systemd-resolved

SYSTEMD-RESOLVED.SERVICE(8)systemd-resolved.serviceSYSTEMD-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 the API Documentation[1] 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 RFC3493[2] 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 host names 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 settings in
       /etc/systemd/network/*.network files (in case systemd-
       networkd.service(8) is used), the per-link dynamic settings received
       over DHCP, user request made 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:

       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) 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).

PROTOCOLS AND ROUTING
       Lookup requests are routed to the available DNS servers, LLMNR and
       MulticastDNS interfaces according to the following rules:

       o   Lookups for the special hostname "localhost" are never routed to
           the network. (A few other, special domains are handled the same
           way.)

       o   Single-label names are routed to all local interfaces capable of IP
           multicasting, using the LLMNR protocol. Lookups for IPv4 addresses
           are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are
           only sent via LLMNR on IPv6. Lookups for the locally configured
           host name and the "_gateway" host name are never routed to LLMNR.

       o   Multi-label names with the domain suffix ".local" are routed to all
           local interfaces capable of IP multicasting, using the MulticastDNS
           protocol. As with LLMNR IPv4 address lookups are sent via IPv4 and
           IPv6 address lookups are sent via IPv6.

       o   Other multi-label names are routed to all local interfaces that
           have a DNS server configured, plus the globally configured DNS
           server if there is one. Address lookups from the link-local address
           range are never routed to DNS. 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 within this DNS domain work. Note that
           today it's generally recommended to avoid defining ".local" in a
           DNS server, as RFC6762[3] reserves this domain for exclusive
           MulticastDNS use.

       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 may be influenced by configuring per-interface
       domain names and other settings. See systemd.network(5) and
       resolvectl(1) for details. 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 search or route-only domains of any
           link (or the globally configured DNS settings), the "best matching"
           search/route-only 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" search/route-only domain. (Note that more than one link
           might have this same "best matching" search/route-only domain
           configured, in which case the query is sent to all of them in
           parallel).

       o   If a query does not match any configured search/route-only domain
           (neither per-link nor global), it is sent to all DNS servers that
           are configured on links with the "DNS default route" option set, as
           well as the globally configured DNS server.

       o   If there is no link configured as "DNS default route" and no global
           DNS server configured, the compiled-in fallback DNS server is used.

       o   Otherwise the query is failed as no suitable DNS servers could be
           determined.

       The "DNS default route" 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
       any route-only domain (not matching "~.") it defaults to false,
       otherwise to true.

       Effectively this means: in order to preferably route all DNS queries
       not explicitly matched by search/route-only domain configuration to a
       specific link, configure a "~."  route-only domain on it. This will
       ensure that other links will not be considered for the queries (unless
       they too carry such a route-only domain). In order to route all such
       DNS queries to a specific link only in case no other link is
       preferable, then set the "DNS default route" 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 search/route-only domains, set the "DNS
       default route" option for it to false.

       See the resolved D-Bus API Documentation[1] for information about the
       APIs systemd-resolved provides.

/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 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 - 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
       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. API Documentation
           https://www.freedesktop.org/wiki/Software/systemd/resolved

        2. RFC3493
           https://tools.ietf.org/html/rfc3493

        3. RFC6762
           https://tools.ietf.org/html/rfc6762

systemd 245                                        SYSTEMD-RESOLVED.SERVICE(8)