systemd-resolved.service(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | SYNTHETIC RECORDS | PROTOCOLS AND ROUTING | COMPATIBILITY WITH THE TRADITIONAL GLIBC STUB RESOLVER | /ETC/RESOLV.CONF | SIGNALS | SEE ALSO | NOTES | COLOPHON

SYSTEMD-RESOLVED.SERVICE(8)temd-resolved.serviceTEMD-RESOLVED.SERVICE(8)

NAME         top

       systemd-resolved.service, systemd-resolved - Network Name
       Resolution manager

SYNOPSIS         top

       systemd-resolved.service

       /usr/lib/systemd/systemd-resolved

DESCRIPTION         top

       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 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 (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         top

       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 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         top

       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         top

       This section provides a short summary of differences in the stub
       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.

/ETC/RESOLV.CONF         top

       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 may be symlinked
           from /etc/resolv.conf. 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         top

       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.

SEE ALSO         top

       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         top

        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.

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have
       a bug report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
       This page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2021-08-27.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2021-08-27.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

systemd 249                                  SYSTEMD-RESOLVED.SERVICE(8)

Pages that refer to this page: resolvectl(1)systemd-nspawn(1)dnssec-trust-anchors.d(5)org.freedesktop.resolve1(5)resolved.conf(5)systemd.dnssd(5)systemd.network(5)nss-resolve(8)