SYSTEMD-RESOLVED.SERVICE(8)           SYSTEMD-RESOLVED.SERVICE(8)

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

     SYNOPSIS
          systemd-resolved.service

          /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:

          +o   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).

          +o   The glibc getaddrinfo(3) API as defined by
              m[blue]RFC3493m[][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.

          +o   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

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          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 synthetizes DNS resource records (RRs) for
          the following cases:

          +o   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).

          +o   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.

          +o   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.

          +o   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
          Lookup requests are routed to the available DNS servers,
          LLMNR, and MulticastDNS interfaces according to the
          following rules:

          +o   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.

          +o   Single-label names are resolved using LLMNR on all local

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              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 synthetized names are not
              routed to LLMNR, MulticastDNS or unicast DNS.

          +o   Queries for the address records (A and AAAA) of
              single-label non-synthetized names are resolved via
              unicast DNS using search domains. For any interface
              which defines search domains, such look-ups are routed
              to that interface, suffixed with each of the search
              domains defined on that interface in turn. When global
              search domains are defined, such look-ups are routed to
              all interfaces, suffixed by each of the global 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 resoulution is
              only possible if search domains are defined.

          +o   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.

          +o   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
              m[blue]RFC6762m[][2] reserves this domain for exclusive
              MulticastDNS use.

          +o   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).

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          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
          traffic:

          +o   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.

          +o   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.

          +o   If there is no link configured as DefaultRoute= and no
              global DNS server configured, one of the compiled-in
              fallback DNS servers is used.

          +o   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 "~.",

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          it defaults to false, otherwise to true.

          Effectively this means: in order to support single-label
          non-synthetized 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
          stub resolver implemented by nss-resolve(8) together with
          systemd-resolved and the tranditional stub resolver
          implemented in nss-dns(8).

          +o   Some names are always resolved internally (see Synthetic
              Records above). Traditionally they would be resolved by
              nss-files, and only if provided in /etc/hosts.

          +o   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).

          +o   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. (There are currently more than 1500
              top-level domain names defined, and new ones are added
              regularly, often using "attractive" names that are also

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              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.)

          +o   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.

          +o   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.

          +o   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).

          +o   Environment variables $LOCALDOMAIN and $RES_OPTIONS
              described in resolv.conf(5) are not supported currently.

     /ETC/RESOLV.CONF
          Four modes of handling /etc/resolv.conf (see resolv.conf(5))
          are supported:

          +o   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.

          +o   A static file /usr/lib/systemd/resolv.conf is provided
              that lists the 127.0.0.53 DNS stub (see above) as only

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              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.

          +o   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.

          +o   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.

          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 en except for debugging purposes en 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

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              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 en except for debugging purposes en 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
          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

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