Provided by: systemd-resolved_255.4-1ubuntu8.4_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 on the 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 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 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 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.
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.
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)
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.