systemd-resolved is a system service that provides network name
resolution to local applications. It implements a caching and validating
DNS/DNSSEC stub resolver, as well as an LLMNR resolver and responder. Local
applications may submit network name resolution requests via three interfaces:
•The native, fully-featured API
systemd-resolved exposes on the bus. See the API
Documentation for details. Usage of this API is generally recommended
to clients as it is asynchronous and fully featured (for example, properly
returns DNSSEC validation status and interface scope for addresses as
necessary for supporting link-local networking).
•The glibc getaddrinfo(3)
API as defined by
 and its related resolver functions, including
. This API is widely supported, including beyond the
Linux platform. In its current form it does not expose DNSSEC validation
status information however, and is synchronous only. This API is backed by the
glibc Name Service Switch (nss(5)
). Usage of the glibc NSS module
is required in order to allow glibc's NSS resolver
functions to resolve host names via systemd-resolved
•Additionally, systemd-resolved provides a
local DNS stub listener on IP address 127.0.0.53 on the local loopback
interface. Programs issuing DNS requests directly, bypassing any local API may
be directed to this stub, in order to connect them to systemd-resolved.
Note however that it is strongly recommended that local programs use the glibc
NSS or bus APIs instead (as described above), as various network resolution
concepts (such as link-local addressing, or LLMNR Unicode domains) cannot be
mapped to the unicast DNS protocol.
The DNS servers contacted are determined from the global settings
in /etc/systemd/resolved.conf, the per-link static settings in
/etc/systemd/network/*.network files, the per-link dynamic settings received
over DHCP and any DNS server information made available by other system
services. See resolved.conf(5) and systemd.network(5) for
details about systemd's own configuration files for DNS servers. To improve
compatibility, /etc/resolv.conf is read in order to discover configured
system DNS servers, but only if it is not a symlink to
/run/systemd/resolve/resolv.conf (see below).
systemd-resolved synthesizes DNS resource records (RRs) for
the following cases:
•The local, configured hostname is resolved to all
locally configured IP addresses ordered by their scope, or — if none
are configured — the IPv4 address 127.0.0.2 (which is on the local
loopback) and the IPv6 address ::1 (which is the local host).
•The hostnames "localhost" and
"localhost.localdomain" (as well as any hostname ending in
".localhost" or ".localhost.localdomain") are resolved to
the IP addresses 127.0.0.1 and ::1.
•The hostname "gateway" is resolved to
all current default routing gateway addresses, ordered by their metric. This
assigns a stable hostname to the current gateway, useful for referencing it
independently of the current network configuration state.
•The mappings defined in /etc/hosts are resolved
to their configured addresses and back.
Lookup requests are routed to the available DNS servers and LLMNR
interfaces according to the following rules:
•Lookups for the special hostname
"localhost" are never routed to the network. (A few other, special
domains are handled the same way.)
•Single-label names are routed to all local
interfaces capable of IP multicasting, using the LLMNR protocol. Lookups for
IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses
are only sent via LLMNR on IPv6. Lookups for the locally configured host name
and the "gateway" host name are never routed to LLMNR.
•Multi-label names are routed to all local
interfaces that have a DNS sever configured, plus the globally configured DNS
server if there is one. Address lookups from the link-local address range are
never routed to DNS.
If lookups are routed to multiple interfaces, the first successful
response is returned (thus effectively merging the lookup zones on all
matching interfaces). If the lookup failed on all interfaces, the last
failing response is returned.
Routing of lookups may be influenced by configuring per-interface
domain names. See systemd.network(5) for details. Lookups for a
hostname ending in one of the per-interface domains are exclusively routed
to the matching interfaces.
See the resolved D-Bus API Documentation for information
about the APIs systemd-resolved provides.
Three modes of handling /etc/resolv.conf (see resolv.conf(5)) are
•A static file /usr/lib/systemd/resolv.conf is
provided that lists the 127.0.0.53 DNS stub (see above) as only DNS server.
This file may be symlinked from /etc/resolv.conf in order to connect all local
clients that bypass local DNS APIs to systemd-resolved. This mode of
operation is recommended.
•systemd-resolved maintains the
/run/systemd/resolve/resolv.conf file for compatibility with traditional Linux
programs. This file may be symlinked from /etc/resolv.conf and is always kept
up-to-date, containing information about all known DNS servers. Note the file
format's limitations: it does not know a concept of per-interface DNS servers
and hence only contains system-wide DNS server definitions. Note that
/run/systemd/resolve/resolv.conf should not be used directly by applications,
but only through a symlink from /etc/resolv.conf. If this mode of operation is
used local clients that bypass any local DNS API will also bypass
systemd-resolved and will talk directly to the known DNS servers.
•Alternatively, /etc/resolv.conf may be managed by
other packages, in which case systemd-resolved will read it for DNS
configuration data. In this mode of operation systemd-resolved is
consumer rather than provider of this configuration file.
Note that the selected mode of operation for this file is detected
fully automatically, depending on whether /etc/resolv.conf is a symlink to
/run/systemd/resolve/resolv.conf or lists 127.0.0.53 as DNS server.