RESOLVCONF(8) resolvconf RESOLVCONF(8)
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 name-
server information and a simple framework for notifying applications of
changes in that information. Resolvconf thus sets itself up as the in-
termediary between programs that supply nameserver information and ap-
plications that use that information.
Information is added to or removed from the database using the resolv-
conf 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 con-
figuration 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/resolv-
conf 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 adminis-
trator must add dns- option lines to the relevant iface stanza in in-
terfaces(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 in-
stalled 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 sub-
scribers to resolvconf's notification service. Subscriber packages
that need to know when nameserver information has changed should in-
stall a script in /etc/resolvconf/update.d/ (or in /etc/resolvconf/up-
date-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 infor-
mation is updated, the script /etc/resolvconf/update.d/libc generates a
new version of the resolver configuration file, /run/resolvconf/re-
solv.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. Specifi-
cally, it writes:
1) up to three nameserver lines, ordered according to /etc/resolv-
conf/interface-order, possibly fewer if one of the addresses is a
loopback address and the TRUNCATE_NAMESERVER_LIST_AFTER_LOOP-
BACK_ADDRESS environment variable is affirmatively set, as dis-
cussed 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 configura-
tion 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/re-
solv.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 configura-
tion file has changed should install a script in /etc/resolvconf/up-
date-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 up-
date-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 up-
dating 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 de-
layed 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 de-
layed 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/de-
fault/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-gener-
ated 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 name-
server 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 re-
solv.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 trunca-
tion. The default is "yes".
A deprecated synonym for this variable is TRUNCATE_NAME-
SERVER_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 con-
figuration file. Normally this is just a comment line.
/etc/resolvconf/resolv.conf.d/tail
File to be appended to the dynamically generated resolver con-
figuration 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 re-
stored to its original state if the resolvconf package is re-
moved.
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 instal-
lation of resolvconf are still reachable after installation of
resolvconf even though at that point not all suppliers of name-
server information may have supplied their information to re-
solvconf(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 pro-
vided 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.
SEE ALSO
interface-order(5), resolv.conf(5), resolver(3).
Read the resolvconf package README file for more in-depth information.
resolvconf 3 Feb 2013 RESOLVCONF(8)
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