systemd-resolved.service
systemd
systemd-resolved.service
8
systemd-resolved.service
systemd-resolved
Network Name Resolution manager
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 on the bus,
see
org.freedesktop.resolve15
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
getaddrinfo3
API as defined by RFC3493 and its related
resolver functions, including
gethostbyname3.
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
(nss5).
Usage of the glibc NSS module
nss-resolve8 is
required in order to allow glibc's NSS resolver functions to resolve host names 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.service8
is used), the per-link dynamic settings received over DHCP, user request made via
resolvectl1, and any
DNS server information made available by other system services. See
resolved.conf5 and
systemd.network5 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).
Protocols and Routing
Lookup requests are routed to the available DNS servers, LLMNR and MulticastDNS interfaces
according to the following rules:
Lookups for the special hostname localhost are never routed to the
network. (A few other, special domains are handled the same way.)
Single-label names are routed to all local interfaces capable of IP multicasting, 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 host name and the
_gateway host name are never routed to LLMNR.
Multi-label names with the domain suffix .local are routed to all
local interfaces capable of IP multicasting, using the MulticastDNS protocol. As with LLMNR IPv4
address lookups are sent via IPv4 and IPv6 address lookups are sent via IPv6.
Other multi-label names are routed to all local interfaces that have a DNS server
configured, plus the globally configured DNS server if there is one. Address lookups from the
link-local address range are never routed to DNS. 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 today it's
generally recommended to avoid defining .local in a DNS server, as RFC6762 reserves this domain for exclusive
MulticastDNS use.
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.network5 and
resolvectl1 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 (or the globally configured DNS settings), the
"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).
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 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 the queries (unless they too
carry such a route-only domain). In order to route all such DNS queries to a specific link only in case
no other link is preferable, then 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 for information about the APIs systemd-resolved provides.
/etc/resolv.conf
Four modes of handling /etc/resolv.conf (see
resolv.conf5) 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.
See Also
systemd1,
resolved.conf5,
dnssec-trust-anchors.d5,
nss-resolve8,
resolvectl1,
resolv.conf5,
hosts5,
systemd.network5,
systemd-networkd.service8