Section: Maintenance Commands (8)
Return to Main Contents
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
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 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, customizations specific
to a particular computer should be done in the
file. If you find that you can't make such a customization without
or using the enter and exit hooks, please submit a bug report.
When it starts, the client script first defines a shell function,
which is later used to create the
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
script, and if present, it invokes the script inline, using the Bourne
shell '.' command. It also invokes all executable scripts in
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
will exit with that error code immediately after the client script exits.
After all processing has completed,
checks for the presence of an executable
script, which if present is invoked using the '.' command. All executable
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
invoked. The rest of the environment as described previously for
dhclient-enter-hooks is also present. The
scripts can modify the value of exit_status to change the exit status
When dhclient needs to invoke the client configuration script, it
defines a set of variables in the environment, and then invokes
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.
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 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, 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
variable 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 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 default route should be set using
$new_routers, and static routes may need to be set up using
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 $new_. Care should be taken that the alias IP
address not be used if it is identical to the bound IP address
($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
parameters from the last lease tested should be deconfigured. This
can be handled in the same way as EXPIRE.
The dhclient has been informed to shut down gracefully, the
dhclient-script should unconfigure or shutdown the interface as
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.
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.
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
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 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.
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
dhclient(8), dhcpd(8), dhcrelay(8), dhclient.conf(5) and
has been written for Internet Systems Consortium
by Ted Lemon in cooperation with Vixie
Enterprises. To learn more about Internet Systems Consortium,
To learn more about Vixie
- SEE ALSO
This document was created by
using the manual pages.
Time: 01:09:51 GMT, April 22, 2018