DHCLIENT-SCRIPT(8) FreeBSD 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.
In general, customizations specific to a particular computer should be
done in the /etc/dhclient.conf file.
OPERATION
When dhclient(8) needs to invoke the client configuration script, it sets
up a number of environment variables and runs 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,
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
configured as required in order to send packets prior to
receiving an actual address. This means configuring the
interface with an IP address of 0.0.0.0 and a broadcast
address of 255.255.255.255. The interface name is passed
in $interface, and the media type in $medium.
If an IP alias has been declared in dhclient.conf(5), 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 is 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 sent using ARPSEND has been received. If one has,
the script should exit with a nonzero status, indicating
that the offered address has already been requested and
should be declined. The $new_ip_address and $interface
variables 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(5), 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
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 "$new_", there is another set of variables
starting with "$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.
Before taking action according to $reason, dhclient-script will check for
the existence of /etc/dhclient-enter-hooks. If found, it will be sourced
(see sh(1)). After taking action according to $reason, dhclient-script
will check for the existence of /etc/dhclient-exit-hooks. If found, it
will be sourced (see sh(1)). These hooks scripts can be used to
dynamically modify the environment at appropriate times during the DHCP
negotiations. For example, if the administrator wishes to disable alias
IP numbers on the DHCP interface, they might want to put the following in
/etc/dhclient-enter-hooks:
[ ."$reason" = .PREINIT ] && ifconfig $interface 0.0.0.0
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.
SEE ALSO
sh(1), dhclient.conf(5), dhclient.leases(5), dhclient(8), dhcpd(8),
dhcrelay(8)
AUTHORS
The original version of dhclient-script was written for the Internet
Software Consortium by Ted Lemon <mellon@fugue.com> in cooperation with
Vixie Enterprises.
The OpenBSD implementation of dhclient-script was written by Kenneth R.
Westerback <krw@openbsd.org>.
BUGS
If more than one interface is being used, there is 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
should not cause any real problems, but it could be confusing.
FreeBSD 13.1-RELEASE-p6 September 6, 2010 FreeBSD 13.1-RELEASE-p6
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