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dhclient-script(8) 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 ini-
tial 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 mod-
ify an existing one. In general, customizations specific to a particular computer
should be done in the /etc/dhclient.conf file. If you find that you can’t make
such a customization without customizing /etc/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/dhclient-enter-hooks script, and if present, it
invokes the script inline, using the Bourne shell ’.’ command. The entire envi-
ronment 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/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/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/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/dhclient-up-${IF}-hooks script will override the generic script and
be sourced when interface $IF has been brought up.
Immediately before dhclient brings an interface DOWN, removing its IP address, sub-
net mask, and routes, in the STOP/RELEASE states, it will check for the existence
of an executable /etc/dhclient-down-hooks script, and source it if found. This
script can handle DHCP options in the environment that are not handled by default.
A per-interface /etc/dhclient-down-${IF}-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 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 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 vari-
able 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 $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_, 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. 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.
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.
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) has been written for Internet Systems Consortium by Ted Lemon in
cooperation with Vixie Enterprises. To learn more about Internet Systems Consor-
tium, see http://www.isc.org. To learn more about Vixie Enterprises, see
http://www.vix.com.
dhclient-script(8)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-08-20 19:55 @38.103.63.61 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)