Open Source Software

systemd-resolved(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 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.

   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

   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

   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

   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

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

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



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