sbin/dhclient/dhclient-script.8

.\"	$OpenBSD: dhclient-script.8,v 1.2 2004/04/09 18:30:15 jmc Exp $
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.\" Copyright (c) 1997 The Internet Software Consortium.
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.\" This software has been written for the Internet Software Consortium
.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
.\" Enterprises.  To learn more about the Internet Software Consortium,
.\" see ``http://www.isc.org/isc''.  To learn more about Vixie
.\" Enterprises, see ``http://www.vix.com''.
.\"
.Dd October 5, 2026
.Dt DHCLIENT-SCRIPT 8
.Os
.Sh NAME
.Nm dhclient-script
.Nd DHCP client network configuration script
.Sh DESCRIPTION
The DHCP client network configuration script is invoked from time to
time by
.Xr 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.
.Pp
.\" 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
.Pa /etc/dhclient.conf
file.
.Sh OPERATION
When
.Xr dhclient 8
needs to invoke the client configuration script, it sets up a number of
environment variables and runs
.Nm .
In all cases,
.Va $reason
is set to the name of the reason why the script has been invoked.
The following reasons are currently defined:
.Li MEDIUM , PREINIT , ARPCHECK , ARPSEND , BOUND , RENEW , REBIND , REBOOT ,
.Li EXPIRE , FAIL
and
.Li TIMEOUT .
.Bl -tag -width ".Li ARPCHECK"
.It Li MEDIUM
The DHCP client is requesting that an interface's media type be set.
The interface name is passed in
.Va $interface ,
and the media type is passed in
.Va $medium .
.It Li 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
.Va $interface ,
and the media type in
.Va $medium .
.Pp
If an IP alias has been declared in
.Xr dhclient.conf 5 ,
its address will be passed in
.Va $alias_ip_address ,
and that IP alias should be deleted from the interface,
along with any routes to it.
.It Li 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
.Va $new_ip_address ,
and the interface name is passed in
.Va $interface .
.It Li ARPCHECK
The DHCP client wants to know if a response to the ARP request sent
using
.Li 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
.Va $new_ip_address
and
.Va $interface
variables are set as with
.Li ARPSEND .
.It Li BOUND
The DHCP client has done an initial binding to a new address.
The new IP address is passed in
.Va $new_ip_address ,
and the interface name is passed in
.Va $interface .
The media type is passed in
.Va $medium .
Any options acquired from the server are passed using the option name
described in
.Xr dhcp-options 5 ,
except that dashes
.Pq Ql -
are replaced by underscores
.Pq Ql _
in order to make valid shell variables, and the variable names start with
.Dq Li new_ .
So for example, the new subnet mask would be passed in
.Va $new_subnet_mask .
.Pp
As an exception, the captive portal API URL from DHCP option 114
.Pq see Xr dhcp-options 5
is passed as
.Va $new_captive_portal_url .
When such a URL is received, the script also writes it to
.Pa /var/run/captive-portal. Ns Va $interface
so that it can be discovered by other programs.
The file is removed when the lease expires or is lost.
.Pp
When a binding has been completed, a lot of network parameters are
likely to need to be set up.
A new
.Pa /etc/resolv.conf
needs to be created, using the values of
.Va $new_domain_name
and
.Va $new_domain_name_servers
(which may list more than one server, separated by spaces).
A default route should be set using
.Va $new_routers ,
and static routes may need to be set up using
.Va $new_static_routes .
.Pp
If an IP alias has been declared, it must be set up here.
The alias IP address will be written as
.Va $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
.Dq Li $alias_
instead of
.Dq Li $new_ .
Care should be taken that the alias IP address not be used if it is identical
to the bound IP address
.Pq Va $new_ip_address ,
since the other alias parameters may be incorrect in this case.
.It Li RENEW
When a binding has been renewed, the script is called as in
.Li BOUND ,
except that in addition to all the variables starting with
.Dq Li $new_ ,
there is another set of variables starting with
.Dq Li $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
.Li BOUND .
.It Li REBIND
The DHCP client has rebound to a new DHCP server.
This can be handled as with
.Li RENEW ,
except that if the IP address has changed,
the ARP table should be cleared.
.It Li REBOOT
The DHCP client has successfully reacquired its old address after a reboot.
This can be processed as with
.Li BOUND .
.It Li 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
.Li RENEW
and
.Li REBIND .
.It Li 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
.Li EXPIRE .
.It Li 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
.Li 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.
.El
.Pp
Before taking action according to
.Va $reason ,
.Nm
will check for the existence of
.Pa /etc/dhclient-enter-hooks .
If found, it will be sourced
.Pq see Xr sh 1 .
After taking action according to
.Va $reason ,
.Nm
will check for the existence of
.Pa /etc/dhclient-exit-hooks .
If found, it will be sourced
.Pq see Xr sh 1 .
These hooks scripts can be used to dynamically modify the environment at
appropriate times during the DHCP negotiations.
For example, if the administrator wishes to disable alias IP numbers on
the DHCP interface, they might want to put the following in
.Pa /etc/dhclient-enter-hooks :
.Bd -literal -offset indent
[ ."$reason" = .PREINIT ] && ifconfig $interface 0.0.0.0
.Ed
.Pp
The usual way to test a lease is to set up the network as with
.Li REBIND
(since this may be called to test more than one lease) and then ping
the first router defined in
.Va $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
.Va $new_routers ,
as well as those listed in
.Va $new_static_routes ,
but current scripts do not do this.
.\" .Sh 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 the Internet Software 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.
.Sh SEE ALSO
.Xr sh 1 ,
.Xr dhclient.conf 5 ,
.Xr dhclient.leases 5 ,
.Xr dhclient 8 ,
.Xr dhcpd 8 ,
.Xr dhcrelay 8
.Sh AUTHORS
.An -nosplit
The original version of
.Nm
was written for the Internet Software Consortium by
.An Ted Lemon Aq Mt mellon@fugue.com
in cooperation with Vixie Enterprises.
.Pp
The
.Ox
implementation of
.Nm
was written by
.An Kenneth R. Westerback Aq Mt krw@openbsd.org .
.Sh 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
.Nm
rewrites
.Pa /etc/resolv.conf .
If more than one interface is being configured,
.Pa /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.