Ubuntu Manpage: systemd-resolved.service, systemd-resolved

name
synopsis
description
/etc/resolv.conf
signals
see also
notes

Provided by: systemd_237-3ubuntu10.57_amd64

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/stub-resolv.conf or /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 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.

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

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 systemd-resolve
           --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
           systemd-resolve --reset-server-features command, however the latter is recommended since it operates
           in a synchronous way.

NOTES

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

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