Difference between revisions of "Resolv.conf"

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[[Category:Networking]]
+
{{Lowercase title}}
 +
[[Category:Domain Name System]]
 +
[[de:Resolv.conf]]
 
[[es:Resolv.conf]]
 
[[es:Resolv.conf]]
[[de:Resolv.conf]]
+
[[fr:Resolv.conf]]
 
[[it:Resolv.conf]]
 
[[it:Resolv.conf]]
[[zh-CN:Resolv.conf]]
+
[[ja:Resolv.conf]]
From from the [http://www.kernel.org/doc/man-pages/online/pages/man5/resolv.conf.5.html resolv.conf(5)] man page:
+
[[zh-hans:Resolv.conf]]
:''"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.''
+
{{Related articles start}}
 +
{{Related|Improving performance#Network}}
 +
{{Related articles end}}
 +
 
 +
The configuration file for DNS resolvers is {{ic|/etc/resolv.conf}}. From {{man|5|resolv.conf}}:
 +
: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.
 +
 
 +
:If this file does not exist, only the name server on the local machine will be queried; the domain name is determined from the hostname and the domain search path is constructed from the domain name.
 +
 
 +
== DNS in Linux ==
 +
 
 +
Your ISP (usually) provides working [[wikipedia:Domain_Name_System|DNS]] servers, and a router may also add an extra DNS server in case it has its 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.
 +
 
 +
=== Testing ===
 +
 
 +
Use ''drill'' (provided by package {{Pkg|ldns}}) before any changes, repeat after making the adjustments and compare the query time(s). The following command uses the nameservers set in {{ic|/etc/resolv.conf}}:
 +
$ drill www.archlinux.org
 +
 
 +
You can also specify a specific nameserver's ip address, bypassing the settings in your {{ic|/etc/resolv.conf}}:
 +
 
 +
$ drill @''ip.of.name.server'' www.archlinux.org
 +
 
 +
For example to test Google's name servers:
 +
 
 +
$ drill @8.8.8.8 www.archlinux.org
 +
 
 +
To test a local name server (such as [[unbound]]) do:
 +
 
 +
$ drill @127.0.0.1 www.archlinux.org
 +
 
 +
== Alternative DNS servers ==
 +
 
 +
To use alternative DNS servers, edit {{ic|/etc/resolv.conf}} and add them at the top of the list so they are used first, optionally removing or commenting out other servers. Currently, you may include a maximum of three nameservers.
 +
 
 +
{{Note|Changes made to {{ic|/etc/resolv.conf}} take effect immediately.}}
 +
 
 +
{{Tip|If you require more flexibility, e.g. more than three nameservers, you can use a local DNS resolver like [[dnsmasq]] or [[unbound]]. In this case the nameserver IP address will likely be {{ic|127.0.0.1}}.}}
  
:''"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."''
+
=== OpenNIC ===
  
==Preserve DNS settings==
+
[http://www.opennicproject.org/ OpenNIC] provides free uncensored nameservers located in multiple countries. The full list of public servers is available at [https://servers.opennic.org/ servers.opennic.org] and a shortlist of nearest nameservers for optimal performance is generated on their [https://www.opennic.org/ home page].
{{Pkg|dhcpcd}}, [[NetworkManager]], and various other processes can overwrite {{ic|/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 NetworkManager, see [https://bbs.archlinux.org/viewtopic.php?id=45394 this thread] on how to prevent it from overriding your {{ic|/etc/resolv.conf}}.
 
  
===Modify the dhcpcd Config===
+
To retrieve a list of nearest nameservers, an API is also available and returns, based on the [https://wiki.opennic.org/api/geoip URL parameters] provided, a list of nameservers in the desired format. For example to get the 200 nearest IPv4 servers, one can use https://api.opennicproject.org/geoip/?list&ipv=4&res=200&adm=0&bl&wl.
dhcpcd's configuration file may be edited to prevent the dhcpcd daemon from overwriting {{ic|/etc/resolv.conf}}. To do this, add the following to the last section of {{ic|/etc/dhcpcd.conf}}:
 
  
  nohook resolv.conf
+
Alternatively, the anycast servers below can be used; while reliable their latency [https://wiki.opennic.org/opennic/dont_anycast fluctuates a lot].
 +
 
 +
# OpenNIC IPv4 nameservers (Worldwide Anycast)
 +
nameserver 185.121.177.177
 +
  nameserver 185.121.177.53
 +
 
 +
# OpenNIC IPv6 nameservers (Worldwide Anycast)
 +
nameserver 2a05:dfc7:5::53
 +
nameserver 2a05:dfc7:5::5353
 +
 
 +
{{Note|Use of OpenNIC DNS servers will allow host name resolution in the traditional Top-Level Domain (TLD) registries, but also in OpenNIC or afiliated operated namespaces (.o, .libre, .dyn...) }}
 +
 
 +
=== Cisco Umbrella (formerly OpenDNS) ===
  
===Use resolv.conf.head===
+
[https://www.opendns.com/home-internet-security/ OpenDNS] provided free alternative nameservers, was [https://umbrella.cisco.com/products/features/opendns-cisco-umbrella bought by Cisco in Nov. 2016] and continues to offer OpenDNS as end-user product of its "Umbrella" product suite with focus on Security Enforcement, Security Intelligence and Web Filtering.
Alternatively, you can create a file called {{ic|/etc/resolv.conf.head}} containing your DNS servers. dhcpcd will prepend this file to the beginning of {{ic|/etc/resolv.conf}}. An example {{ic|/etc/resolv.conf.head}} for someone using [[OpenDNS]] would be:
+
The old nameservers [https://www.opendns.com/setupguide/ still work] but are [https://www.opendns.com/home-internet-security/ pre-configured to block adult content]:
  
  # OpenDNS servers
+
  # OpenDNS IPv4 nameservers
 
  nameserver 208.67.222.222
 
  nameserver 208.67.222.222
 
  nameserver 208.67.220.220
 
  nameserver 208.67.220.220
  
If you are not pleased with the OpenDNS servers, you might try [https://developers.google.com/speed/public-dns/ Google's nameservers] as an alternative.
+
# OpenDNS IPv6 nameservers
  # Google nameservers
+
nameserver 2620:0:ccc::2
 +
nameserver 2620:0:ccd::2
 +
 
 +
=== Google ===
 +
 
 +
[https://developers.google.com/speed/public-dns/ Google's nameservers] can be used as an alternative:
 +
 
 +
  # Google IPv4 nameservers
 
  nameserver 8.8.8.8
 
  nameserver 8.8.8.8
 
  nameserver 8.8.4.4
 
  nameserver 8.8.4.4
  
===Write-protect /etc/resolv.conf===
+
# Google IPv6 nameservers
Another way to protect your {{ic|/etc/resolv.conf}} from being modified by anything is setting the write-protection attribute:
+
nameserver 2001:4860:4860::8888
  chattr +i /etc/resolv.conf
+
nameserver 2001:4860:4860::8844
 +
 
 +
=== Comodo ===
 +
[http://securedns.dnsbycomodo.com/ Comodo] provides another IPv4 set, with optional (non-free) web-filtering. Implied in this feature is that the service hijacks the queries.
 +
 
 +
# Comodo nameservers
 +
nameserver 8.26.56.26
 +
nameserver 8.20.247.20
 +
 
 +
=== Yandex ===
 +
[https://dns.yandex.com/advanced/ Yandex.DNS] has servers in Russia, Eastern and Western Europe and has three options, ''Basic'', ''Safe'' and ''Family'':
 +
 
 +
# Basic Yandex.DNS - Quick and reliable DNS
 +
nameserver 77.88.8.8              # Preferred IPv4 DNS
 +
nameserver 77.88.8.1              # Alternate IPv4 DNS
 +
 +
nameserver 2a02:6b8::feed:0ff    # Preferred IPv6 DNS
 +
nameserver 2a02:6b8:0:1::feed:0ff # Alternate IPv6 DNS
 +
 
 +
# Safe Yandex.DNS - Protection from virus and fraudulent content
 +
nameserver 77.88.8.88            # Preferred IPv4 DNS
 +
nameserver 77.88.8.2              # Alternate IPv4 DNS
 +
 +
nameserver 2a02:6b8::feed:bad    # Preferred IPv6 DNS
 +
nameserver 2a02:6b8:0:1::feed:bad # Alternate IPv6 DNS
 +
 
 +
# Family Yandex.DNS - Without adult content
 +
nameserver 77.88.8.7              # Preferred IPv4 DNS
 +
nameserver 77.88.8.3              # Alternate IPv4 DNS
 +
 +
nameserver 2a02:6b8::feed:a11    # Preferred IPv6 DNS
 +
nameserver 2a02:6b8:0:1::feed:a11 # Alternate IPv6 DNS
 +
 
 +
Yandex.DNS' speed is the same in the three modes. In ''Basic'' mode, there is no traffic filtering. In ''Safe'' mode, protection from infected and fraudulent sites is provided. ''Family'' mode enables protection from dangerous sites and blocks sites with adult content.
 +
 
 +
=== DNS.WATCH ===
 +
[https://dns.watch/ DNS.WATCH] focuses on neutrality and security and provides two servers located in Germany with no logging and with DNSSEC enabled. Note they welcome commercial sponsorship.
 +
 
 +
# dns.watch IPv4 nameservers
 +
nameserver 84.200.69.80    # resolver1.dns.watch
 +
nameserver 84.200.70.40    # resolver2.dns.watch
 +
 
 +
=== UncensoredDNS ===
 +
 
 +
[http://censurfridns.dk UncensoredDNS] is a free uncensored DNS service. It is run by a private individual and consists in one anycast served by multiple servers and one unicast node hosted in Denmark.
 +
 
 +
# censurfridns.dk IPv4 nameservers
 +
nameserver 91.239.100.100    ## anycast.censurfridns.dk
 +
nameserver 89.233.43.71      ## unicast.censurfridns.dk
 +
 
 +
# censurfridns.dk IPv6 nameservers
 +
nameserver 2001:67c:28a4::  ## anycast.censurfridns.dk
 +
nameserver 2a01:3a0:53:53::  ## unicast.censurfridns.dk
 +
 
 +
{{Note|Its servers listen to port 5353 as well as the standard port 53. This can be used in case your ISP hijacks port 53.}}
 +
 
 +
== Preserve DNS settings ==
 +
 
 +
[[dhcpcd]], [[netctl]], [[NetworkManager]], and various other processes can overwrite {{ic|/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 [[netctl]] and static IP address assignment, do not use the {{ic|DNS*}} options in your profile, otherwise ''resolvconf'' is called and {{ic|/etc/resolv.conf}} overwritten.
 +
 
 +
=== Prevent NetworkManager modifications ===
 +
 
 +
To stop NetworkManager from modifying {{ic|/etc/resolv.conf}}, edit {{ic|/etc/NetworkManager/NetworkManager.conf}} and add the following in the {{ic|[main]}} section:
 +
 
 +
dns=none
 +
 
 +
{{ic|/etc/resolv.conf}} might be a broken symlink that you will need to remove after doing that. Then, just create a new {{ic|/etc/resolv.conf}} file.
 +
 
 +
=== Use openresolv ===
 +
 
 +
{{Pkg|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 {{ic|/etc/resolvconf.conf}} and running {{ic|resolvconf -u}} will generate {{ic|/etc/resolv.conf}}.
 +
 
 +
=== Modify the dhcpcd config ===
 +
 
 +
''dhcpcd'''s configuration file may be edited to prevent the ''dhcpcd'' daemon from overwriting {{ic|/etc/resolv.conf}}. To do this, add the following to the last section of {{ic|/etc/dhcpcd.conf}}:
 +
 
 +
nohook resolv.conf
 +
 
 +
Alternatively, you can create a file called {{ic|/etc/resolv.conf.head}} containing your DNS servers. ''dhcpcd'' will prepend this file to the beginning of {{ic|/etc/resolv.conf}}.
 +
 
 +
Or you can configure dhcpcd to use the same DNS servers every time. To do this, add the following line at the end of your {{ic|/etc/dhcpcd.conf}}, where {{ic|''dns-server-ip-addressses''}} is a space separated list of DNS IP addresses.
 +
 
 +
static domain_name_servers=''dns-server-ip-addresses''
 +
 
 +
For example, to set it to Google's DNS servers:
 +
 
 +
static domain_name_servers=8.8.8.8 8.8.4.4
 +
 
 +
=== Write-protect /etc/resolv.conf ===
 +
 
 +
Another way to protect your {{ic|/etc/resolv.conf}} from being modified by anything is setting the immutable (write-protection) attribute:
 +
 
 +
  # chattr +i /etc/resolv.conf
 +
 
 +
=== Limit lookup time ===
 +
 
 +
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, put the following in {{ic|/etc/resolv.conf}}.
  
===Use timeout option to reduce hostname lookup time===
 
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 {{ic|/etc/resolv.conf.tail}} and add the following line:
 
 
  options timeout:1
 
  options timeout:1
  
Then restart your network daemon and see if it works better.
+
== Tips and tricks ==
 +
 
 +
=== Local domain names ===
 +
 
 +
If you want to be able to use the hostname of local machine names without the fully qualified domain names, then add a line to {{ic|resolv.conf}} with the local domain such as:
 +
 
 +
domain example.com
 +
 
 +
That way you can refer to local hosts such as {{ic|mainmachine1.example.com}} as simply {{ic|mainmachine1}} when using the ''ssh'' command, but the ''drill'' command still requires the fully qualified domain names in order to perform lookups.

Latest revision as of 16:59, 12 November 2017

The configuration file for DNS resolvers is /etc/resolv.conf. From resolv.conf(5):

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.
If this file does not exist, only the name server on the local machine will be queried; the domain name is determined from the hostname 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 it has its 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.

Testing

Use drill (provided by package ldns) before any changes, repeat after making the adjustments and compare the query time(s). The following command uses the nameservers set in /etc/resolv.conf:

$ drill www.archlinux.org

You can also specify a specific nameserver's ip address, bypassing the settings in your /etc/resolv.conf:

$ drill @ip.of.name.server www.archlinux.org

For example to test Google's name servers:

$ drill @8.8.8.8 www.archlinux.org

To test a local name server (such as unbound) do:

$ drill @127.0.0.1 www.archlinux.org

Alternative DNS servers

To use alternative DNS servers, edit /etc/resolv.conf and add them at the top of the list so they are used first, optionally removing or commenting out other servers. Currently, you may include a maximum of three nameservers.

Note: Changes made to /etc/resolv.conf take effect immediately.
Tip: If you require more flexibility, e.g. more than three nameservers, you can use a local DNS resolver like dnsmasq or unbound. In this case the nameserver IP address will likely be 127.0.0.1.

OpenNIC

OpenNIC provides free uncensored nameservers located in multiple countries. The full list of public servers is available at servers.opennic.org and a shortlist of nearest nameservers for optimal performance is generated on their home page.

To retrieve a list of nearest nameservers, an API is also available and returns, based on the URL parameters provided, a list of nameservers in the desired format. For example to get the 200 nearest IPv4 servers, one can use https://api.opennicproject.org/geoip/?list&ipv=4&res=200&adm=0&bl&wl.

Alternatively, the anycast servers below can be used; while reliable their latency fluctuates a lot.

# OpenNIC IPv4 nameservers (Worldwide Anycast)
nameserver 185.121.177.177
nameserver 185.121.177.53
# OpenNIC IPv6 nameservers (Worldwide Anycast)
nameserver 2a05:dfc7:5::53
nameserver 2a05:dfc7:5::5353
Note: Use of OpenNIC DNS servers will allow host name resolution in the traditional Top-Level Domain (TLD) registries, but also in OpenNIC or afiliated operated namespaces (.o, .libre, .dyn...)

Cisco Umbrella (formerly OpenDNS)

OpenDNS provided free alternative nameservers, was bought by Cisco in Nov. 2016 and continues to offer OpenDNS as end-user product of its "Umbrella" product suite with focus on Security Enforcement, Security Intelligence and Web Filtering. The old nameservers still work but are pre-configured to block adult content:

# OpenDNS IPv4 nameservers
nameserver 208.67.222.222
nameserver 208.67.220.220
# OpenDNS IPv6 nameservers
nameserver 2620:0:ccc::2
nameserver 2620:0:ccd::2

Google

Google's nameservers can be used as an alternative:

# Google IPv4 nameservers
nameserver 8.8.8.8
nameserver 8.8.4.4
# Google IPv6 nameservers
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.

# Comodo nameservers 
nameserver 8.26.56.26 
nameserver 8.20.247.20

Yandex

Yandex.DNS has servers in Russia, Eastern and Western Europe and has three options, Basic, Safe and Family:

# Basic Yandex.DNS - Quick and reliable DNS
nameserver 77.88.8.8              # Preferred IPv4 DNS
nameserver 77.88.8.1              # Alternate IPv4 DNS

nameserver 2a02:6b8::feed:0ff     # Preferred IPv6 DNS
nameserver 2a02:6b8:0:1::feed:0ff # Alternate IPv6 DNS
# Safe Yandex.DNS - Protection from virus and fraudulent content
nameserver 77.88.8.88             # Preferred IPv4 DNS
nameserver 77.88.8.2              # Alternate IPv4 DNS

nameserver 2a02:6b8::feed:bad     # Preferred IPv6 DNS
nameserver 2a02:6b8:0:1::feed:bad # Alternate IPv6 DNS
# Family Yandex.DNS - Without adult content
nameserver 77.88.8.7              # Preferred IPv4 DNS
nameserver 77.88.8.3              # Alternate IPv4 DNS

nameserver 2a02:6b8::feed:a11     # Preferred IPv6 DNS
nameserver 2a02:6b8:0:1::feed:a11 # Alternate IPv6 DNS

Yandex.DNS' speed is the same in the three modes. In Basic mode, there is no traffic filtering. In Safe mode, protection from infected and fraudulent sites is provided. Family mode enables protection from dangerous sites and blocks sites with adult content.

DNS.WATCH

DNS.WATCH focuses on neutrality and security and provides two servers located in Germany with no logging and with DNSSEC enabled. Note they welcome commercial sponsorship.

# dns.watch IPv4 nameservers
nameserver 84.200.69.80    # resolver1.dns.watch 
nameserver 84.200.70.40    # resolver2.dns.watch

UncensoredDNS

UncensoredDNS is a free uncensored DNS service. It is run by a private individual and consists in one anycast served by multiple servers and one unicast node hosted in Denmark.

# censurfridns.dk IPv4 nameservers
nameserver 91.239.100.100    ## anycast.censurfridns.dk
nameserver 89.233.43.71      ## unicast.censurfridns.dk
# censurfridns.dk IPv6 nameservers
nameserver 2001:67c:28a4::   ## anycast.censurfridns.dk
nameserver 2a01:3a0:53:53::  ## unicast.censurfridns.dk
Note: Its servers listen to port 5353 as well as the standard port 53. This can be used in case your ISP hijacks port 53.

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 netctl and static IP address assignment, do not use the DNS* options in your profile, otherwise resolvconf is called and /etc/resolv.conf overwritten.

Prevent NetworkManager modifications

To stop NetworkManager from modifying /etc/resolv.conf, edit /etc/NetworkManager/NetworkManager.conf and add the following in the [main] section:

dns=none

/etc/resolv.conf might be a broken symlink that you will need to remove after doing that. Then, just create a new /etc/resolv.conf file.

Use openresolv

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

Or you can configure dhcpcd to use the same DNS servers every time. To do this, add the following line at the end of your /etc/dhcpcd.conf, where dns-server-ip-addressses is a space separated list of DNS IP addresses.

static domain_name_servers=dns-server-ip-addresses

For example, to set it to Google's DNS servers:

static domain_name_servers=8.8.8.8 8.8.4.4

Write-protect /etc/resolv.conf

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

# chattr +i /etc/resolv.conf

Limit lookup time

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, put the following in /etc/resolv.conf.

options timeout:1

Tips and tricks

Local domain names

If you want to be able to use the hostname of local machine names without the fully qualified domain names, then add a line to resolv.conf with the local domain such as:

domain example.com

That way you can refer to local hosts such as mainmachine1.example.com as simply mainmachine1 when using the ssh command, but the drill command still requires the fully qualified domain names in order to perform lookups.