dhclient-script - DHCP client network configuration script
The DHCP client network configuration script is invoked from time to time by
. 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 /etc/dhcp/dhclient.conf
If you find that you can't make such a customization without customizing
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 /etc/dhcp/dhclient-enter-hooks
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 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/dhcp/dhclient-exit-hooks
if present is invoked using the '.' command. All executable scripts in
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 /etc/dhcp/dhclient-exit-hooks
scripts can modify the value 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
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
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.
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 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_, and $requested_ 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 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
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
The dhclient has been informed to shut down gracefully, the dhclient-script
should unconfigure or shutdown the interface as appropriate.
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 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 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 confusing.
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