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 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).

       •   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.

       •   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:

       •   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).

       •   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 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:

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

       •   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.

       •   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.

       •   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:

       •   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).

       •   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.

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

       •   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:

       •   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

       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