Domain name resolution
- 1 Name Service Switch
- 2 Glibc resolver
- 3 Lookup utilities
- 4 Resolver performance
- 5 Privacy and security
- 6 Third-party DNS services
- 7 DNS servers
- 8 See also
Name Service Switch
The Name Service Switch (NSS) facility is part of the GNU C Library ( ) and backs the 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 . The database responsible for domain name resolution is the hosts database, for which glibc offers the following services:
- file: reads the
- dns: the glibc resolver which reads
Systemd provides three NSS services for hostname resolution:
- systemd-resolved - a caching DNS stub resolver, described in
/etc/hosts, described in Network configuration#Local hostname resolution
- provides hostname resolution without having to edit
- - provides hostname resolution for the names of local containers
Resolve a domain name using NSS
NSS databases can be queried with. A domain name can be resolved through NSS using:
$ getent hosts domain_name
/etc/hostsdirectly. See Network configuration#Local hostname resolution.
The glibc resolver reads
/etc/resolv.conf for every resolution to determine the nameservers and options to use.
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.
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, it is also possible to write-protect it by setting the immutable file attribute:
# chattr +i /etc/resolv.conf
/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
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. You can fix that by setting the following option in
Local domain names
To be able to use the hostname of local machine names without the fully qualified domain name, add a line to
/etc/resolv.conf with the local domain such as:
That way you can refer to local hosts such as
mainmachine1.example.org as simply
mainmachine1 when using the ssh command, but the drill command still requires the fully qualified domain names in order to perform lookups.
- provides , 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
Unless a DNS server is specified, drill will use the nameservers defined in
dnssec-tools. provides , , and a bunch of
The Glibc resolver does not cache queries. To implement local caching, use systemd-resolved or set up a local caching DNS server and use it as the name server by setting
::1 as the name servers in
/etc/resolv.conf or in
/etc/resolvconf.conf if using openresolv.
Privacy and security
The DNS protocol is unencrypted and does not account for confidentiality, integrity or authentication, so if you use an untrusted network or a malicious ISP, your DNS queries can be eavesdropped and the responses manipulated. Furthermore, DNS servers can conduct DNS hijacking.
You need to trust your DNS server to treat your queries confidentially. DNS servers are provided by ISPs and third-parties. Alternatively you can run your own recursive name server, which however takes more effort. If you use a DHCP client in untrusted networks, be sure to set static name servers to avoid using and being subject to arbitrary DNS servers. To secure your communication with a remote DNS server you can use an encrypted protocol, like DNS over TLS, DNS over HTTPS or DNSCrypt, provided that both the upstream server and your resolver support the protocol. To verify that responses are actually from authoritative name servers, you can validate DNSSEC, provided that both the upstream server(s) and your resolver support it.
Be aware that client software, such as major web browsers, may also (start to) implement some of the encrypted DNS protocols. While the encryption of queries may often be seen as a bonus, it also means the software sidetracks queries around the system resolver configuration. Mozilla has proposed disabling application-level DNS if the system resolver cannot resolve the domain "use-application-dns.net". Currently this check is only implemented in Firefox.
Third-party DNS services
There are various third-party DNS services available, some of which also have dedicated software:
- dingo — A DNS client for Google DNS over HTTPS
- opennic-up — Automates the renewal of the DNS servers with the most responsive OpenNIC servers
DNS servers can be authoritative and recursive. If they are neither, they are called stub resolvers and simply forward all queries to another recursive name server. Stub resolvers are typically used to introduce DNS caching on the local host or network. Note that the same can also be achieved with a fully-fledged name server. This section compares the available DNS servers, for a more detailed comparison, refer to Wikipedia:Comparison of DNS server software.
|systemd-resolved||No||No||Yes||Yes||Yes||Resolver and limited server||No||Insecure resolver2||No|
|BIND||Yes||Yes||Yes||Yes||Yes||Yes||No||stunnel#DNS over TLS||No|
- Only forwards using DNS over HTTPS when Rescached itself is queried using DNS over HTTPS.
- From  : Note as the resolver is not capable of authenticating the server, it is vulnerable for "man-in-the-middle" attacks.
- From Wikipedia: dnsmasq has limited authoritative support, intended for internal network use rather than public Internet use.
It is possible to use specific DNS resolvers when querying specific domain names. This is particularly useful when connecting to a VPN, so that queries to the VPN network are resolved by the VPN's DNS, while queries to the internet will still be resolved by your standard DNS resolver. It can also be used on local networks.
To implement it, you need to use a local resolver because glibc does not support it.
In a dynamic environment (laptops and to some extents desktops), you need to configure your resolver based on the network(s) you are connected to. The best way to do that is to use openresolv because it supports multiple subscribers. Some network managers support it, either through openresolv, or by configuring the resolver directly.