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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 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 with-
out 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 mod-
ify 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 $IF has been brought up.
Immediately before dhclient brings an interface DOWN, removing its IP address, subnet
mask, and routes, in the STOP/RELEASE states, it will check for the existence of an exe-
cutable /etc/dhcp/dhclient-down-hooks script, and source it if found. This script can han-
dle DHCP options in the environment that are not handled by default. A per-interface
/etc/dhcp/dhclient-${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 vari-
ables in the environment, and then invokes /usr/sbin/dhclient-script. In all cases, $rea-
son is set to the name of the reason why the script has been invoked. The following rea-
sons 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 broad-
cast address of 255.255.255.255. For other clients, it may be possible to simply config-
ure 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 under-
scores ('_') 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 desir-
able.
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 vari-
ables 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. Other-
wise, 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 uncon-
figure 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 con-
figuration 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.
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.
Normally, if dhclient was compiled with libcap-ng support, dhclient drops most capabili-
ties 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 capabilities 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 coop-
eration 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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