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 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).
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) 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).
PROTOCOLS AND ROUTING¶
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.
/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 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.
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.
NOTES¶
- 1.
- RFC3493
- 2.
- RFC6762
- 3.
- resolved D-Bus API Documentation
