Provided by: systemd_249.11-0ubuntu3_amd64

NAME

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

     systemd-resolved.service

     /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 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).

     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).

     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.

     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.

     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.

     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.

     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. 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.