dhclient-script - DHCP client network configuration script
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 details). These hooks will allow
the user to override the default behaviour of the client in creating a
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, cus-
tomizations specific to a particular computer should be done in the
/etc/dhclient.conf file. If you find that you can't make such a cus-
tomization without customizing /etc/dhclient.conf or using the enter
and exit hooks, please submit a bug report.
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.
On after defining the make_resolv_conf function, the client script
checks for the presence of an executable /etc/dhclient-enter-hooks
script, and if present, it invokes the script inline, using the Bourne
shell '.' command. 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 executable /etc/dhclient-exit-hooks script, which if
present is invoked using the '.' command. 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 invoked. The rest of the environment as described
previously for dhclient-enter-hooks is also present. The
/etc/dhclient-exit-hooks script can modify the valid of exit_status to
change the exit status of dhclient-script.
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 and TIMEOUT.
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.
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 con-
figure the interface up without actually giving it an IP address at
all. The interface name is passed in $interface, and the media type
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.
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 order to make valid shell variables, and the vari-
able names start with new_. So for example, the new subnet mask would
be passed in $new_subnet_mask.
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 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
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 cre-
ated, using the values of $new_domain_name and $new_domain_name_servers
(which may list more than one server, seperated 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 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 vari-
ables named as described previously except starting with $alias_
instead of $new_. Care should be taken that the alias IP address not
be used if it is identical to the bound IP address ($new_ip_address),
since the other alias parameters may be incorrect in this case.
When a binding has been renewed, the script is called as in BOUND,
except that in addition to all the variables starting with $new_, there
is another set of variables starting 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
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.
The DHCP client has successfully reacquired its old address after a
reboot. This can be processed as with BOUND.
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.
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 parame-
ters from the last lease tested should be deconfigured. This can be
handled in the same way as EXPIRE.
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 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.
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 the Internet Software
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.
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 confus-
dhclient(8), dhcpd(8), dhcrelay(8), dhclient.conf(5) and
dhclient-script(8) has been written for the Internet Software Consor-
tium by Ted Lemon in cooperation with Vixie Enterprises. To learn more
about the Internet Software Consortium, see http://www.isc.org. To
learn more about Vixie Enterprises, see http://www.vix.com.