Manual: systemd-resolved.service
SYSTEMD-RESOLVED.SERVICE(8) systemd-resolved.service SYSTEMD-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:
• 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).
SYNTHETIC RECORDS
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 the preferred source addresses of default gateways if specified, or
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).
PROTOCOLS AND ROUTING
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 is 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 does not 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 configured as the default route, as well as the globally configured DNS
server.
• If there are no DNS servers configured on any link also configured as the default route 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.
Whether a link is the default route or not can be configured with resolvectl default-route command or
DNSDefaultRoute= setting in .network files. If not configured explicitly, 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, configure the link as the default route and do not configure a "~." route-only domain on
it. Finally, in order to avoid a specific link receiving any DNS traffic not matching any of its configured routing
domains, do not make it a default route.
See org.freedesktop.resolve1(5) for information about the D-Bus APIs systemd-resolved provides.
COMPATIBILITY WITH THE TRADITIONAL GLIBC STUB RESOLVER
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 however maintains state, and
will continuously talk to the same server for all queries in a particular lookup scope until some form of error
is seen at which point it will switch to the next server, and then stay 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 takes time. 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.
/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 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.
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.
CREDENTIALS
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.
KERNEL COMMAND LINE
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.
IP PORTS
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.
SEE ALSO
systemd(1), resolved.conf(5), systemd.dns-delegate(5), systemd.dnssd(5), dnssec-trust-anchors.d(5), nss-resolve(8),
resolvectl(1), resolv.conf(5), hosts(5), systemd.network(5), systemd-networkd.service(8), org.freedesktop.resolve1(5)
NOTES
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.
systemd 259.5 SYSTEMD-RESOLVED.SERVICE(8)