NAME
systemd-resolved.service, systemd-resolved - Network Name Resolution managerSYNOPSIS
systemd-resolved.service /lib/systemd/systemd-resolvedDESCRIPTION
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
SIGUSR1Upon 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
- 2.
- RFC6762
- 3.
- For example, if /etc/resolv.conf has
nameserver 127.0.0.53 search foobar.com barbar.com
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 |