Manual: systemd-resolved.service
SYSTEMD-RESOLVED.SERVICE(8) systemd-resolved.service SYSTEMD-RESOLVED.SERVICE(8)
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 is 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 does not 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 avoid a specific link receiving any DNS traffic not matching any of its
configured routing domains, do not make it a 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 however maintains state, and will continuously talk to the same server for all
queries in a particular lookup scope until some form of error is seen at which point it will
switch to the next server, and then stay 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 takes time. 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), systemd.dns-delegate(5), systemd.dnssd(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 259.5 SYSTEMD-RESOLVED.SERVICE(8)