Difference between revisions of "Domain name resolution"

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{{Lowercase title}}
 
 
[[Category:Domain Name System]]
 
[[Category:Domain Name System]]
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[[Category:Network configuration]]
 
[[de:Resolv.conf]]
 
[[de:Resolv.conf]]
[[es:Resolv.conf]]
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[[es:Domain name resolution]]
 
[[fr:Resolv.conf]]
 
[[fr:Resolv.conf]]
 
[[it:Resolv.conf]]
 
[[it:Resolv.conf]]
 
[[ja:Resolv.conf]]
 
[[ja:Resolv.conf]]
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[[pt:Domain name resolution]]
 
[[zh-hans:Resolv.conf]]
 
[[zh-hans:Resolv.conf]]
 
{{Related articles start}}
 
{{Related articles start}}
{{Related|Improving performance#Network}}
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{{Related|Alternative DNS services}}
 +
{{Related|Network configuration}}
 
{{Related articles end}}
 
{{Related articles end}}
 +
In general, a [[Wikipedia:Domain name|domain name]] represents an IP address and is associated to it in the [[Wikipedia:Domain Name System|Domain Name System]] (DNS).
 +
This article explains how to configure domain name resolution and resolve domain names.
  
The configuration file for DNS resolvers is {{ic|/etc/resolv.conf}}. From {{man|5|resolv.conf}}:
+
== Name Service Switch ==
: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.
+
The [[Wikipedia:Name Service Switch|Name Service Switch]] (NSS) facility is part of the GNU C Library ({{Pkg|glibc}}) and backs the {{man|3|getaddrinfo}} API, used to resolve domain names. NSS allows system databases to be provided by separate services, whose search order can be configured by the administrator in {{man|5|nsswitch.conf}}. The database responsible for domain name resolution is the ''hosts'' database, for which glibc offers the following services:
  
== DNS in Linux ==
+
* ''file'': reads the {{ic|/etc/hosts}} file, see {{man|5|hosts}}
 +
* ''dns'': the [[#Glibc resolver|glibc resolver]] which reads {{ic|/etc/resolv.conf}}, see {{man|5|resolv.conf}}
  
Your ISP (usually) provides working [[wikipedia:Domain_Name_System|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.
+
[[Systemd]] provides three NSS services for hostname resolution:
  
=== Testing ===
+
* {{man|8|nss-resolve}} - a caching DNS stub resolver, described in [[systemd-resolved]]
 +
* {{man|8|nss-myhostname}} - provides hostname resolution without having to edit {{ic|/etc/hosts}}, described in [[Network configuration#Local hostname resolution]]
 +
* {{man|8|nss-mymachines}} - provides hostname resolution for the names of local {{man|8|systemd-machined}} containers
  
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}}:
+
=== Resolve a domain name using NSS ===
$ drill www5.yahoo.com
 
  
You can also specify a specific nameserver's ip address, bypassing the settings in your {{ic|/etc/resolv.conf}}:
+
NSS databases can be queried with {{man|1|getent}}. A domain name can be resolved through NSS using:
  
  $ drill @''ip.of.name.server'' www5.yahoo.com
+
  $ getent hosts ''domain_name''
  
For example to test Google's name servers:
+
{{Note|While most programs resolve domain names using NSS, some may read {{ic|/etc/resolv.conf}} and/or {{ic|/etc/hosts}} directly. See [[Network configuration#Local hostname resolution]].}}
  
$ drill @8.8.8.8 www5.yahoo.com
+
== Glibc resolver ==
  
To test a local name server (such as [[unbound]]) do:
+
The glibc resolver reads {{ic|/etc/resolv.conf}} for every resolution to determine the nameservers and options to use.
  
$ drill @127.0.0.1 www5.yahoo.com
+
{{man|5|resolv.conf}} lists nameservers together with some configuration options.
 +
Nameservers listed first are tried first, up to three nameservers may be listed. Lines starting with a number sign ({{ic|#}}) are ignored.
  
== Alternative DNS servers ==
+
{{Note|The glibc resolver does not cache queries. To improve query lookup time you can set up a caching resolver. See [[#Resolvers]] for more information.}}
  
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.
+
=== Overwriting of /etc/resolv.conf ===
  
{{Note|Changes made to {{ic|/etc/resolv.conf}} take effect immediately.}}
+
[[Network manager]]s tend to overwrite {{ic|/etc/resolv.conf}}, for specifics see the corresponding section:
  
{{Tip|If you require more flexibility, e.g. more than three nameservers, you can use a local DNS like [[dnsmasq]] or [[unbound]]. In this case the nameserver IP address will likely be {{ic|127.0.0.1}}.}}
+
* [[dhcpcd#resolv.conf]]
 +
* [[netctl#resolv.conf]]
 +
* [[NetworkManager#resolv.conf]]
  
=== OpenNIC ===
+
To prevent programs from overwriting {{ic|/etc/resolv.conf}} you can also write-protect it by setting the immutable [[file attribute]]:
  
[http://www.opennicproject.org/ OpenNIC] provides free uncensored nameservers with additional features.
+
# chattr +i /etc/resolv.conf
  
{{Tip|OpenNIC offers many [https://servers.opennic.org/ different nameservers] located in multiple countries. Pick some of the [https://www.opennic.org/ nearest nameservers] for optimal performance. Alternatively, the anycast servers below can be used; while reliable their latency [https://wiki.opennic.org/opennic/dont_anycast fluctuates a lot]. }}
+
{{Tip|If you want multiple processes to write to {{ic|/etc/resolv.conf}}, you can use [[resolvconf]].}}
  
# OpenNIC IPv4 nameservers (Worldwide Anycast)
+
=== Limit lookup time ===
nameserver 185.121.177.177
 
nameserver 185.121.177.53
 
  
# OpenNIC IPv6 nameservers (Worldwide Anycast)
+
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}}.
nameserver 2a05:dfc7:5::53
 
nameserver 2a05:dfc7:5::5353
 
  
=== Cisco Umbrella (formerly OpenDNS) ===
+
options timeout:1
  
[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.
+
=== Hostname lookup delayed with IPv6 ===
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 IPv4 nameservers
+
If you experience a 5 second delay when resolving hostnames it might be due to a DNS-server/Firewall misbehaving and only giving one reply to a parallel A and AAAA request.[https://udrepper.livejournal.com/20948.html] You can fix that by setting the following option in {{ic|/etc/resolv.conf}}:
nameserver 208.67.222.222
 
nameserver 208.67.220.220
 
  
  # OpenDNS IPv6 nameservers
+
  options single-request
nameserver 2620:0:ccc::2
 
nameserver 2620:0:ccd::2
 
  
=== Google ===
+
=== Local domain names ===
  
[https://developers.google.com/speed/public-dns/ Google's nameservers] can be used as an alternative:
+
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|/etc/resolv.conf}} with the local domain such as:
  
  # Google IPv4 nameservers
+
  domain example.com
nameserver 8.8.8.8
 
nameserver 8.8.4.4
 
  
# Google IPv6 nameservers
+
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.
nameserver 2001:4860:4860::8888
 
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] have three options:
 
 
 
# 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 all 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.
 
 
 
=== UncensoredDNS ===
 
 
 
[http://censurfridns.dk UncensoredDNS] is a free uncensored DNS resolver which also answers queries on port 5353 if you are behind a firewall blocking outgoing port 53. It is run by a private individual and it consists in one anycast served by multiple servers and one unicast node hosted in Denmark.
 
  
# censurfridns.dk IPv4 nameservers
+
== Resolvers ==
nameserver 91.239.100.100    ## anycast.censurfridns.dk
 
nameserver 89.233.43.71      ## unicast.censurfridns.dk
 
  
# censurfridns.dk IPv6 nameservers
+
{{Expansion|Fill in the unknowns. Mention {{Pkg|dingo}}, maybe below table because it only works with one provider and therefore doesn't really fit into the table.}}
nameserver 2001:67c:28a4::  ## anycast.censurfridns.dk
 
nameserver 2a01:3a0:53:53::  ## unicast.censurfridns.dk
 
  
== Preserve DNS settings ==
+
The Glibc resolver provides only the most basic necessities, it does not cache queries nor provides any security features. If you require more functionality, use another resolver.
  
[[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.
+
{{Tip|
*If you are using ''dhcpcd'', see [[#Modify the dhcpcd config]] below.
+
* The ''drill'' or ''dig'' [[#Lookup utilities|lookup utilities]] report the query time.
*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.
+
* A router usually sets its own caching resolver as the network's DNS server thus providing DNS cache for the whole network.
 +
* If it takes too long to switch to the next DNS server you can try [[#Limit lookup time|decreasing the timeout]].
 +
}}
  
=== With NetworkManager===
+
In the table below, the columns have the following meaning:
  
To stop NetworkManager from modifying {{ic|/etc/resolv.conf}}, edit {{ic|/etc/NetworkManager/NetworkManager.conf}} and add the following in the {{ic|[main]}} section:
+
* ''Cache'': [[Wikipedia:Name server#Caching name server|caches]] the DNS queries to improve lookup times of subsequent identical requests.
 +
* ''Recursor'': can [[Wikipedia:Name server#Recursive query|recursively query]] the domain name starting from the [[Wikipedia:DNS root zone|DNS root zone]].
 +
* ''resolvconf compatibility'': can acquire name servers and search domains, to use for forwarding requests, from software that sets them using [[resolvconf]].
 +
* ''Validates DNSSEC'': [[Wikipedia:Domain Name System Security Extensions#The lookup procedure|validates]] DNS query responses using [[DNSSEC]].
 +
* ''DNS over TLS'': supports forwarding using the [[Wikipedia:DNS over TLS|DNS over TLS]] protocol.
 +
* ''DNS over HTTPS'': supports forwarding using the [[Wikipedia:DNS over HTTPS|DNS over HTTPS]] protocol.
  
  dns=none
+
{| class="wikitable sortable" style="text-align:center"
 +
! Resolver !! Cache !! Recursor !! ''resolvconf'' compatibility !! Validates DNSSEC !! DNS over TLS !! DNS over HTTPS
 +
|-
 +
! [[#Glibc resolver|glibc]]
 +
| {{No}} || {{No}} || {{G|[[openresolv]]}} || {{No}} || {{No}} || {{No}}
 +
|-
 +
! [[BIND]]
 +
| {{Yes}} || {{Yes}} || {{G|[[openresolv]] subscriber}} || {{Yes}} || ? || ?
 +
|-
 +
! [[dnscrypt-proxy]]<sup>1</sup>
 +
| {{Yes}} || {{No}} || {{No}} || {{No}} || {{No}} || {{Yes}}
 +
|-
 +
! [[dnsmasq]]
 +
| {{Yes}} || {{No}} || {{G|[[openresolv]] subscriber}} || {{Yes}} || {{No|http://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2018q2/012131.html}} || {{No}}
 +
|-
 +
! [[Knot Resolver]]
 +
| {{Yes}} || {{Yes}} || {{No}} || {{Yes}} || {{Yes}} || {{No|https://gitlab.labs.nic.cz/knot/knot-resolver/issues/243}}
 +
|-
 +
! [[pdnsd]]
 +
| {{Yes}} || {{Yes}} || {{G|[[openresolv]] subscriber}} || {{No}} || {{No}} || {{No}}
 +
|-
 +
! {{Pkg|powerdns-recursor}}
 +
| {{Yes}} || {{Yes}} || {{No|https://roy.marples.name/projects/openresolv/config#pdns_recursor}} || {{Yes}} || ? || ?
 +
|-
 +
! [[Rescached]]
 +
| {{Yes}} || {{No}} || {{G|[[openresolv]] subscriber}}<sup>2</sup> || {{No}} || {{No}} || {{Y|Limited}}<sup>3</sup>
 +
|-
 +
! [[Stubby]]
 +
| {{No}} || {{No}} || {{No}} || {{Yes}} || {{Yes}} || {{No}}
 +
|-
 +
!style="white-space: nowrap;"| [[systemd-resolved]]
 +
| {{Yes}} || {{No}} || {{G|[[systemd-resolvconf]]}} || {{Yes}} || {{Y|Insecure}}<sup>4</sup> || {{No|https://github.com/systemd/systemd/issues/8639}}
 +
|-
 +
! [[Unbound]]
 +
| {{Yes}} || {{Yes}} || {{G|[[openresolv]] subscriber}} || {{Yes}} || {{Yes}} || {{No|1=https://nlnetlabs.nl/bugs-script/show_bug.cgi?id=1200}}
 +
|}
  
{{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.
+
# Implements a [[Wikipedia:DNSCrypt|DNSCrypt]] protocol client.
 +
# Can use the subscribers of dnsmasq, pdns and unbound.[https://github.com/shuLhan/rescached-go#integration-with-openresolv]
 +
# Only forwards using DNS over HTTPS when Rescached itself is queried using DNS over HTTPS.[https://github.com/shuLhan/rescached-go#integration-with-dns-over-https]
 +
# From {{man|5|resolved.conf}}: ''Note as the resolver is not capable of authenticating the server, it is vulnerable for "man-in-the-middle" attacks.''[https://github.com/systemd/systemd/issues/9397] Also, the only supported mode is "opportunistic", which ''makes DNS-over-TLS vulnerable to "downgrade" attacks''.[https://github.com/systemd/systemd/issues/10755]
  
=== Using openresolv ===
+
== Privacy ==
  
{{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.
+
DNS is not encrypted, so you may want to use a [[#Resolvers|resolver]] that supports an encrypted protocol, like [[Wikipedia:DNS over TLS|DNS over TLS]], [[Wikipedia:DNS over HTTPS|DNS over HTTPS]] or [[Wikipedia:DNSCrypt|DNSCrypt]].
  
The configuration is done in {{ic|/etc/resolvconf.conf}} and running {{ic|resolvconf -u}} will generate {{ic|/etc/resolv.conf}}.
+
Most DNS servers keep a log of IP addresses and sites visited on a more or less temporary basis. The data collected can be used to perform various statistical studies. Personally-identifying information have value and can also be rented or sold to third parties. [[Alternative DNS services]] provides a list of popular services, check their privacy policy for information about how user data is handled.
  
=== Modify the dhcpcd config ===
+
== Lookup utilities ==
  
''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}}:
+
To query specific DNS servers and DNS/[[DNSSEC]] records you can use dedicated DNS lookup utilities. These tools implement DNS themselves and do not use [[#Name Service Switch|NSS]].
  
nohook resolv.conf
+
* {{Pkg|ldns}} provides {{man|1|drill}}, which is a tool designed to retrieve information out of the DNS.
  
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}}.
+
For example, to query a specific nameserver with ''drill'' for the TXT records of a domain:
  
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.
+
$ drill @''nameserver'' TXT ''domain''
  
static domain_name_servers=''dns-server-ip-addresses''
+
If you do not specify a DNS server ''drill'' uses the nameservers defined in {{ic|/etc/resolv.conf}}.
  
For example, to set it to Google's DNS servers:
+
* {{Pkg|bind-tools}} provides {{man|1|dig}}, {{man|1|host}}, {{man|1|nslookup}} and a bunch of {{ic|dnssec-}} tools.
  
static domain_name_servers=8.8.8.8 8.8.4.4
+
== See also ==
  
=== Write-protect /etc/resolv.conf ===
+
* [https://www.tldp.org/LDP/nag2/x-087-2-resolv.html Linux Network Administrators Guide]
 
+
* [https://www.debian.org/doc/manuals/debian-handbook/sect.hostname-name-service.en.html#sect.name-resolution Debian Handbook]
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
 
 
 
=== 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, put the following in {{ic|/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 {{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 13:49, 17 November 2018

In general, a domain name represents an IP address and is associated to it in the Domain Name System (DNS). This article explains how to configure domain name resolution and resolve domain names.

Name Service Switch

The Name Service Switch (NSS) facility is part of the GNU C Library (glibc) and backs the getaddrinfo(3) API, used to resolve domain names. NSS allows system databases to be provided by separate services, whose search order can be configured by the administrator in nsswitch.conf(5). The database responsible for domain name resolution is the hosts database, for which glibc offers the following services:

Systemd provides three NSS services for hostname resolution:

Resolve a domain name using NSS

NSS databases can be queried with getent(1). A domain name can be resolved through NSS using:

$ getent hosts domain_name
Note: While most programs resolve domain names using NSS, some may read /etc/resolv.conf and/or /etc/hosts directly. See Network configuration#Local hostname resolution.

Glibc resolver

The glibc resolver reads /etc/resolv.conf for every resolution to determine the nameservers and options to use.

resolv.conf(5) lists nameservers together with some configuration options. Nameservers listed first are tried first, up to three nameservers may be listed. Lines starting with a number sign (#) are ignored.

Note: The glibc resolver does not cache queries. To improve query lookup time you can set up a caching resolver. See #Resolvers for more information.

Overwriting of /etc/resolv.conf

Network managers tend to overwrite /etc/resolv.conf, for specifics see the corresponding section:

To prevent programs from overwriting /etc/resolv.conf you can also write-protect it by setting the immutable file attribute:

# chattr +i /etc/resolv.conf
Tip: If you want multiple processes to write to /etc/resolv.conf, you can use resolvconf.

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

Hostname lookup delayed with IPv6

If you experience a 5 second delay when resolving hostnames it might be due to a DNS-server/Firewall misbehaving and only giving one reply to a parallel A and AAAA request.[1] You can fix that by setting the following option in /etc/resolv.conf:

options single-request

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 /etc/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.

Resolvers

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Fill in the unknowns. Mention dingo, maybe below table because it only works with one provider and therefore doesn't really fit into the table. (Discuss in Talk:Domain name resolution#)

The Glibc resolver provides only the most basic necessities, it does not cache queries nor provides any security features. If you require more functionality, use another resolver.

Tip:
  • The drill or dig lookup utilities report the query time.
  • A router usually sets its own caching resolver as the network's DNS server thus providing DNS cache for the whole network.
  • If it takes too long to switch to the next DNS server you can try decreasing the timeout.

In the table below, the columns have the following meaning:

  • Cache: caches the DNS queries to improve lookup times of subsequent identical requests.
  • Recursor: can recursively query the domain name starting from the DNS root zone.
  • resolvconf compatibility: can acquire name servers and search domains, to use for forwarding requests, from software that sets them using resolvconf.
  • Validates DNSSEC: validates DNS query responses using DNSSEC.
  • DNS over TLS: supports forwarding using the DNS over TLS protocol.
  • DNS over HTTPS: supports forwarding using the DNS over HTTPS protocol.
Resolver Cache Recursor resolvconf compatibility Validates DNSSEC DNS over TLS DNS over HTTPS
glibc No No openresolv No No No
BIND Yes Yes openresolv subscriber Yes ? ?
dnscrypt-proxy1 Yes No No No No Yes
dnsmasq Yes No openresolv subscriber Yes No No
Knot Resolver Yes Yes No Yes Yes No
pdnsd Yes Yes openresolv subscriber No No No
powerdns-recursor Yes Yes No Yes ? ?
Rescached Yes No openresolv subscriber2 No No Limited3
Stubby No No No Yes Yes No
systemd-resolved Yes No systemd-resolvconf Yes Insecure4 No
Unbound Yes Yes openresolv subscriber Yes Yes No
  1. Implements a DNSCrypt protocol client.
  2. Can use the subscribers of dnsmasq, pdns and unbound.[2]
  3. Only forwards using DNS over HTTPS when Rescached itself is queried using DNS over HTTPS.[3]
  4. From resolved.conf(5): Note as the resolver is not capable of authenticating the server, it is vulnerable for "man-in-the-middle" attacks.[4] Also, the only supported mode is "opportunistic", which makes DNS-over-TLS vulnerable to "downgrade" attacks.[5]

Privacy

DNS is not encrypted, so you may want to use a resolver that supports an encrypted protocol, like DNS over TLS, DNS over HTTPS or DNSCrypt.

Most DNS servers keep a log of IP addresses and sites visited on a more or less temporary basis. The data collected can be used to perform various statistical studies. Personally-identifying information have value and can also be rented or sold to third parties. Alternative DNS services provides a list of popular services, check their privacy policy for information about how user data is handled.

Lookup utilities

To query specific DNS servers and DNS/DNSSEC records you can use dedicated DNS lookup utilities. These tools implement DNS themselves and do not use NSS.

  • ldns provides drill(1), which is a tool designed to retrieve information out of the DNS.

For example, to query a specific nameserver with drill for the TXT records of a domain:

$ drill @nameserver TXT domain

If you do not specify a DNS server drill uses the nameservers defined in /etc/resolv.conf.

See also