Systemd-resolved

Related articles

• systemd-networkd
• resolv.conf
• Avahi
• Stubby
• openresolv

systemd-resolved is a systemd service that provides network name resolution to local applications via a D-Bus interface, the resolve NSS service (nss-resolve(8)), and a local DNS stub listener on 127.0.0.53. See systemd-resolved(8) for the usage.

Installation

systemd-resolved is a part of the systemd package that is installed by default.

Configuration

systemd-resolved provides resolver services for Domain Name System (DNS) (including DNSSEC and DNS over TLS), Multicast DNS (mDNS) and Link-Local Multicast Name Resolution (LLMNR).

The resolver can be configured by editing /etc/systemd/resolved.conf and/or drop-in .conf files in /etc/systemd/resolved.conf.d/. See resolved.conf(5).

To use systemd-resolved start and enable systemd-resolved.service.

Tip: To understand the context around the choices and switches, one can turn on detailed debug information for systemd-resolved as described in systemd#Diagnosing a service.

DNS

systemd-resolved has four different modes for handling the Domain name resolution (the four modes are described in systemd-resolved(8) § /ETC/RESOLV.CONF). We will focus here on the two most relevant modes.

1. Using the systemd DNS stub file - the systemd DNS stub file /run/systemd/resolve/stub-resolv.conf contains the local stub 127.0.0.53 as the only DNS server and a list of search domains. This is the recommended mode of operation. The service users are advised to redirect the /etc/resolv.conf file to the local stub DNS resolver file /run/systemd/resolve/stub-resolv.conf managed by systemd-resolved. This propagates the systemd managed configuration to all the clients. This can be done by replacing /etc/resolv.conf with a symbolic link to the systemd stub:

   # ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf

2. Preserving resolv.conf - this mode preserves /etc/resolv.conf and systemd-resolved is simply a client of this file. This mode is less disruptive as /etc/resolv.conf can continue to be managed by other packages.

Tip: The mode of operation of systemd-resolved is detected automatically, depending on whether /etc/resolv.conf is a symlink to the local stub DNS resolver file or contains server names.

Setting DNS servers

Tip: In order to check the DNS actually used by systemd-resolved, the command to use is:

$ resolvectl status

Automatically

systemd-resolved will work out of the box with a network manager using /etc/resolv.conf. No particular configuration is required since systemd-resolved will be detected by following the /etc/resolv.conf symlink. This is going to be the case with systemd-networkd or NetworkManager.

However, if the DHCP and VPN clients use the resolvconf program to set name servers and search domains (see openresolv#Users for a list of software that use resolvconf), the additional package systemd-resolvconf is needed to provide the /usr/bin/resolvconf symlink.

Note:

• systemd-resolved has a limited resolvconf interface and may not work with all the clients, see resolvectl(1) § COMPATIBILITY WITH RESOLVCONF(8) for more information.
• The resolvconf interface in systemd 239 does not set nameservers. See FS#59459 and systemd issue 9423.

Manually

In local DNS stub mode, alternative DNS servers are provided in the resolved.conf(5) file:

/etc/systemd/resolved.conf.d/dns_servers.conf

[Resolve]
DNS=91.239.100.100 89.233.43.71

Note: Network managers have their own DNS settings that override systemd-resolved's default.

Fallback

If systemd-resolved does not receive DNS server addresses from the network manager and no DNS servers are configured manually then systemd-resolved falls back to the fallback DNS addresses to ensure that DNS resolution always works.

Note: The fallback DNS are in this order: Cloudflare, Quad9 (without filtering and without DNSSEC) and Google; see the systemd PKGBUILD where the servers are defined.

The addresses can be changed by setting FallbackDNS= in resolved.conf(5). E.g.:

/etc/systemd/resolved.conf.d/fallback_dns.conf

[Resolve]
FallbackDNS=127.0.0.1 ::1

To disable the fallback DNS funtionality set the FallbackDNS option without specifying any addresses:

/etc/systemd/resolved.conf.d/fallback_dns.conf

[Resolve]
FallbackDNS=

DNSSEC

By default DNSSEC validation will only be enabled if the upstream DNS server supports it. If you want to always validate DNSSEC, thus breaking DNS resolution with name servers that do not support it, set DNSSEC=true:

/etc/systemd/resolved.conf.d/dnssec.conf

[Resolve]
DNSSEC=true

Test DNSSEC validation by querying a domain with a invalid signature:

$ resolvectl query sigfail.verteiltesysteme.net

sigfail.verteiltesysteme.net: resolve call failed: DNSSEC validation failed: invalid

Now test a domain with valid signature:

$ resolvectl query sigok.verteiltesysteme.net

sigok.verteiltesysteme.net: 134.91.78.139

-- Information acquired via protocol DNS in 266.3ms.
-- Data is authenticated: yes

DNS over TLS

Warning: As of version 239:

• Only opportunistic mode is supported making systemd-resolved vulnerable to downgrade attacks.
• DNS server certificates are not checked making systemd-resolved vulnerable to man-in-the-middle attacks. See systemd issue 9397.

DNS over TLS is disabled by default. To enable it change the DNSOverTLS= setting in the [Resolve] section in resolved.conf(5).

/etc/systemd/resolved.conf.d/dns_over_tls.conf

[Resolve]
DNSOverTLS=opportunistic

Note: The used DNS server must support DNS over TLS otherwise systemd-resolved will disable DNS over TLS for the connection.

mDNS

systemd-resolved is capable of working as a multicast DNS resolver and responder.

The resolver provides hostname resolution using a "hostname.local" naming scheme.

mDNS will only be activated for the connection if both the systemd-resolved's global setting (MulticastDNS= in resolved.conf(5)) and the network manager's per-connection setting is enabled. By default systemd-resolved enables mDNS responder, but both systemd-networkd and NetworkManager do not enable it for connections:

• For systemd-networkd the setting is MulticastDNS= in the [Network] section. See systemd.network(5).
• For NetworkManager the setting is mdns= in the [connection] section, see nm-settings(5). The values are 0 - disabled, 1 - resolver only, 2 - resolver and responder. [1]

Note: If Avahi has been installed, consider disabling avahi-daemon.service and avahi-daemon.socket to prevent conflicts with systemd-resolved.

Tip: The default for all NetworkManager connections can be set by creating a configuration file in /etc/NetworkManager/conf.d/ and setting connection.mdns= in the [connection] section. For example the following will enable mDNS resolver for all connections:

/etc/NetworkManager/conf.d/mdns.conf

[connection]
connection.mdns=1

See NetworkManager.conf(5).

If you plan to use mDNS and use a firewall, make sure to open UDP port 5353.

LLMNR

Link-Local Multicast Name Resolution is a hostname resolution protocol created by Microsoft.

LLMNR will only be activated for the connection if both the systemd-resolved's global setting (LLMNR= in resolved.conf(5)) and the network manager's per-connection setting is enabled. By default systemd-resolved enables LLMNR responder; systemd-networkd and NetworkManager enable it for connections.

• For systemd-networkd the setting is LLMNR= in the [Network] section. See systemd.network(5).
• For NetworkManager the setting is llmnr= in the [connection] section, see nm-settings(5). The values are 0 - disabled, 1 - resolver only, 2 - resolver and responder.

Tip: The default for all NetworkManager connections can be set by creating a configuration file in /etc/NetworkManager/conf.d/ and setting connection.llmnr= in the [connection] section. For example the following will disable LLMNR for all connections:

/etc/NetworkManager/conf.d/llmnr.conf

[connection]
connection.llmnr=0

See NetworkManager.conf(5).

If you plan to use LLMNR and use a firewall, make sure to open UDP and TCP ports 5355.

Lookup

To query DNS records, mDNS or LLMNR hosts you can use the resolvectl utility.

For example, to query a DNS record:

$ resolvectl query archlinux.org

archlinux.org: 2a01:4f8:172:1d86::1
               138.201.81.199

-- Information acquired via protocol DNS in 48.4ms.
-- Data is authenticated: no

See resolvectl(1) § EXAMPLES for more examples.