Provided by: systemd-resolved_257.4-1ubuntu3.2_amd64 
NAME
systemd-resolved.service, systemd-resolved - Network Name Resolution manager
SYNOPSIS
systemd-resolved.service
/usr/lib/systemd/systemd-resolved
DESCRIPTION
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 and MulticastDNS
resolver and responder. Local applications may submit network name resolution requests via three
interfaces:
• The native, fully-featured API systemd-resolved exposes via D-Bus, see org.freedesktop.resolve1(5)
and org.freedesktop.LogControl1(5) 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 native API systemd-resolved exposes via Varlink on the /run/systemd/resolve/io.systemd.Resolve
AF_UNIX socket. This provides similar functionality as the D-Bus interface, but is available during
the entire runtime, without requiring a running D-Bus system bus broker service.
• The glibc getaddrinfo(3) API as defined by RFC3493[1] and its related resolver functions, including
gethostbyname(3). 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 nss-resolve(8)
is required in order to allow glibc's NSS resolver functions to resolve hostnames via
systemd-resolved.
• Additionally, systemd-resolved provides a local DNS stub listener on the IP addresses 127.0.0.53 and
127.0.0.54 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 stub resolver on 127.0.0.53 provides the full feature set of the local resolver, which
includes offering LLMNR/MulticastDNS resolution. The DNS stub resolver on 127.0.0.54 provides a more
limited resolver, that operates in "proxy" mode only, i.e. it will pass most DNS messages relatively
unmodified to the current upstream DNS servers and back, but not try to process the messages locally,
and hence does not validate DNSSEC, or offer up LLMNR/MulticastDNS. (It will translate to
DNS-over-TLS communication if needed however.)
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 (in case systemd-networkd.service(8) is
used), the per-link dynamic settings received over DHCP, information provided via resolvectl(1), 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/stub-resolv.conf, /usr/lib/systemd/resolv.conf or
/run/systemd/resolve/resolv.conf (see below).
SYNTHETIC RECORDS
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
interface) 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 hostname "_outbound" is resolved to the local IPv4 and IPv6 addresses that are most likely used
for communication with other hosts. This is the preferred source addresses of default gateways if
specified, or determined by requesting a routing decision to the configured default gateways from the
kernel and then using the local IP addresses selected by this decision. This hostname is only
available if there is at least one local default gateway configured. This assigns a stable hostname
to the local outbound IP addresses, useful for referencing them independently of the current network
configuration state.
• The hostname "_localdnsstub" is resolved to the IP address 127.0.0.53, i.e. the address the local DNS
stub (see above) is listening on.
• The hostname "_localdnsproxy" is resolved to the IP address 127.0.0.54, i.e. the address the local
DNS proxy (see above) is listening on.
• The mappings defined in /etc/hosts are resolved to their configured addresses and back, but they will
not affect lookups for non-address types (like MX). Support for /etc/hosts may be disabled with
ReadEtcHosts=no, see resolved.conf(5).
PROTOCOLS AND ROUTING
The lookup requests that systemd-resolved.service receives are routed to the available DNS servers,
LLMNR, and MulticastDNS interfaces according to the following rules:
• Names for which synthetic records are generated (the local hostname, "localhost" and "localdomain",
local gateway, as listed in the previous section) and addresses configured in /etc/hosts are never
routed to the network and a reply is sent immediately.
• Single-label names are resolved using LLMNR on all local interfaces where LLMNR is enabled. Lookups
for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are only sent via
LLMNR on IPv6. Note that lookups for single-label synthesized names are not routed to LLMNR,
MulticastDNS or unicast DNS.
• Queries for the address records (A and AAAA) of single-label non-synthesized names are resolved via
unicast DNS using search domains. For any interface which defines search domains, such look-ups are
routed to the servers defined for that interface, suffixed with each of those search domains. When
global search domains are defined, such look-ups are routed to the global servers. For each search
domain, queries are performed by suffixing the name with each of the search domains in turn.
Additionally, lookup of single-label names via unicast DNS may be enabled with the
ResolveUnicastSingleLabel=yes setting. The details of which servers are queried and how the final
reply is chosen are described below. Note that this means that address queries for single-label names
are never sent out to remote DNS servers by default, and resolution is only possible if search
domains are defined.
• Multi-label names with the domain suffix ".local" are resolved using MulticastDNS on all local
interfaces where MulticastDNS is enabled. As with LLMNR, IPv4 address lookups are sent via IPv4 and
IPv6 address lookups are sent via IPv6.
• Queries for multi-label names are routed via unicast DNS on local interfaces that have a DNS server
configured, plus the globally configured DNS servers if there are any. Which interfaces are used is
determined by the routing logic based on search and route-only domains, described below. Note that by
default, lookups for domains with the ".local" suffix are not routed to DNS servers, unless the
domain is specified explicitly as routing or search domain for the DNS server and interface. This
means that on networks where the ".local" domain is defined in a site-specific DNS server, explicit
search or routing domains need to be configured to make lookups work within this DNS domain. Note
that these days, it's generally recommended to avoid defining ".local" in a DNS server, as RFC6762[2]
reserves this domain for exclusive MulticastDNS use.
• Address lookups (reverse lookups) are routed similarly to multi-label names, with the exception that
addresses from the link-local address range are never routed to unicast DNS and are only resolved
using LLMNR and MulticastDNS (when enabled).
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 is determined by the per-interface routing domains (search and route-only) and global
search domains. See systemd.network(5) and resolvectl(1) for a description how those settings are set
dynamically and the discussion of Domains= in resolved.conf(5) for a description of globally configured
DNS settings.
The following query routing logic applies for unicast DNS lookups initiated by systemd-resolved.service:
• If a name to look up matches (that is: is equal to or has as suffix) any of the configured routing
domains (search or route-only) of any link, or the globally configured DNS settings, "best matching"
routing domain is determined: the matching one with the most labels. The query is then sent to all
DNS servers of any links or the globally configured DNS servers associated with this "best matching"
routing domain. (Note that more than one link might have this same "best matching" routing domain
configured, in which case the query is sent to all of them in parallel).
In case of single-label names, when search domains are defined, the same logic applies, except that
the name is first suffixed by each of the search domains in turn. Note that this search logic doesn't
apply to any names with at least one dot. Also see the discussion about compatibility with the
traditional glibc resolver below.
• If a query does not match any configured routing domain (either per-link or global), it is sent to
all DNS servers that are configured on links configured as the default route, as well as the globally
configured DNS server.
• If there are no DNS servers configured on any link also configured as the default route and no global
DNS server configured, one of the compiled-in fallback DNS servers is used.
• Otherwise the unicast DNS query fails, as no suitable DNS servers can be determined.
Whether a link is the default route or not can be configured with resolvectl default-route command or
DNSDefaultRoute= setting in .network files. If not configured explicitly, it is implicitly determined
based on the configured DNS domains for a link: if there's a route-only domain other than "~.", it
defaults to false, otherwise to true.
Effectively this means: in order to support single-label non-synthesized names, define appropriate search
domains. In order to preferably route all DNS queries not explicitly matched by routing domain
configuration to a specific link, configure a "~." route-only domain on it. This will ensure that other
links will not be considered for these queries (unless they too carry such a routing domain). In order to
route all such DNS queries to a specific link only if no other link is preferred, configure the link as
the default route and do not configure a "~." route-only domain on it. Finally, in order to ensure that
a specific link never receives any DNS traffic not matching any of its configured routing domains, make
it not the default route.
See org.freedesktop.resolve1(5) for information about the D-Bus APIs systemd-resolved provides.
COMPATIBILITY WITH THE TRADITIONAL GLIBC STUB RESOLVER
This section provides a short summary of differences in the resolver implemented by nss-resolve(8)
together with systemd-resolved and the traditional stub resolver implemented in nss-dns.
• Some names are always resolved internally (see Synthetic Records above). Traditionally they would be
resolved by nss-files if provided in /etc/hosts. But note that the details of how a query is
constructed are under the control of the client library. nss-dns will first try to resolve names
using search domains and even if those queries are routed to systemd-resolved, it will send them out
over the network using the usual rules for multi-label name routing [3].
• Single-label names are not resolved for A and AAAA records using unicast DNS (unless overridden with
ResolveUnicastSingleLabel=, see resolved.conf(5)). This is similar to the no-tld-query option being
set in resolv.conf(5).
• Search domains are not used for suffixing of multi-label names. (Search domains are nevertheless used
for lookup routing, for names that were originally specified as single-label or multi-label.) Any
name with at least one dot is always interpreted as a FQDN. nss-dns would resolve names both as
relative (using search domains) and absolute FQDN names. Some names would be resolved as relative
first, and after that query has failed, as absolute, while other names would be resolved in opposite
order. The ndots option in /etc/resolv.conf was used to control how many dots the name needs to have
to be resolved as relative first. This stub resolver does not implement this at all: multi-label
names are only resolved as FQDNs.[4]
• This resolver has a notion of the special ".local" domain used for MulticastDNS, and will not route
queries with that suffix to unicast DNS servers unless explicitly configured, see above. Also,
reverse lookups for link-local addresses are not sent to unicast DNS servers.
• This resolver reads and caches /etc/hosts internally. (In other words, nss-resolve replaces nss-files
in addition to nss-dns). Entries in /etc/hosts have highest priority.
• This resolver also implements LLMNR and MulticastDNS in addition to the classic unicast DNS protocol,
and will resolve single-label names using LLMNR (when enabled) and names ending in ".local" using
MulticastDNS (when enabled).
• Environment variables $LOCALDOMAIN and $RES_OPTIONS described in resolv.conf(5) are not supported
currently.
• The nss-dns resolver maintains little state between subsequent DNS queries, and for each query always
talks to the first listed DNS server from /etc/resolv.conf first, and on failure continues with the
next until reaching the end of the list which is when the query fails. The resolver in
systemd-resolved.service however maintains state, and will continuously talk to the same server for
all queries on a particular lookup scope until some form of error is seen at which point it switches
to the next, and then continuously stays with it for all queries on the scope until the next failure,
and so on, eventually returning to the first configured server. This is done to optimize lookup
times, in particular given that the resolver typically must first probe server feature sets when
talking to a server, which is time consuming. This different behaviour implies that listed DNS
servers per lookup scope must be equivalent in the zones they serve, so that sending a query to one
of them will yield the same results as sending it to another configured DNS server.
/ETC/RESOLV.CONF
Four modes of handling /etc/resolv.conf (see resolv.conf(5)) are supported:
• systemd-resolved maintains the /run/systemd/resolve/stub-resolv.conf file for compatibility with
traditional Linux programs. This file lists the 127.0.0.53 DNS stub (see above) as the only DNS
server. It also contains a list of search domains that are in use by systemd-resolved. The list of
search domains is always kept up-to-date. Note that /run/systemd/resolve/stub-resolv.conf should not
be used directly by applications, but only through a symlink from /etc/resolv.conf. This file may be
symlinked from /etc/resolv.conf in order to connect all local clients that bypass local DNS APIs to
systemd-resolved with correct search domains settings. This mode of operation is recommended.
• 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 file does not contain any search
domains.
• 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.
SIGNALS
SIGUSR1
Upon reception of the SIGUSR1 process signal systemd-resolved will dump the contents of all DNS
resource record caches it maintains, as well as all feature level information it learnt about
configured DNS servers into the system logs.
Added in version 231.
SIGUSR2
Upon reception of the SIGUSR2 process signal systemd-resolved will flush all caches it maintains.
Note that it should normally not be necessary to request this explicitly – except for debugging
purposes – as systemd-resolved flushes the caches automatically anyway any time the host's network
configuration changes. Sending this signal to systemd-resolved is equivalent to the resolvectl
flush-caches command, however the latter is recommended since it operates in a synchronous way.
Added in version 231.
SIGRTMIN+1
Upon reception of the SIGRTMIN+1 process signal systemd-resolved will forget everything it learnt
about the configured DNS servers. Specifically any information about server feature support is
flushed out, and the server feature probing logic is restarted on the next request, starting with the
most fully featured level. Note that it should normally not be necessary to request this explicitly –
except for debugging purposes – as systemd-resolved automatically forgets learnt information any time
the DNS server configuration changes. Sending this signal to systemd-resolved is equivalent to the
resolvectl reset-server-features command, however the latter is recommended since it operates in a
synchronous way.
Added in version 235.
SIGHUP
Upon reception of the SIGHUP process signal systemd-resolved will flush all caches it maintains, drop
all open TCP connections (if any), and reload its configuration files.
Added in version 256.
CREDENTIALS
systemd-resolved supports the service credentials logic as implemented by
ImportCredential=/LoadCredential=/SetCredential= (see systemd.exec(5) for details). The following
credentials are used when passed in:
network.dns, network.search_domains
May contain a space separated list of DNS server IP addresses and DNS search domains. This
information is only used when no explicit configuration via /etc/systemd/resolved.conf,
/etc/resolv.conf or the kernel command line has been provided.
Added in version 253.
KERNEL COMMAND LINE
systemd-resolved also honours two kernel command line options:
nameserver=, domain=
Takes the IP address of a DNS server (in case of nameserver=), and a DNS search domain (in case of
domain=). May be used multiple times, to define multiple DNS servers/search domains. If either of
these options are specified /etc/resolv.conf will not be read and the DNS= and Domains= settings of
resolved.conf(5) will be ignored. These two kernel command line options hence override system
configuration.
Added in version 253.
IP PORTS
The systemd-resolved service listens on the following IP ports:
• Port 53 on IPv4 addresses 127.0.0.53 and 127.0.0.54 (both are on the local loopback interface "lo").
This is the local DNS stub, as discussed above. Both UDP and TCP are covered.
• Port 5353 on all local addresses, both IPv4 and IPv6 (0.0.0.0 and ::0), for MulticastDNS on UDP. Note
that even though the socket is bound to all local interfaces via the selected "wildcard" IP
addresses, the incoming datagrams are filtered by the network interface they are coming in on, and
separate MulticastDNS link-local scopes are maintained for each, taking into consideration whether
MulticastDNS is enabled for the interface or not.
• Port 5355 on all local addresses, both IPv4 and IP6 (0.0.0.0 and ::0), for LLMNR, on both TCP and
UDP. As with MulticastDNS filtering by incoming network interface is applied.
SEE ALSO
systemd(1), resolved.conf(5), dnssec-trust-anchors.d(5), nss-resolve(8), resolvectl(1), resolv.conf(5),
hosts(5), systemd.network(5), systemd-networkd.service(8), org.freedesktop.resolve1(5)
NOTES
1. RFC3493
https://tools.ietf.org/html/rfc3493
2. RFC6762
https://tools.ietf.org/html/rfc6762
3. For example, if /etc/resolv.conf has
nameserver 127.0.0.53
search foobar.com barbar.com
and we look up "localhost", nss-dns will send the following queries to systemd-resolved listening on
127.0.0.53:53: first "localhost.foobar.com", then "localhost.barbar.com", and finally "localhost". If
(hopefully) the first two queries fail, systemd-resolved will synthesize an answer for the third
query.
When using nss-dns with any search domains, it is thus crucial to always configure nss-files with
higher priority and provide mappings for names that should not be resolved using search domains.
4. There are currently more than 1500 top-level domain names defined, and new ones are added regularly,
often using "attractive" names that are also likely to be used locally. Not looking up multi-label
names in this fashion avoids fragility in both directions: a valid global name could be obscured by a
local name, and resolution of a relative local name could suddenly break when a new top-level domain
is created, or when a new subdomain of a top-level domain in registered. Resolving any given name as
either relative or absolute avoids this ambiguity.
systemd 257.4 SYSTEMD-RESOLVED.SERVICE(8)