NAME | SYNOPSIS | DESCRIPTION | /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 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, the per-link dynamic settings
       received over DHCP 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/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) 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).

       Lookup requests are routed to the available DNS servers and LLMNR
       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 are routed to all local interfaces that have a
           DNS sever configured, plus the globally configured DNS server if
           there is one. Address lookups from the link-local address range
           are never routed to DNS.

       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. See systemd.network(5) for details. Lookups for a
       hostname ending in one of the per-interface domains are exclusively
       routed to the matching interfaces.

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

/ETC/RESOLV.CONF         top

       Three modes of handling /etc/resolv.conf (see resolv.conf(5)) are
       supported:

       ·   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 mode of operation is recommended.

       ·   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 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.

SEE ALSO         top

       systemd(1), resolved.conf(5), dnssec-trust-anchors.d(5),
       nss-resolve(8), systemd-resolve(1), resolv.conf(5), hosts(5),
       systemd.network(5), systemd-networkd.service(8)

NOTES         top

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

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

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
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       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a bug
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systemd 233                                      SYSTEMD-RESOLVED.SERVICE(8)