Resolv.conf

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Revision as of 12:52, 9 February 2014 by Indigo (Talk | contribs) (adding mv of note regarding opendns to its section; adding couple of sentences and other set of example servers)

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The configuration file for DNS resolvers is /etc/resolv.conf. From its man page:

"The resolver is a set of routines in the C library that provide access to the Internet Domain Name System (DNS). The resolver configuration file contains information that is read by the resolver routines the first time they are invoked by a process. The file is designed to be human readable and contains a list of keywords with values that provide various types of resolver information.
"On a normally configured system this file should not be necessary. The only name server to be queried will be on the local machine; the domain name is determined from the host name and the domain search path is constructed from the domain name."

DNS in Linux

Your ISP (usually) provides working DNS servers, and a router may also add an extra DNS server in case you have your own cache server. Switching between DNS servers does not represent a problem for Windows users, because if a DNS server is slow or does not work it will immediately switch to a better one. However, Linux usually takes longer to timeout, which could be the reason why you are getting a delay.

Use dig (provided by package dnsutils) before any changes, repeat after making the adjustments in the section below and compare the query time(s):

$ dig www5.yahoo.com

You can also specify a nameserver:

$ dig @ip.of.name.server www5.yahoo.com

Alternative DNS servers

To use alternative DNS servers, edit /etc/resolv.conf and add them to the top of the file so they are used first, optionally removing or commenting out already listed servers.

Note:
  • Changes made to /etc/resolv.conf take effect immediately.
  • Bracket notation must be used for IPv6 addresses in /etc/resolv.conf.

OpenDNS

OpenDNS provides free alternative nameservers:

# OpenDNS nameservers
nameserver 208.67.222.222
nameserver 208.67.220.220

There are also IPv6 servers available:

nameserver [2620:0:ccc::2]
nameserver [2620:0:ccd::2]
Warning: The OpenDNS servers ALWAYS respond with an IP address for any query, even if the domain or DNS record doesn't exist. This can cause problems when debugging network issues.

Fixing problems with Google

OpenDNS hijacks Google-searches by routing all queries through their own servers first. This can be annoying because Google searches may slow down noticeably and it also breaks Google's FeelingLucky feature (e.g., entering digg in your adress bar will open www.digg.com). For the latter, there is a Firefox-addon that brings back the original behaviour. A more elegant solution is to redirect all queries for Google exclusively to your ISP's DNS Server. This can be done with dnsmasq (see Speeding up DNS with dnsmasq for more information).

Google

Google's nameservers can be used as an alternative:

# Google nameservers
nameserver 8.8.8.8
nameserver 8.8.4.4

There are also IPv6 servers available:

nameserver [2001:4860:4860::8888]
nameserver [2001:4860:4860::8844]

Comodo

Comodo provides another IPv4 set, with optional (non-free) web-filtering. Implied in this feature is that the service hijacks the queries like OpenDNS does.

# Comodo nameservers 
nameserver 8.26.56.26 
nameserver 8.20.247.20

Preserve DNS settings

dhcpcd, netctl, NetworkManager, and various other processes can overwrite /etc/resolv.conf. This is usually desirable behavior, but sometimes DNS settings need to be set manually (e.g. when using a static IP address). There are several ways to accomplish this.

  • If you are using dhcpcd, see #Modify the dhcpcd config below.
  • If you are using NetworkManager, see this thread on how to prevent it from overriding your /etc/resolv.conf.
  • If you are using netctl and static IP address assignment, do not use the DNS* options in your profile, otherwise resolvconf is called and /etc/resolv.conf overwritten.

Using openresolv

openresolv provides a utility resolvconf, which is a framework for managing multiple DNS configurations. See man 8 resolvconf and man 5 resolvconf.conf for more information.

The configuration is done in /etc/resolvconf.conf and running resolvconf -u will generate /etc/resolv.conf.

Modify the dhcpcd config

dhcpcd's configuration file may be edited to prevent the dhcpcd daemon from overwriting /etc/resolv.conf. To do this, add the following to the last section of /etc/dhcpcd.conf:

nohook resolv.conf

Alternatively, you can create a file called /etc/resolv.conf.head containing your DNS servers. dhcpcd will prepend this file to the beginning of /etc/resolv.conf.

Write-protect /etc/resolv.conf

Another way to protect your /etc/resolv.conf from being modified by anything is setting the write-protection attribute:

# chattr +i /etc/resolv.conf

Use timeout option to reduce hostname lookup time

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: /etc/resolv.conf.tail is specific to dhcpcd, other programs don't use it (Discuss in Talk:Resolv.conf#)

If you are confronted with a very long hostname lookup (may it be in pacman or while browsing), it often helps to define a small timeout after which an alternative nameserver is used. To do so, create a file called /etc/resolv.conf.tail and add the following line:

options timeout:1

Then restart your network daemon and see if it works better.

See also