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RESOLVCONF

Section: resolvconf (8)
Updated: 6 Dec 2011
Index Return to Main Contents

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NAME

resolvconf - manage nameserver information  

SYNOPSIS

cat FILE | resolvconf -a IFACE.PROG

resolvconf -d IFACE.PROG

resolvconf -u

resolvconf --enable-updates | --disable-updates | --updates-are-enabled | --create-runtime-directories | --wipe-runtime-directories  

DESCRIPTION

Overwrite (-a) or delete (-d) the nameserver information record IFACE.PROG. Also run the update scripts in /etc/resolvconf/update.d/ if the nameserver information has changed and updating is enabled.

With -u, just run the update scripts if updating is enabled.

With -a, -d or -u schedule a delayed update if updating is not currently enabled.  

PUBLICATION

Normally resolvconf is run only by hook scripts attached to network interface configurers such as pppd(8) (for ppp interfaces), to DHCP clients such as dhclient(8), to ifup(8) and ifdown, and to DNS caches such as dnsmasq(8) (for the loopback interface). These hook scripts furnish resolvconf with information about nameservers. For example, dhclient receives one or more nameserver addresses during its negotiation with the DHCP server; its hook script /etc/dhcp/dhclient-enter-hooks.d/resolvconf pushes this information to resolvconf.

The ifup(8) program can be used to configure network interfaces according to settings in /etc/network/interfaces(5). To make ifup push nameserver information to resolvconf when it configures an interface, add dns- lines to the relevant iface stanza in /etc/network/interfaces. To add nameserver addresses add a line beginning with dns-nameservers.

    dns-nameservers 11.22.33.44 55.66.77.88

Note that one or more addresses can be given, and note the `s' at the end of the option name. (Contrast with the resolv.conf(5) option nameserver.) For each other valid resolv.conf(5) configuration option, you can include, in the stanza, one line beginning with that option name with a dns- prefix. The resulting stanza might look like the following example.

    iface eth0 inet static
        address 192.168.3.3
        netmask 255.255.255.0
        gateway 192.168.3.1
        dns-nameservers 192.168.3.45 192.168.8.10
        dns-search foo.org bar.com

See the resolvconf package's README file for more detailed information. For more information about the dns-* options, e.g., dns-search, see resolv.conf(5).

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

SUBSCRIPTION

Nameserver information provided to resolvconf is stored for use by subscribers to resolvconf's notification service. Subscribers 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.

The most important piece of software that subscribes to the notification service is the set of functions that make up the GNU C Library resolver(3). When nameserver information is updated, the script /etc/resolvconf/update.d/libc writes a new resolver configuration file to /run/resolvconf/resolv.conf and then runs the scripts in /etc/resolvconf/update-libc.d/. To make the resolver use the 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 never modified by resolvconf(8). If you find that /etc/resolv.conf is not being updated, 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 with the GNU C Library resolver) should work with resolvconf(8).

Subscribers 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/. 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/. Examples of packages that do this are fetchmail(1) and squid(8).

Client hook scripts will find the files containing nameserver information in the current directory.  

OPTIONS

-a IFACE.PROG

Add or overwrite the record IFACE.PROG. 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.

The string IFACE.PROG name 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 PROG, the name of the interface configuration program, e.g., `eth0.dhclient'.

Following the addition or deletion of the record, run the update scripts (if updating is enabled).

-u

Just run the update scripts (if updating is 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.

--create-runtime-directories

Create the directories where resolver information records and other run-time files are stored.

--wipe-runtime-directories

Delete everything in the directories where resolver information records and other run-time files are stored.

 

ENVIRONMENT VARIABLES

The following variables can be set in the default-override 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(8)-generated resolv.conf 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 update.d/libc script will include no more nameserver addresses after the first loopback address. In IPv4 a loopback address is an address that starts with "127." This is usually the preferable behavior if the nameserver at 127.* is a local caching nameserver since it inhibits unnecessary changes to resolv.conf. When an interface is brought up the local caching nameserver is informed by resolvconf(8) of any new nameserver addresses and the additional name service is made available to applications that make use of the resolver and the local caching nameserver; the applications themselves 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 this truncation feature. The default is "yes". A deprecated synonym for this variable is TRUNCATE_NAMESERVER_LIST_AFTER_127.

 

FILES

/etc/default/resolvconf

See the ENVIRONMENT VARIABLES section.

/run/resolvconf

This is the location where nameserver information is stored. It will be created if resolvconf is called with the --create-runtime-directories option. Clients should not make any assumptions about the hierarchy that is constructed under this directory.

/etc/resolvconf/interface-order

Determines the order in which nameserver information records are processed. See 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 if 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 the latter are always added to the end of the dynamically generated resolv.conf.

 

BUGS

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

AUTHOR

Written by Thomas Hood <jdthood@gmail.com>.  

COPYRIGHT

Copyright © 2004, 2011 Thomas Hood
This is free software; see the source for copying conditions.  

SEE ALSO

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

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Index

NAME

SYNOPSIS

DESCRIPTION

PUBLICATION

SUBSCRIPTION

OPTIONS

ENVIRONMENT VARIABLES

FILES

BUGS

AUTHOR

COPYRIGHT

SEE ALSO

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