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resolvconf

manage nameserver information

cat FILE | resolvconf -a IFACE.PROG

resolvconf -d IFACE.PROG

resolvconf -u

resolvconf --enable-updates | --disable-updates | --updates-are-enabled

The resolvconf package comprises a simple database for run-time nameserver information and a simple framework for notifying applications of changes in that information. Resolvconf thus sets itself up as the intermediary between programs that supply nameserver information and applications that use that information.

Information is added to or removed from the database using the resolvconf program. See the OPTIONS section below for a discussion of the available options.

Normally the resolvconf program is run only by network interface configuration programs such as ifup(8), ifdown, NetworkManager(8), dhclient(8), and pppd(8); and by local nameservers such as dnsmasq(8). These programs obtain nameserver information from some source and push it to resolvconf.

dhclient

The dhclient program, for example, may receive nameserver addresses and domain search list information during its negotiation with the DHCP server; if so, its hook script /etc/dhcp/dhclient-enter-hooks.d/resolvconf pushes this information to resolvconf.

ifup

The ifup program can be used to configure network interfaces according to settings in /etc/network/interfaces. To make ifup push nameserver information to resolvconf when it configures an interface the administrator must add dns- option lines to the relevant iface stanza in interfaces(5). The following option names are accepted: dns-nameserver, dns-search, and dns-sortlist.

To add a nameserver IP address, add an option line consisting of dns-nameserver and the address. To add multiple nameserver addresses, include multiple such dns-nameserver lines.

dns-nameserver 192.168.1.254 dns-nameserver 8.8.8.8

To add search domain names, add a line beginning with dns-search.

dns-search foo.org bar.com

The dns-nameservers option is also accepted and, unlike dns-nameserver, can be given multiple arguments, separated by spaces.

The dns-domain option is deprecated in favor of dns-search.

The resulting stanza might look like the following example.

iface eth0 inet static address 192.168.1.3 netmask 255.255.255.0 gateway 192.168.1.1 dns-nameserver 192.168.1.254 dns-nameserver 8.8.8.8 dns-search foo.org bar.com

N.B.: On a machine where resolvconf has just been or is about to be installed and which previously relied on a static /etc/resolv.conf file,

the nameserver information in that static file, (which is to say the information on nameserver, domain, search and sortlist lines) should be migrated to the appropriate iface stanza(s) in /etc/network/interfaces(5) as just described;
options (which is to say, any options lines) should be migrated to /etc/resolvconf/resolv.conf.d/base.

Command line

The administrator can run resolvconf from the command line to add or delete nameserver information, but this is not normally necessary or advisable.

Nameserver information provided to resolvconf is stored for use by subscribers to resolvconf's notification service. Subscriber packages that need to know when nameserver information has changed should install a script in /etc/resolvconf/update.d/ (or in /etc/resolvconf/update-libc.d/: see below). For example, DNS caches such as dnsmasq(8) and pdnsd(8) subscribe to the notification service so that they know whither to forward queries. Client hook scripts will find the files containing nameserver information in the current directory.

libc

The most important software package that subscribes to the notification service is the GNU C Library resolver(3). This library is used by many applications that need to resolve domain names. When nameserver information is updated, the script /etc/resolvconf/update.d/libc generates a new version of the resolver configuration file, /run/resolvconf/resolv.conf, as described below. If the new version of the file differs from the previously generated one then the hook scripts found in /etc/resolvconf/update-libc.d/ are executed.

The dynamically generated resolver configuration file always starts with the contents of /etc/resolvconf/resolv.conf.d/head and ends with the contents of /etc/resolvconf/resolv.conf.d/tail. Between head and tail the libc script inserts dynamic nameserver information compiled from, first, information provided for configured interfaces; second, static information from /etc/resolvconf/resolv.conf.d/base. Specifically, it writes:

1)
up to three nameserver lines, ordered according to /etc/resolvconf/interface-order, possibly fewer if one of the addresses is a loopback address and the TRUNCATE_NAMESERVER_LIST_AFTER_LOOPBACK_ADDRESS environment variable is affirmatively set, as discussed in the ENVIRONMENT VARIABLES section;
2)
up to one search line containing the combined domain search list from all "domain" and "search" input lines, also ordered according to interface-order(5);
3)
all other non-comment input lines.

To make the resolver use this dynamically generated resolver configuration file the administrator should ensure that /etc/resolv.conf is a symbolic link to /run/resolvconf/resolv.conf. This link is normally created on installation of the resolvconf package. The link is never modified by the resolvconf program itself. If you find that /etc/resolv.conf is not being updated, please check to make sure that the link is intact.

The GNU C Library resolver library isn't the only resolver library available. However, any resolver library that reads /etc/resolv.conf (and most of them do, in order to be compatible) should work fine with resolvconf.

Subscriber packages that need to know only when the resolver configuration file has changed should install a script in /etc/resolvconf/update-libc.d/ rather than in /etc/resolvconf/update.d/. (For example, two packages that install update-libc.d/ hook scripts are fetchmail and squid.) This is important for synchronization purposes: scripts in update-libc.d/ are run after resolv.conf has been updated; the same is not necessarily true of scripts in update.d/.

-a IFACE.PROG
Add or overwrite the record IFACE.PROG then run the update scripts if updating is enabled. When this option is used the information must be provided to resolvconf on its standard input in the format of the resolv.conf(5) file. Each line in the file must be terminated by a newline.
-d IFACE.PROG
Delete the record IFACE.PROG then run the update scripts if updating is enabled.

The string IFACE.PROG may not contain spaces, slashes, an initial dot, an initial hyphen or an initial tilde. It is conventionally formed from IFACE, the name of the interface involved, a dot, and IPROG, the name of the interface configuration program, e.g., "eth0.dhclient".

-u
Just run the update scripts (if updating is enabled).
With -a, -d or -u:
if updating is not enabled, schedule a delayed update. The delayed update will be carried out when updates are enabled.
--enable-updates
Set the flag indicating that resolvconf should run update scripts when invoked in the future with -a, -d or -u. If a delayed update was scheduled then run update scripts.
--disable-updates
Clear the flag.
--updates-are-enabled
Return 0 if the flag is set, otherwise return 1.

The following variables can be set in the configuration file /etc/default/resolvconf. If the file does not exist you will have to create it.

REPORT_ABSENT_SYMLINK
If set to "yes" then resolvconf will print a message when /etc/resolv.conf is not a symbolic link to the resolvconf-generated resolver configuration file. Set to "no" to prevent the printing of this message. The default is "yes".
TRUNCATE_NAMESERVER_LIST_AFTER_LOOPBACK_ADDRESS
If set to "yes" then the libc script will include no more nameserver addresses after the first nameserver address that is a loopback address. (In IPv4 a loopback address is any one that starts with "127.". In IPv6 the loopback address is "::1".)
The advantage of truncating the nameserver list after a loopback address is that doing so inhibits unnecessary changes to resolv.conf and thus reduces the number of instances in which the update-libc.d/ scripts have to be run. When an interface is brought up or down the local caching nameserver that listens on the loopback address is still informed of the change and adapts accordingly; the clients of the resolver which use the local caching nameserver do not need to be notified of the change. A disadvantage of this mode of operation is that applications have no secondary or tertiary nameserver address to fall back on should the local caching nameserver crash. Insofar as a local nameserver crash can be regarded as an unlikely event, this is a relatively minor disadvantage. Set to "no" to disable truncation. The default is "yes".
A deprecated synonym for this variable is TRUNCATE_NAMESERVER_LIST_AFTER_127.

/etc/default/resolvconf
See the ENVIRONMENT VARIABLES section.
/etc/resolvconf/interface-order
Determines the order of precedence of nameserver addresses and search domain names. See above and interface-order(5).
/etc/resolvconf/resolv.conf.d/base
File containing basic resolver information. The lines in this file are included in the resolver configuration file even when no interfaces are configured.
/etc/resolvconf/resolv.conf.d/head
File to be prepended to the dynamically generated resolver configuration file. Normally this is just a comment line.
/etc/resolvconf/resolv.conf.d/tail
File to be appended to the dynamically generated resolver configuration file. To append nothing, make this an empty file. This file is a good place to put a resolver options line if one is needed, e.g.,

options inet6

/etc/resolvconf/resolv.conf.d/original
Copy of the /etc/resolv.conf file before the resolvconf package was installed. This file has no effect on the functioning of resolvconf; it is retained so that /etc/resolv.conf can be restored to its original state if the resolvconf package is removed.

Note also that a copy of this file is included in the database until the first reboot after installation of the resolvconf package; this ensures that nameservers reachable before installation of resolvconf are still reachable after installation of resolvconf even though at that point not all suppliers of nameserver information may have supplied their information to resolvconf(8).

Note also that the administrator can choose to create a symbolic link in /etc/resolvconf/resolv.conf.d/ from tail to original so that the contents of original are always added to the end of the dynamically generated file.

Currently resolvconf does not check the sanity of the information provided to it.

Written by Thomas Hood <jdthood@gmail.com> with contributions by Nathan Stratton Treadway.

Copyright © 2004-2013 Thomas Hood. This is free software; see the source for copying conditions.

interface-order(5), resolv.conf(5), resolver(3).

Read the resolvconf package README file for more in-depth information.

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