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dhclient-script.8NAME
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 configura
tion 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 possi
ble 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 sys
tems, even though the actual client may work, so a pio
neering 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
ETCDIR/dhclient.conf file. If you find that you can't
make such a customization without customizing dhclient-
script 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
ETCDIR/dhclient-enter-hooks script, and if present, it
invokes the script inline, using the Bourne shell '.' com
mand. The entire environment documented under OPERATION
is available to this script, which may modify the environ
ment 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
ETCDIR/dhclient-script will exit with that error code
immediately after the client script exits.
After all processing has completed, ETCDIR/dhclient-script
checks for the presence of an executable ETCDIR/dhclient-
exit-hooks script, which if present is invoked using the
'.' command. The exit status is passed in the
exit_status shell variable, and will always be zero if the
script succeeded at the task for which it was invoked.
OPERATION
When dhclient needs to invoke the client configuration
script, it writes a shell script into /tmp which defines a
variety of variables. 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,
ARPCHECK, ARPSEND, 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 config
ured as required in order to send packets prior to receiv
ing 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 giv
ing 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.
ARPSEND
The DHCP client is requesting that an address that has
been offered to it be checked to see if somebody else is
using it, by sending an ARP request for that address.
It's not clear how to implement this, so no examples exist
yet. The IP address to check is passed in
$new_ip_address, and the interface name is passed in
$interface.
ARPCHECK
The DHCP client wants to know if a response to the ARP
request send using ARPSEND has been received. If one
has, the script should exit with a nonzero status, indi
cating that the offered address has already been requested
and should be declined. $new_ip_address and $interface
are set as with ARPSEND.
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.
When a binding has been completed, a lot of network param
eters 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, seperated 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 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.
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 sys
tems may be similar or even identical. The script files
included in the Internet Software Consortium DHCP distri
bution appear in the distribution tree under
client/scripts, and bear the names of the operating sys
tems on which they are intended to work.
BUGS
If more than one interface is being used, there's no obvi
ous way to avoid clashes between server-supplied configu
ration 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 ini
tialized 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 the Internet Soft
ware Consortium by Ted Lemon <mellon@fugue.com> in cooper
ation with Vixie Enterprises. To learn more about the
Internet Software Consortium, see http://www.vix.com/isc.
To learn more about Vixie Enterprises, see
http://www.vix.com.
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