Provided by: systemd-resolved_253.5-1ubuntu7_amd64 bug

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:

       •   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 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

       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.

       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.

CREDENTIALS

       systemd-resolved supports the service credentials logic as implemented by
       LoadCredential=/SetCredential= (see systemd.exec(1) 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.

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.

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.