DHCP client network configuration script
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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
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 /etc/resolv.conf
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 ETCDIR/dhclient.conf file. If
you find that you can’t make such a customization
without customizing ETCDIR/dhclient.conf or using the
enter and exit hooks, please submit a bug report.
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.
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.
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.
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
ETCDIR/dhclient-enter-hooks script, and if present, it
invokes the script inline, using the Bourne shell ´.´
command. It also invokes all executable scripts in
ETCDIR/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 CLIENTBINDIR/dhclient-script will exit
with that error code immediately after the client script exits.
After all processing has completed,
CLIENTBINDIR/dhclient-script checks for the presence of an
executable ETCDIR/dhclient-exit-hooks script, which if
present is invoked using the ’.’ command. All
executable scripts in ETCDIR/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 invoked. The rest of the environment as described
previously for dhclient-enter-hooks is also present. The
ETCDIR/dhclient-exit-hooks.d/* scripts can modify the
value of exit_status to change the exit status of
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.
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
When dhclient needs to invoke the client configuration script, it
defines a set of variables in the environment, and then invokes
CLIENTBINDIR/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
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 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 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.
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 BOUND.
The dhclient has been informed to shut down gracefully, the
dhclient-script should unconfigure or shutdown the interface as
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
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
dhcpd, dhcrelay, dhclient.conf and
has been written for Internet Systems Consortium by Ted
Lemon in cooperation with Vixie Enterprises. To learn more
about Internet Systems Consortium, see
https://www.isc.org. To learn more about Vixie
Enterprises, see http://www.vix.com.