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NAME | SYNOPSIS | DESCRIPTION | /ETC/RESOLV.CONF | SIGNALS | SEE ALSO | NOTES | COLOPHON |
SYSTEMD-RESOLVED.SERVICE(8)ystemd-resolved.serviceYSTEMD-RESOLVED.SERVICE(8)
systemd-resolved.service, systemd-resolved - Network Name Resolution
manager
systemd-resolved.service
/usr/lib/systemd/systemd-resolved
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.
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.
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.
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)
1. API Documentation
https://www.freedesktop.org/wiki/Software/systemd/resolved
2. RFC3493
https://tools.ietf.org/html/rfc3493
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 2017-07-05. If you dis‐
cover 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 234 SYSTEMD-RESOLVED.SERVICE(8)
Pages that refer to this page: systemd-resolve(1), dnssec-trust-anchors.d(5), resolved.conf(5), systemd.network(5), systemd.directives(7), systemd.index(7), nss-resolve(8)