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NAME | SYNOPSIS | DESCRIPTION | SYNTHETIC RECORDS | PROTOCOLS AND ROUTING | /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 and
MulticastDNS 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 org.freedesktop.resolve1(5) and
org.freedesktop.LogControl1(5) 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[1] 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 hostnames 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 (in case
systemd-networkd.service(8) is used), the per-link dynamic settings
received over DHCP, information provided via resolvectl(1), 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, /usr/lib/systemd/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). Support for /etc/hosts may be
disabled with ReadEtcHosts=no, see resolved.conf(5).
Lookup requests are routed to the available DNS servers, LLMNR, and
MulticastDNS interfaces according to the following rules:
· Names for which synthetic records are generated (as listed in the
previous section) are never routed to the network and a reply is
sent immediately. In particular this means that lookups for
"localhost" are never routed to the network.
· Single-label names are routed to all local interfaces capable of
IP multicasting, where LLMNR is not disabled, 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 hostname and the
"_gateway" hostname are never routed to LLMNR.
· Multi-label names with the domain suffix ".local" are routed to
all local interfaces capable of IP multicasting, where
MulticastDNS is not disabled, using the MulticastDNS protocol. As
with LLMNR, IPv4 address lookups are sent via IPv4 and IPv6
address lookups are sent via IPv6.
· Resolution of address records (A and AAAA) via unicast DNS (i.e.
not LLMNR or MulticastDNS) for non-synthesized single-label names
is allowed for non-top-level domains. This means that such
records can be resolved when search domains are defined. For any
interface which defines search domains, such look-ups are routed
to that interface, suffixed with each of the search domains
defined on that interface in turn. When global search domains are
defined, such look-ups are routed to all interfaces, suffixed by
each of the global search domains in turn. Additionally, lookup
of single-label names via unicast DNS may be enabled with the
ResolveUnicastSingleLabel=yes setting. The details of which
servers are queried and how the final reply is chosen are
described below. Note that this means that address queries for
single-label names are never sent out to remote DNS servers by
default, and if no search domains are defined, resolution will
fail.
· Other multi-label names are routed to all local interfaces that
have a DNS server configured, plus the globally configured DNS
servers if there are any. 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
these days, it's generally recommended to avoid defining ".local"
in a DNS server, as RFC6762[2] reserves this domain for exclusive
MulticastDNS use.
· Address lookups are routed similarly to multi-label names, with
the exception that addresses from the link-local address range
are never routed to unicast DNS and are only resolved using LLMNR
and MulticastDNS (when enabled).
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 and other settings. See systemd.network(5) and
resolvectl(1) for details. The following query routing logic applies
for unicast DNS traffic:
· If a name to look up matches (that is: is equal to or has as
suffix) any of the configured search or route-only domains of any
link (see systemd.network(5)), or the globally configured DNS
settings (see the discussion of Domains= in resolved.conf(5)),
"best matching" search/route-only domain is determined: the
matching one with the most labels. The query is then sent to all
DNS servers of any links or the globally configured DNS servers
associated with this "best matching" search/route-only domain.
(Note that more than one link might have this same "best
matching" search/route-only domain configured, in which case the
query is sent to all of them in parallel).
In case of single-label names, when search domains are defined,
the same logic applies, except that the name is first suffixed by
the search domain.
· If a query does not match any configured search/route-only domain
(neither per-link nor global), it is sent to all DNS servers that
are configured on links with the "DNS default route" option set,
as well as the globally configured DNS server.
· If there is no link configured as "DNS default route" and no
global DNS server configured, the compiled-in fallback DNS server
is used.
· Otherwise the query is failed as no suitable DNS servers could be
determined.
The "DNS default route" option is a boolean setting configurable with
resolvectl or in .network files. If not set, it is implicitly
determined based on the configured DNS domains for a link: if there's
any route-only domain (not matching "~.") it defaults to false,
otherwise to true.
Effectively this means: in order to support single-label
non-synthetized names, define appropriate search domains. In order to
preferably route all DNS queries not explicitly matched by
search/route-only domain configuration to a specific link, configure
a "~." route-only domain on it. This will ensure that other links
will not be considered for these queries (unless they too carry such
a route-only domain). In order to route all such DNS queries to a
specific link only if no other link is preferable, set the "DNS
default route" option for the link to true and do not configure a
"~." route-only domain on it. Finally, in order to ensure that a
specific link never receives any DNS traffic not matching any of its
configured search/route-only domains, set the "DNS default route"
option for it to false.
See the resolved D-Bus API Documentation[3] for information about the
APIs systemd-resolved provides.
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.
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.
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)
1. RFC3493
https://tools.ietf.org/html/rfc3493
2. RFC6762
https://tools.ietf.org/html/rfc6762
3. resolved D-Bus API Documentation
https://www.freedesktop.org/wiki/Software/systemd/resolved
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 2020-08-13. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2020-08-11.) If you discover 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 246 SYSTEMD-RESOLVED.SERVICE(8)
Pages that refer to this page: systemd.dnssd(5) , 30-systemd-environment-d-generator(7) , systemd.index(7) , libnss_resolve.so.2(8) , nss-resolve(8)