SYSTEMD-RESOLVED.SERVICE(8)systemd-resolved.serviceSYSTEMD-RESOLVED.SERVICE(8)
NAME
systemd-resolved.service, systemd-resolved - Network Name Resolution
manager
SYNOPSIS
systemd-resolved.service
/usr/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 and
MulticastDNS resolver and responder. Local applications may submit
network name resolution requests via three interfaces:
o 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).
o 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.
o 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, 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:
o 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).
o 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.
o 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.
o 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, LLMNR and
MulticastDNS interfaces according to the following rules:
o Lookups for the special hostname "localhost" are never routed to
the network. (A few other, special domains are handled the same
way.)
o 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.
o Multi-label names with the domain suffix ".local" are routed to all
local interfaces capable of IP multicasting, using the MulticastDNS
protocol. As with LLMNR IPv4 address lookups are sent via IPv4 and
IPv6 address lookups are sent via IPv6.
o Other 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. 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
today it's generally recommended to avoid defining ".local" in a
DNS server, as RFC6762[3] reserves this domain for exclusive
MulticastDNS use.
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:
o 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.
o 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.
o 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.
o 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 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
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
1. API Documentation
https://www.freedesktop.org/wiki/Software/systemd/resolved
2. RFC3493
https://tools.ietf.org/html/rfc3493
3. RFC6762
https://tools.ietf.org/html/rfc6762
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