SYSTEMD-RESOLVED.SERVICE(8)systemd-resolved.serviceSYSTEMD-RESOLVED.SERVICE(8)
NAME
systemd-resolved.service, systemd-resolved - Network Name Resolution
manager
SYNOPSIS
systemd-resolved.service
/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 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 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 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 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'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.
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.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.
/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.
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. 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 252 SYSTEMD-RESOLVED.SERVICE(8)
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