Provided by: isc-dhcp-client_4.4.3-P1-4ubuntu2_amd64 
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 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 /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 inline, 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 executable
/etc/dhcp/dhclient-exit-hooks script, which if present is invoked using the '.' command. 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 invoked. 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 $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.
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 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 $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 $new_. Care should
be taken that the alias IP address not be used if it is identical to the bound IP address
($new_ip_address), since the other alias parameters may be incorrect 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 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.
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 expired. 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 deconfigured. This can be
handled in the same way as EXPIRE.
STOP
The dhclient has been informed to shut down gracefully, the dhclient-script should unconfigure 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.
V6ONLY
The DHCPv4 client has received a requested valid v6-only-preferred option. The system should disable IPv4
on the interface. On its side the dhclient waits for V6ONLY_WAIT seconds (the timer is carried by the
option with a minimum of MIN_V6ONLY_WAIT) before returning in the INIT state.
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.
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)