclient.conf(5), its
address will be passed in $alias_ip_address, and that IP address
alias should be deleted from the interface, along with any routes to it.
1.3.3.3. BOUND¶
The DHCP client has done an initial binding to a new address. The new IP
address is passed in $new_ip_address, and the interface name is
passed in $interface. The media type is passed in $medium. Any
options acquired from the server are passed using the option name
described in dhcp-options(5), except that dashes (-) are
replaced by underscores (_) in order to make valid shell variables,
and the variable names start with new_. So, for example, the new
subnet mask would be passed in $new_subnet_mask. Options from a
non-default universe will have the universe name prepended to the option
name, for example $new_dhcp6_server_id. The options that the client
explicitly requested via a PRL or ORO option are passed with the same
option name as above but prepended with requested_ and with a value
of 1, for example requested_subnet_mask=1. No such variable is
defined for options not requested by the client or options that don't
require a request option, such as the IP address (*_ip_address) or
expiration time (*_expiry).
Before actually configuring the address, dhclient-script` should somehow ARP for it and exit with a non-zero status if it receives a reply. In this case, the client will send a DHCPDECLINE message to the server and acquire a different address. This may also be done in the RENEW, REBIND, or REBOOT states, but is not required, and indeed may not be desirable.
When a binding has been completed, a lot of network parameters are
likely to need to be set up. A new /etc/resolv.conf file needs
to be created, using the values of $new_domain_name and
$new_domain_name_servers (which may list more than one server,
separated by spaces). A default route should be set using
$new_routers, and static routes may need to be set up using
$new_static_routes.
If an IP address alias has been declared, it must be set up here. The
alias IP address will be written as $alias_ip_address, and other
DHCP options that are set for the alias (e.g., subnet mask) will be
passed in variables named as described previously except prefixed with
$alias_ instead of $new_. Care should be taken that the alias IP
address not be used if it is identical to the bound IP address
($new_ip_address), since the other alias parameters may be incorrect
in this case.
1.3.3.4. RENEW¶
When a binding has been renewed, the script is called as in
BOUND, except that in addition to all
the variables whose names are prefixed with $new_ and
$requested_, there is another set of variables whose names are
prefixed with $old_. Persistent settings that may have changed need
to be deleted; for example, if a local route to the bound address is
being configured, the old local route should be deleted. If the default
route has changed, the old default route should be deleted. If the
static routes have changed, the old ones should be deleted. Otherwise,
processing can be done as with BOUND.
1.3.3.5. REBIND¶
The DHCP client has rebound to a new DHCP server. This can be handled as with RENEW, except that if the IP address has changed, the ARP table should be cleared.
1.3.3.6. REBOOT¶
The DHCP client has successfully reacquired its old address after a reboot. This can be processed as with BOUND.
1.3.3.7. EXPIRE¶
The DHCP client has failed to renew its lease or acquire a new one, and the lease has expired. The IP address must be relinquished, and all related parameters should be deleted, as in RENEW and REBIND.
1.3.3.8. FAIL¶
The DHCP client has been unable to contact any DHCP servers, and any leases that have been tested have not proved to be valid. The parameters from the last lease tested should be deconfigured. This can be handled in the same way as EXPIRE.
1.3.3.9. STOP¶
dhclient(8) has been informed to shut down gracefully. dhclient-script should unconfigure or shutdown the interface as appropriate.
1.3.3.10. RELEASE¶
dhclient(8) has been executed using the -r command line
argument, indicating that the administrator wishes it to release its
lease(s). dhclient-script should unconfigure or shutdown the
interface.
1.3.3.11. NBI¶
NBI stands for No-Broadcast-Interfaces. dhclient(8) was unable to find any interfaces upon which it believed it should commence DHCP. What dhclient-script should do in this situation is entirely up to the implementor.
1.3.3.12. TIMEOUT¶
The DHCP client has been unable to contact any DHCP servers. However, an old lease has been identified, and its parameters have been passed in as with BOUND. The client configuration script should test these parameters and, if it has reason to believe they are valid, should exit with a value of 0. If not, it should exit with a non-zero value.
The usual way to test a lease is to set up the network as with
REBIND (since this may be called to test
more than one lease) and then ping the first router defined in
$routers. If a response is received, the lease must be valid for the
network to which the interface is currently connected. It would be more
complete to try to ping all of the routers listed in $new_routers,
as well as those listed in $new_static_routes, but current scripts
do not do this.
1.3.4. Bugs¶
If more than one interface is being used, there's no obvious way to
avoid clashes between server-supplied configuration parameters; for
example, the stock dhclient-script rewrites
/etc/resolv.conf. If more than one interface is being
configured, /etc/resolv.conf will be repeatedly initialized to
the values provided by one server, and then the other. Assuming the
information provided by both servers is valid, this shouldn't cause any
real problems, but it could be confusing.
1.3.5. See also¶
dhclient(8), dhcpd(8), dhclient.conf(5), dhclient.leases(5)
1.3.6. Copyright¶
Copyright (C) 2025 Banu Systems Private Limited. All rights reserved.
Copyright (c) 2012,2014,2016 by Internet Systems Consortium, Inc. ("ISC").
Copyright (c) 2009-2010 by Internet Systems Consortium, Inc. ("ISC").
Copyright (c) 2004-2005 by Internet Systems Consortium, Inc. ("ISC").
Copyright (c) 1996-2003 by Internet Software Consortium.