systemd-resolved.service, systemd-resolved — Network Name Resolution manager
systemd-resolved.service
/usr/lib/systemd/systemd-resolved
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 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).
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 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).
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 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 with the DefaultRoute=
option set, as well as the globally configured DNS server.
If there is no link configured as DefaultRoute=
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.
The DefaultRoute=
option is a boolean setting configurable with
resolvectl or in .network
files. If not set, 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,
set the DefaultRoute=
option for the link to true 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, set the
DefaultRoute=
option for it to false.
See
org.freedesktop.resolve1(5)
for information about the D-Bus APIs systemd-resolved
provides.
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 [1].
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.[2]
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.
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.
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.
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.
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.
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.
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.
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.
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)
[1] 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.
[2] 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.