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dhclient-script(8) System Manager's Manual dhclient-script(8)

NAME
dhclient-script - DHCP client network configuration script

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 inter‐
face'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 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 HOOKS for de‐
tails). These hooks will allow the user to override the default behaviour of the client in
creating a /etc/resolv.conf file.

No standard client script exists for some operating systems, even though the actual client
may work, so a pioneering user may well need to create a new script or modify an existing
one. In general, customizations specific to a particular computer should be done in the
/etc/dhcp/dhclient.conf file. If you find that you can't make such a customization without
customizing /etc/dhcp/dhclient.conf or using the enter and exit hooks, please submit a bug
report.

HOOKS
When it starts, the client script 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.

After defining the make_resolv_conf function, the client script checks for the presence of an
executable /etc/dhcp/dhclient-enter-hooks script, and if present, it invokes the script in‐
line, using the Bourne shell ´.´ command. It also invokes all executable scripts in
/etc/dhcp/dhclient-enter-hooks.d/* in the same way. The entire environment documented under
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 /sbin/dhclient-script will exit with
that error code immediately after the client script exits.

After all processing has completed, /sbin/dhclient-script checks for the presence of an exe‐
cutable /etc/dhcp/dhclient-exit-hooks script, which if present is invoked using the '.' com‐
mand. All executable scripts in /etc/dhcp/dhclient-exit-hooks.d/* are also invoked. The
exit status of dhclient-script will be passed to dhclient-exit-hooks in the exit_status shell
variable, and will always be zero if the script succeeded at the task for which it was in‐
voked. The rest of the environment as described previously for dhclient-enter-hooks is also
present. The /etc/dhcp/dhclient-exit-hooks and /etc/dhcp/dhclient-exit-hooks.d/* scripts
can modify the value of exit_status to change the exit status of dhclient-script.

OPERATION
When dhclient needs to invoke the client configuration script, it defines a set of variables
in the environment, and then invokes /sbin/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.

MEDIUM
The DHCP client is requesting that an interface's media type be set. The interface name is
passed in $interface, and the media type is passed in $medium.

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 $inter‐
face, and the media type in $medium.

If an IP alias has been declared in dhclient.conf, its address will be passed in
$alias_ip_address, and that ip alias should be deleted from the interface, along with any
routes to it.

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, except that dashes (´-´) are replaced by underscores (´_´) in or‐
der 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 ex‐
piration time (*_expiry).

Before actually configuring the address, dhclient-script should somehow ARP for it and exit
with a nonzero status if it receives a reply. In this case, the client will send a DHCPDE‐
CLINE 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 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 de‐
fault route should be set using $new_routers, and static routes may need to be set up using
$new_static_routes.

If an IP 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 starting with $alias_
instead of $new_. Care should be taken that the alias IP address not be used if it is iden‐
tical to the bound IP address ($new_ip_address), since the other alias parameters may be in‐
correct in this case.

RENEW
When a binding has been renewed, the script is called as in BOUND, except that in addition to
all the variables starting with $new_, and $requested_ there is another set of variables
starting with $old_. Persistent settings that may have changed need to be deleted - for ex‐
ample, 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.

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.

REBOOT
The DHCP client has successfully reacquired its old address after a reboot. This can be
processed as with BOUND.

EXPIRE
The DHCP client has failed to renew its lease or acquire a new one, and the lease has ex‐
pired. The IP address must be relinquished, and all related parameters should be deleted,
as in RENEW and REBIND.

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 de‐
configured. This can be handled in the same way as EXPIRE.

STOP
The dhclient has been informed to shut down gracefully, the dhclient-script should unconfig‐
ure or shutdown the interface as appropriate.

RELEASE
The dhclient has been executed using the -r flag, indicating that the administrator wishes it
to release its lease(s). dhclient-script should unconfigure or shutdown the interface.

NBI
No-Broadcast-Interfaces...dhclient 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.

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 zero. If not, it should exit with a nonzero 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 cur‐
rently 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.

FILES
Each operating system should generally have its own script file, although the script files
for similar operating systems may be similar or even identical. The script files included
in Internet Systems Consortium DHCP distribution appear in the distribution tree under
client/scripts, and bear the names of the operating systems on which they are intended to
work.

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.

SEE ALSO
dhclient(8), dhcpd(8), dhcrelay(8), dhclient.conf(5) and dhclient.leases(5).

AUTHOR
dhclient-script(8) To learn more about Internet Systems Consortium, see https://www.isc.org.

dhclient-script(8)