Ubuntu Manpage: resolvconf - manage nameserver information

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

DESCRIPTION

       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.

SUPPLIERS OF NAMESERVER INFORMATION

       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.

CONSUMERS OF NAMESERVER INFORMATION

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/.

OPTIONS

       -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.

ENVIRONMENT VARIABLES

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.

FILES

       /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.

BUGS

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

AUTHOR

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

COPYRIGHT

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