systemd-resolved.service(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | SYNTHETIC RECORDS | PROTOCOLS AND ROUTING | /ETC/RESOLV.CONF | SIGNALS | SEE ALSO | NOTES | COLOPHON

SYSTEMD-RESOLVED.SERVICE(8)ystemd-resolved.serviceYSTEMD-RESOLVED.SERVICE(8)

NAME         top

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

SYNOPSIS         top

       systemd-resolved.service

       /usr/lib/systemd/systemd-resolved

DESCRIPTION         top

       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).

SYNTHETIC RECORDS         top

       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). Support for /etc/hosts may be
           disabled with ReadEtcHosts=no, see resolved.conf(5).

PROTOCOLS AND ROUTING         top

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

       ·   Names for which synthetic records are generated (as listed in the
           previous section) are never routed to the network and a reply is
           sent immediately. In particular this means that lookups for
           "localhost" are never routed to the network.

       ·   Single-label names are routed to all local interfaces capable of
           IP multicasting, where LLMNR is not disabled, 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 hostname and the
           "_gateway" hostname are never routed to LLMNR.

       ·   Multi-label names with the domain suffix ".local" are routed to
           all local interfaces capable of IP multicasting, where
           MulticastDNS is not disabled, using the MulticastDNS protocol. As
           with LLMNR, IPv4 address lookups are sent via IPv4 and IPv6
           address lookups are sent via IPv6.

       ·   Resolution of address records (A and AAAA) via unicast DNS (i.e.
           not LLMNR or MulticastDNS) for non-synthesized single-label names
           is allowed for non-top-level domains. This means that such
           records can be resolved when search domains are defined. 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 if no search domains are defined, resolution will
           fail.

       ·   Other multi-label names are routed to all local interfaces that
           have a DNS server configured, plus the globally configured DNS
           servers if there are any. 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
           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 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 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 (see systemd.network(5)), or the globally configured DNS
           settings (see the discussion of Domains= in resolved.conf(5)),
           "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).

           In case of single-label names, when search domains are defined,
           the same logic applies, except that the name is first suffixed by
           the search domain.

       ·   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 support single-label
       non-synthetized names, define appropriate search domains. 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 these queries (unless they too carry such
       a route-only domain). In order to route all such DNS queries to a
       specific link only if no other link is preferable, 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[3] for information about the
       APIs systemd-resolved provides.

/ETC/RESOLV.CONF         top

       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         top

       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         top

       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         top

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

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

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

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a bug
       report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2020-09-18.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-09-18.)  If you discover any rendering problems in
       this HTML version of the page, or you believe there is a better or
       more up-to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not part
       of the original manual page), send a mail to man-pages@man7.org

systemd 246                                      SYSTEMD-RESOLVED.SERVICE(8)

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