dhclient-script(8) System Manager's Manual dhclient-script(8)
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, and to handle DHCP options
not handled by default.
No standard client script exists for some operating systems, even though the actual client may work, so a pio‐
neering user may well need to create a new script or modify an existing one. In general, customizations spe‐
cific 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.
On 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. 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 /usr/sbin/dhclient-script will exit
with that error code immediately after the client script exits.
After all processing has completed, /usr/sbin/dhclient-script checks for the presence of an executable
/etc/dhcp/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/dhcp/dhclient-exit-hooks script can modify the
valid of exit_status to change the exit status of dhclient-script.
Immediately after dhclient brings an interface UP with a new IP address, subnet mask, and routes, in the
REBOOT/BOUND states, it will check for the existence of an executable /etc/dhcp/dhclient-up-hooks script, and
source it if found. This script can handle DHCP options in the environment that are not handled by default. A
per-interface. /etc/dhcp/dhclient-${IF}-up-hooks script will override the generic script and be sourced when
interface $IF has been brought up.
Immediately before dhclient brings an interface DOWN, removing its IP address, subnet mask, and routes, in the
STOP/RELEASE states, it will check for the existence of an executable /etc/dhcp/dhclient-down-hooks script,
and source it if found. This script can handle DHCP options in the environment that are not handled by
default. A per-interface /etc/dhcp/dhclient-${IF}-down-hooks script will override the generic script and be
sourced when interface $IF is about to be brought down.
OPERATION
When dhclient needs to invoke the client configuration script, it defines a set of variables in the environ‐
ment, and then invokes /usr/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,
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. 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, or 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 vari‐
ables 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, pro‐
cessing 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
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 parame‐
ters 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.
FILES
Each operating system should generally have its own script file, although the script files for similar operat‐
ing 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 con‐
figuration 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.
Normally, if dhclient was compiled with libcap-ng support, dhclient drops most capabilities immediately upon
startup. While more secure, this greatly restricts the additional actions that hooks in dhclient-script can
take. For example, any daemons that dhclient-script starts or restarts will inherit the restricted capabili‐
ties as well, which may interfere with their correct operation. Thus, the -nc option can be used to prevent
dhclient from dropping capabilities.
SEE ALSO
dhclient(8), dhcpd(8), dhcrelay(8), dhclient.conf(5) and dhclient.leases(5).
AUTHOR
dhclient-script(8) 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.
dhclient-script(8)
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