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SYSTEMD-....SERVICE(8) systemd-resolved.service SYSTEMD-....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 via
D-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 native API systemd-resolved exposes via Varlink on the
/run/systemd/resolve/io.systemd.Resolve AF_UNIX socket. This
provides similar functionality as the D-Bus interface, but is
available during the entire runtime, without requiring a
running D-Bus system bus broker service.
• 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 the IP addresses 127.0.0.53 and 127.0.0.54 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 stub resolver on 127.0.0.53 provides the full feature
set of the local resolver, which includes offering
LLMNR/MulticastDNS resolution. The DNS stub resolver on
127.0.0.54 provides a more limited resolver, that operates in
"proxy" mode only, i.e. it will pass most DNS messages
relatively unmodified to the current upstream DNS servers and
back, but not try to process the messages locally, and hence
does not validate DNSSEC, or offer up LLMNR/MulticastDNS. (It
will translate to DNS-over-TLS communication if needed
however.)
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 interface) 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 hostname "_outbound" is resolved to the local IPv4 and
IPv6 addresses that are most likely used for communication
with other hosts. This is determined by requesting a routing
decision to the configured default gateways from the kernel
and then using the local IP addresses selected by this
decision. This hostname is only available if there is at
least one local default gateway configured. This assigns a
stable hostname to the local outbound IP addresses, useful
for referencing them independently of the current network
configuration state.
• The hostname "_localdnsstub" is resolved to the IP address
127.0.0.53, i.e. the address the local DNS stub (see above)
is listening on.
• The hostname "_localdnsproxy" is resolved to the IP address
127.0.0.54, i.e. the address the local DNS proxy (see above)
is listening on.
• 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).
The lookup requests that systemd-resolved.service receives are
routed to the available DNS servers, LLMNR, and MulticastDNS
interfaces according to the following rules:
• Names for which synthetic records are generated (the local
hostname, "localhost" and "localdomain", local gateway, as
listed in the previous section) and addresses configured in
/etc/hosts are never routed to the network and a reply is
sent immediately.
• Single-label names are resolved using LLMNR on all local
interfaces where LLMNR is enabled. Lookups for IPv4 addresses
are only sent via LLMNR on IPv4, and lookups for IPv6
addresses are only sent via LLMNR on IPv6. Note that lookups
for single-label synthesized names are not routed to LLMNR,
MulticastDNS or unicast DNS.
• Queries for the address records (A and AAAA) of single-label
non-synthesized names are resolved via unicast DNS using
search domains. For any interface which defines search
domains, such look-ups are routed to the servers defined for
that interface, suffixed with each of those search domains.
When global search domains are defined, such look-ups are
routed to the global servers. For each search domain, queries
are performed by suffixing the name with each of the 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 resolution is only possible if search
domains are defined.
• Multi-label names with the domain suffix ".local" are
resolved using MulticastDNS on all local interfaces where
MulticastDNS is enabled. As with LLMNR, IPv4 address lookups
are sent via IPv4 and IPv6 address lookups are sent via IPv6.
• Queries for multi-label names are routed via unicast DNS on
local interfaces that have a DNS server configured, plus the
globally configured DNS servers if there are any. Which
interfaces are used is determined by the routing logic based
on search and route-only domains, described below. 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 work within this DNS domain. 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 (reverse 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 is determined by the per-interface routing
domains (search and route-only) and global search domains. See
systemd.network(5) and resolvectl(1) for a description how those
settings are set dynamically and the discussion of Domains= in
resolved.conf(5) for a description of globally configured DNS
settings.
The following query routing logic applies for unicast DNS lookups
initiated by systemd-resolved.service:
• If a name to look up matches (that is: is equal to or has as
suffix) any of the configured routing domains (search or
route-only) of any link, or the globally configured DNS
settings, "best matching" routing 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" routing domain.
(Note that more than one link might have this same "best
matching" routing 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 each of the search domains in turn. Note
that this search logic doesn't apply to any names with at
least one dot. Also see the discussion about compatibility
with the traditional glibc resolver below.
• If a query does not match any configured routing domain
(either per-link or global), it is sent to all DNS servers
that are configured on links with the DefaultRoute= option
set, as well as the globally configured DNS server.
• If there is no link configured as DefaultRoute= and no global
DNS server configured, one of the compiled-in fallback DNS
servers is used.
• Otherwise the unicast DNS query fails, as no suitable DNS
servers can be determined.
The DefaultRoute= 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 a route-only domain other than "~.", it defaults to
false, otherwise to true.
Effectively this means: in order to support single-label
non-synthesized names, define appropriate search domains. In
order to preferably route all DNS queries not explicitly matched
by routing 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 routing domain). In order to route all such DNS queries to
a specific link only if no other link is preferred, set the
DefaultRoute= 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 routing domains, set the DefaultRoute= option for
it to false.
See org.freedesktop.resolve1(5) for information about the D-Bus
APIs systemd-resolved provides.
This section provides a short summary of differences in the
resolver implemented by nss-resolve(8) together with
systemd-resolved and the traditional stub resolver implemented in
nss-dns.
• Some names are always resolved internally (see Synthetic
Records above). Traditionally they would be resolved by
nss-files if provided in /etc/hosts. But note that the
details of how a query is constructed are under the control
of the client library. nss-dns will first try to resolve
names using search domains and even if those queries are
routed to systemd-resolved, it will send them out over the
network using the usual rules for multi-label name routing
[3].
• Single-label names are not resolved for A and AAAA records
using unicast DNS (unless overridden with
ResolveUnicastSingleLabel=, see resolved.conf(5)). This is
similar to the no-tld-query option being set in
resolv.conf(5).
• Search domains are not used for suffixing of multi-label
names. (Search domains are nevertheless used for lookup
routing, for names that were originally specified as
single-label or multi-label.) Any name with at least one dot
is always interpreted as a FQDN. nss-dns would resolve names
both as relative (using search domains) and absolute FQDN
names. Some names would be resolved as relative first, and
after that query has failed, as absolute, while other names
would be resolved in opposite order. The ndots option in
/etc/resolv.conf was used to control how many dots the name
needs to have to be resolved as relative first. This stub
resolver does not implement this at all: multi-label names
are only resolved as FQDNs.[4]
• This resolver has a notion of the special ".local" domain
used for MulticastDNS, and will not route queries with that
suffix to unicast DNS servers unless explicitly configured,
see above. Also, reverse lookups for link-local addresses are
not sent to unicast DNS servers.
• This resolver reads and caches /etc/hosts internally. (In
other words, nss-resolve replaces nss-files in addition to
nss-dns). Entries in /etc/hosts have highest priority.
• This resolver also implements LLMNR and MulticastDNS in
addition to the classic unicast DNS protocol, and will
resolve single-label names using LLMNR (when enabled) and
names ending in ".local" using MulticastDNS (when enabled).
• Environment variables $LOCALDOMAIN and $RES_OPTIONS described
in resolv.conf(5) are not supported currently.
• The nss-dns resolver maintains little state between
subsequent DNS queries, and for each query always talks to
the first listed DNS server from /etc/resolv.conf first, and
on failure continues with the next until reaching the end of
the list which is when the query fails. The resolver in
systemd-resolved.service however maintains state, and will
continuously talk to the same server for all queries on a
particular lookup scope until some form of error is seen at
which point it switches to the next, and then continuously
stays with it for all queries on the scope until the next
failure, and so on, eventually returning to the first
configured server. This is done to optimize lookup times, in
particular given that the resolver typically must first probe
server feature sets when talking to a server, which is time
consuming. This different behaviour implies that listed DNS
servers per lookup scope must be equivalent in the zones they
serve, so that sending a query to one of them will yield the
same results as sending it to another configured DNS server.
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 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.
Added in version 231.
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.
Added in version 231.
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.
Added in version 235.
SIGHUP
Upon reception of the SIGHUP process signal systemd-resolved
will flush all caches it maintains, drop all open TCP
connections (if any), and reload its configuration files.
Added in version 256.
systemd-resolved supports the service credentials logic as
implemented by ImportCredential=/LoadCredential=/SetCredential=
(see systemd.exec(5) for details). The following credentials are
used when passed in:
network.dns, network.search_domains
May contain a space separated list of DNS server IP addresses
and DNS search domains. This information is only used when no
explicit configuration via /etc/systemd/resolved.conf,
/etc/resolv.conf or the kernel command line has been
provided.
Added in version 253.
systemd-resolved also honours two kernel command line options:
nameserver=, domain=
Takes the IP address of a DNS server (in case of
nameserver=), and a DNS search domain (in case of domain=).
May be used multiple times, to define multiple DNS
servers/search domains. If either of these options are
specified /etc/resolv.conf will not be read and the DNS= and
Domains= settings of resolved.conf(5) will be ignored. These
two kernel command line options hence override system
configuration.
Added in version 253.
The systemd-resolved service listens on the following IP ports:
• Port 53 on IPv4 addresses 127.0.0.53 and 127.0.0.54 (both are
on the local loopback interface "lo"). This is the local DNS
stub, as discussed above. Both UDP and TCP are covered.
• Port 5353 on all local addresses, both IPv4 and IPv6 (0.0.0.0
and ::0), for MulticastDNS on UDP. Note that even though the
socket is bound to all local interfaces via the selected
"wildcard" IP addresses, the incoming datagrams are filtered
by the network interface they are coming in on, and separate
MulticastDNS link-local scopes are maintained for each,
taking into consideration whether MulticastDNS is enabled for
the interface or not.
• Port 5355 on all local addresses, both IPv4 and IP6 (0.0.0.0
and ::0), for LLMNR, on both TCP and UDP. As with
MulticastDNS filtering by incoming network interface is
applied.
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. For example, if /etc/resolv.conf has
nameserver 127.0.0.53
search foobar.com barbar.com
and we look up "localhost", nss-dns will send the following
queries to systemd-resolved listening on 127.0.0.53:53: first
"localhost.foobar.com", then "localhost.barbar.com", and
finally "localhost". If (hopefully) the first two queries
fail, systemd-resolved will synthesize an answer for the
third query.
When using nss-dns with any search domains, it is thus
crucial to always configure nss-files with higher priority
and provide mappings for names that should not be resolved
using search domains.
4. There are currently more than 1500 top-level domain names
defined, and new ones are added regularly, often using
"attractive" names that are also likely to be used locally.
Not looking up multi-label names in this fashion avoids
fragility in both directions: a valid global name could be
obscured by a local name, and resolution of a relative local
name could suddenly break when a new top-level domain is
created, or when a new subdomain of a top-level domain in
registered. Resolving any given name as either relative or
absolute avoids this ambiguity.
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 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) 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
[email protected]
systemd 257~devel SYSTEMD-....SERVICE(8)
Pages that refer to this page: resolvectl(1), systemd-nspawn(1), varlinkctl(1), dnssec-trust-anchors.d(5), org.freedesktop.resolve1(5), resolved.conf(5), systemd.dnssd(5), systemd.network(5), kernel-command-line(7), systemd.directives(7), systemd.index(7), systemd.system-credentials(7), nss-resolve(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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