200 lines
9.5 KiB
Groff
200 lines
9.5 KiB
Groff
.\" dhclient-script.8
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.\"
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.\" Copyright (c) 1996-1999 Internet Software Consortium.
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.\" Use is subject to license terms which appear in the file named
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.\" ISC-LICENSE that should have accompanied this file when you
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.\" received it. If a file named ISC-LICENSE did not accompany this
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.\" file, or you are not sure the one you have is correct, you may
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.\" obtain an applicable copy of the license at:
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.\"
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.\" http://www.isc.org/isc-license-1.0.html.
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.\"
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.\" This file is part of the ISC DHCP distribution. The documentation
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.\" associated with this file is listed in the file DOCUMENTATION,
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.\" included in the top-level directory of this release.
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.\"
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.\" Support and other services are available for ISC products - see
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.\" http://www.isc.org for more information.
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.TH dhclient-script 8
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.SH NAME
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dhclient-script - DHCP client network configuration script
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.SH DESCRIPTION
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The DHCP client network configuration script is invoked from time to
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time by \fBdhclient(8)\fR. This script is used by the dhcp client to
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set each interface's initial configuration prior to requesting an
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address, to test the address once it has been offered, and to set the
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interface's final configuration once a lease has been acquired. If no
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lease is acquired, the script is used to test predefined leases, if
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any, and also called once if no valid lease can be identified.
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.PP
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This script is not meant to be customized by the end user. If local
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customizations are needed, they should be possible using the enter and
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exit hooks provided (see HOOKS for details). These hooks will allow the
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user to override the default behaviour of the client in creating a
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.B /etc/resolv.conf
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file.
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.PP
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No standard client script exists for some operating systems, even though
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the actual client may work, so a pioneering user may well need to create
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a new script or modify an existing one. In general, customizations specific
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to a particular computer should be done in the
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.B ETCDIR/dhclient.conf
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file. If you find that you can't make such a customization without
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customizing
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.B ETCDIR/dhclient.conf
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or using the enter and exit hooks, please submit a bug report.
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.SH HOOKS
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When it starts, the client script first defines a shell function,
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.B make_resolv_conf ,
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which is later used to create the
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.B /etc/resolv.conf
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file. To override the default behaviour, redefine this function in
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the enter hook script.
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.PP
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On after defining the make_resolv_conf function, the client script checks
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for the presence of an executable
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.B ETCDIR/dhclient-enter-hooks
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script, and if present, it invokes the script inline, using the Bourne
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shell '.' command. The entire environment documented under OPERATION
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is available to this script, which may modify the environment if needed
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to change the behaviour of the script. If an error occurs during the
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execution of the script, it can set the exit_status variable to a nonzero
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value, and
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.B ETCDIR/dhclient-script
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will exit with that error code immediately after the client script exits.
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.PP
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After all processing has completed,
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.B ETCDIR/dhclient-script
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checks for the presence of an executable
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.B ETCDIR/dhclient-exit-hooks
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script, which if present is invoked using the '.' command. The exit status
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is passed in the exit_status shell variable, and will always be zero if the
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script succeeded at the task for which it was invoked.
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.SH OPERATION
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When dhclient needs to invoke the client configuration script, it
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writes a shell script into /tmp which defines a variety of variables.
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In all cases, $reason is set to the name of the reason why the script
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has been invoked. The following reasons are currently defined:
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MEDIUM, PREINIT, BOUND, RENEW, REBIND, REBOOT, EXPIRE, FAIL and TIMEOUT.
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.PP
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.SH MEDIUM
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The DHCP client is requesting that an interface's media type
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be set. The interface name is passed in $interface, and the media
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type is passed in $medium.
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.SH PREINIT
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The DHCP client is requesting that an interface be configured as
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required in order to send packets prior to receiving an actual
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address. For clients which use the BSD socket library, this means
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configuring the interface with an IP address of 0.0.0.0 and a
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broadcast address of 255.255.255.255. For other clients, it may be
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possible to simply configure the interface up without actually giving
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it an IP address at all. The interface name is passed in $interface,
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and the media type in $medium.
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.PP
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If an IP alias has been declared in dhclient.conf, its address will be
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passed in $alias_ip_address, and that ip alias should be deleted from
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the interface, along with any routes to it.
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.SH BOUND
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The DHCP client has done an initial binding to a new address. The
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new ip address is passed in $new_ip_address, and the interface name is
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passed in $interface. The media type is passed in $medium. Any
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options acquired from the server are passed using the option name
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described in \fBdhcp-options\fR, except that dashes ('-') are replaced
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by underscores ('_') in order to make valid shell variables, and the
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variable names start with new_. So for example, the new subnet mask
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would be passed in $new_subnet_mask.
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.PP
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Before actually configuring the address, dhclient-script should
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somehow ARP for it and exit with a nonzero status if it receives a
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reply. In this case, the client will send a DHCPDECLINE message to
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the server and acquire a different address. This may also be done in
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the RENEW, REBIND, or REBOOT states, but is not required, and indeed
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may not be desirable.
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.PP
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When a binding has been completed, a lot of network parameters are
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likely to need to be set up. A new /etc/resolv.conf needs to be
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created, using the values of $new_domain_name and
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$new_domain_name_servers (which may list more than one server,
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seperated by spaces). A default route should be set using
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$new_routers, and static routes may need to be set up using
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$new_static_routes.
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.PP
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If an IP alias has been declared, it must be set up here. The alias
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IP address will be written as $alias_ip_address, and other DHCP
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options that are set for the alias (e.g., subnet mask) will be passed
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in variables named as described previously except starting with
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$alias_ instead of $new_. Care should be taken that the alias IP
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address not be used if it is identical to the bound IP address
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($new_ip_address), since the other alias parameters may be incorrect
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in this case.
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.SH RENEW
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When a binding has been renewed, the script is called as in BOUND,
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except that in addition to all the variables starting with $new_,
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there is another set of variables starting with $old_. Persistent
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settings that may have changed need to be deleted - for example, if a
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local route to the bound address is being configured, the old local
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route should be deleted. If the default route has changed, the old default
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route should be deleted. If the static routes have changed, the old
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ones should be deleted. Otherwise, processing can be done as with
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BOUND.
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.SH REBIND
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The DHCP client has rebound to a new DHCP server. This can be handled
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as with RENEW, except that if the IP address has changed, the ARP
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table should be cleared.
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.SH REBOOT
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The DHCP client has successfully reacquired its old address after a
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reboot. This can be processed as with BOUND.
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.SH EXPIRE
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The DHCP client has failed to renew its lease or acquire a new one,
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and the lease has expired. The IP address must be relinquished, and
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all related parameters should be deleted, as in RENEW and REBIND.
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.SH FAIL
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The DHCP client has been unable to contact any DHCP servers, and any
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leases that have been tested have not proved to be valid. The
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parameters from the last lease tested should be deconfigured. This
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can be handled in the same way as EXPIRE.
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.SH TIMEOUT
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The DHCP client has been unable to contact any DHCP servers.
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However, an old lease has been identified, and its parameters have
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been passed in as with BOUND. The client configuration script should
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test these parameters and, if it has reason to believe they are valid,
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should exit with a value of zero. If not, it should exit with a
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nonzero value.
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.PP
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The usual way to test a lease is to set up the network as with REBIND
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(since this may be called to test more than one lease) and then ping
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the first router defined in $routers. If a response is received, the
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lease must be valid for the network to which the interface is
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currently connected. It would be more complete to try to ping all of
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the routers listed in $new_routers, as well as those listed in
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$new_static_routes, but current scripts do not do this.
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.SH FILES
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Each operating system should generally have its own script file,
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although the script files for similar operating systems may be similar
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or even identical. The script files included in the Internet
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Software Consortium DHCP distribution appear in the distribution tree
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under client/scripts, and bear the names of the operating systems on
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which they are intended to work.
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.SH BUGS
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If more than one interface is being used, there's no obvious way to
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avoid clashes between server-supplied configuration parameters - for
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example, the stock dhclient-script rewrites /etc/resolv.conf. If
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more than one interface is being configured, /etc/resolv.conf will be
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repeatedly initialized to the values provided by one server, and then
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the other. Assuming the information provided by both servers is
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valid, this shouldn't cause any real problems, but it could be
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confusing.
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.SH SEE ALSO
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dhclient(8), dhcpd(8), dhcrelay(8), dhclient.conf(5) and
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dhclient.leases(5).
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.SH AUTHOR
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.B dhclient-script(8)
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has been written for the Internet Software Consortium
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by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
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Enterprises. To learn more about the Internet Software Consortium,
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see
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.B http://www.vix.com/isc.
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To learn more about Vixie
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Enterprises, see
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.B http://www.vix.com.
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