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NAME | SYNOPSIS | DESCRIPTION | CONFIGURATION DIRECTORIES AND PRECEDENCE | OPTIONS | SEE ALSO | NOTES | COLOPHON |
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RESOLVED.CONF(5) resolved.conf RESOLVED.CONF(5)
resolved.conf, resolved.conf.d - Network Name Resolution
configuration files
/etc/systemd/resolved.conf
/run/systemd/resolved.conf
/usr/local/lib/systemd/resolved.conf
/usr/lib/systemd/resolved.conf
/etc/systemd/resolved.conf.d/*.conf
/run/systemd/resolved.conf.d/*.conf
/usr/local/lib/systemd/resolved.conf.d/*.conf
/usr/lib/systemd/resolved.conf.d/*.conf
These configuration files control local DNS and LLMNR name
resolution.
The default configuration is set during compilation, so
configuration is only needed when it is necessary to deviate from
those defaults. The main configuration file is loaded from one of
the listed directories in order of priority, only the first file
found is used: /etc/systemd/, /run/systemd/,
/usr/local/lib/systemd/ [1], /usr/lib/systemd/. The vendor
version of the file contains commented out entries showing the
defaults as a guide to the administrator. Local overrides can
also be created by creating drop-ins, as described below. The
main configuration file can also be edited for this purpose (or a
copy in /etc/ if it's shipped under /usr/), however using
drop-ins for local configuration is recommended over
modifications to the main configuration file.
In addition to the main configuration file, drop-in configuration
snippets are read from /usr/lib/systemd/*.conf.d/,
/usr/local/lib/systemd/*.conf.d/, and /etc/systemd/*.conf.d/.
Those drop-ins have higher precedence and override the main
configuration file. Files in the *.conf.d/ configuration
subdirectories are sorted by their filename in lexicographic
order, regardless of in which of the subdirectories they reside.
When multiple files specify the same option, for options which
accept just a single value, the entry in the file sorted last
takes precedence, and for options which accept a list of values,
entries are collected as they occur in the sorted files.
When packages need to customize the configuration, they can
install drop-ins under /usr/. Files in /etc/ are reserved for the
local administrator, who may use this logic to override the
configuration files installed by vendor packages. Drop-ins have
to be used to override package drop-ins, since the main
configuration file has lower precedence. It is recommended to
prefix all filenames in those subdirectories with a two-digit
number and a dash, to simplify the ordering. This also defines a
concept of drop-in priorities to allow OS vendors to ship
drop-ins within a specific range lower than the range used by
users. This should lower the risk of package drop-ins overriding
accidentally drop-ins defined by users. It is recommended to use
the range 10-40 for drop-ins in /usr/ and the range 60-90 for
drop-ins in /etc/ and /run/, to make sure that local and
transient drop-ins take priority over drop-ins shipped by the OS
vendor.
To disable a configuration file supplied by the vendor, the
recommended way is to place a symlink to /dev/null in the
configuration directory in /etc/, with the same filename as the
vendor configuration file.
The following options are available in the [Resolve] section:
DNS=
A space-separated list of IPv4 and IPv6 addresses to use as
system DNS servers. Each address can optionally take a port
number separated with ":", a network interface name or index
separated with "%", and a Server Name Indication (SNI)
separated with "#". When IPv6 address is specified with a
port number, then the address must be in the square brackets.
That is, the acceptable full formats are
"111.222.333.444:9953%ifname#example.com" for IPv4 and
"[1111:2222::3333]:9953%ifname#example.com" for IPv6. DNS
requests are sent to one of the listed DNS servers in
parallel to suitable per-link DNS servers acquired from
systemd-networkd.service(8) or set at runtime by external
applications. For compatibility reasons, if this setting is
not specified, the DNS servers listed in /etc/resolv.conf are
used instead, if that file exists and any servers are
configured in it. This setting defaults to the empty list.
Added in version 213.
FallbackDNS=
A space-separated list of IPv4 and IPv6 addresses to use as
the fallback DNS servers. Please see DNS= for acceptable
format of addresses. Any per-link DNS servers obtained from
systemd-networkd.service(8) take precedence over this
setting, as do any servers set via DNS= above or
/etc/resolv.conf. This setting is hence only used if no other
DNS server information is known. If this option is not given,
a compiled-in list of DNS servers is used instead.
Added in version 216.
Domains=
A space-separated list of domains, optionally prefixed with
"~", used for two distinct purposes described below. Defaults
to the empty list.
Any domains not prefixed with "~" are used as search suffixes
when resolving single-label hostnames (domain names which
contain no dot), in order to qualify them into
fully-qualified domain names (FQDNs). These "search domains"
are strictly processed in the order they are specified in,
until the name with the suffix appended is found. For
compatibility reasons, if this setting is not specified, the
search domains listed in /etc/resolv.conf with the search
keyword are used instead, if that file exists and any domains
are configured in it.
The domains prefixed with "~" are called "route-only
domains". All domains listed here (both search domains and
route-only domains after removing the "~" prefix) define a
search path that preferably directs DNS queries to this
interface. This search path has an effect only when suitable
per-link DNS servers are known. Such servers may be defined
through the DNS= setting (see above) and dynamically at run
time, for example from DHCP leases. If no per-link DNS
servers are known, route-only domains have no effect.
Use the construct "~." (which is composed from "~" to
indicate a route-only domain and "." to indicate the DNS
root domain that is the implied suffix of all DNS domains) to
use the DNS servers defined for this link preferably for all
domains.
See "Protocols and Routing" in systemd-resolved.service(8)
for details of how search and route-only domains are used.
Note that configuring the MulticastDNS domain "local" as
search or routing domain has the effect of routing lookups
for this domain to classic unicast DNS. This may be used to
provide compatibility with legacy installations that use this
domain in a unicast DNS context, against the IANA assignment
of this domain to pure MulticastDNS purposes. Search and
routing domains are a unicast DNS concept, they cannot be
used to resolve single-label lookups via MulticastDNS.
Added in version 229.
LLMNR=
Takes a boolean argument or "resolve". Controls Link-Local
Multicast Name Resolution support (RFC 4795[2]) on the local
host. If true, enables full LLMNR responder and resolver
support. If false, disables both. If set to "resolve", only
resolution support is enabled, but responding is disabled.
Note that systemd-networkd.service(8) also maintains per-link
LLMNR settings. LLMNR will be enabled on a link only if the
per-link and the global setting is on.
Added in version 216.
MulticastDNS=
Takes a boolean argument or "resolve". Controls Multicast DNS
support (RFC 6762[3]) on the local host. If true, enables
full Multicast DNS responder and resolver support. If false,
disables both. If set to "resolve", only resolution support
is enabled, but responding is disabled. Note that
systemd-networkd.service(8) also maintains per-link Multicast
DNS settings. Multicast DNS will be enabled on a link only if
the per-link and the global setting is on.
Added in version 234.
DNSSEC=
Takes a boolean argument or "allow-downgrade".
If set to true, all DNS lookups are DNSSEC-validated locally
(excluding LLMNR and Multicast DNS). If the response to a
lookup request is detected to be invalid a lookup failure is
returned to applications. Note that this mode requires a DNS
server that supports DNSSEC. If the DNS server does not
properly support DNSSEC all validations will fail.
If set to "allow-downgrade", DNSSEC validation is attempted,
but if the server does not support DNSSEC properly, DNSSEC
mode is automatically disabled. Note that this mode makes
DNSSEC validation vulnerable to "downgrade" attacks, where an
attacker might be able to trigger a downgrade to non-DNSSEC
mode by synthesizing a DNS response that suggests DNSSEC was
not supported.
If set to false, DNS lookups are not DNSSEC validated.
Note that DNSSEC validation requires retrieval of additional
DNS data, and thus results in a small DNS lookup time
penalty.
DNSSEC requires knowledge of "trust anchors" to prove data
integrity. The trust anchor for the Internet root domain is
built into the resolver, additional trust anchors may be
defined with dnssec-trust-anchors.d(5). Trust anchors may
change at regular intervals, and old trust anchors may be
revoked. In such a case DNSSEC validation is not possible
until new trust anchors are configured locally or the
resolver software package is updated with the new root trust
anchor. In effect, when the built-in trust anchor is revoked
and DNSSEC= is true, all further lookups will fail, as it
cannot be proved anymore whether lookups are correctly
signed, or validly unsigned. If DNSSEC= is set to
"allow-downgrade" the resolver will automatically turn off
DNSSEC validation in such a case.
Client programs looking up DNS data will be informed whether
lookups could be verified using DNSSEC, or whether the
returned data could not be verified (either because the data
was found unsigned in the DNS, or the DNS server did not
support DNSSEC or no appropriate trust anchors were known).
In the latter case it is assumed that client programs employ
a secondary scheme to validate the returned DNS data, should
this be required.
It is recommended to set DNSSEC= to true on systems where it
is known that the DNS server supports DNSSEC correctly, and
where software or trust anchor updates happen regularly. On
other systems it is recommended to set DNSSEC= to
"allow-downgrade".
In addition to this global DNSSEC setting
systemd-networkd.service(8) also maintains per-link DNSSEC
settings. For system DNS servers (see above), only the global
DNSSEC setting is in effect. For per-link DNS servers the
per-link setting is in effect, unless it is unset in which
case the global setting is used instead.
Site-private DNS zones generally conflict with DNSSEC
operation, unless a negative (if the private zone is not
signed) or positive (if the private zone is signed) trust
anchor is configured for them. If "allow-downgrade" mode is
selected, it is attempted to detect site-private DNS zones
using top-level domains (TLDs) that are not known by the DNS
root server. This logic does not work in all private zone
setups.
Defaults to "allow-downgrade".
Added in version 229.
DNSOverTLS=
Takes a boolean argument or "opportunistic". If true all
connections to the server will be encrypted. Note that this
mode requires a DNS server that supports DNS-over-TLS and has
a valid certificate. If the hostname was specified in DNS= by
using the format "address#server_name" it is used to validate
its certificate and also to enable Server Name Indication
(SNI) when opening a TLS connection. Otherwise the
certificate is checked against the server's IP. If the DNS
server does not support DNS-over-TLS all DNS requests will
fail.
When set to "opportunistic" DNS request are attempted to send
encrypted with DNS-over-TLS. If the DNS server does not
support TLS, DNS-over-TLS is disabled. Note that this mode
makes DNS-over-TLS vulnerable to "downgrade" attacks, where
an attacker might be able to trigger a downgrade to
non-encrypted mode by synthesizing a response that suggests
DNS-over-TLS was not supported. If set to false, DNS lookups
are send over UDP.
Note that DNS-over-TLS requires additional data to be send
for setting up an encrypted connection, and thus results in a
small DNS look-up time penalty.
Note that in "opportunistic" mode the resolver is not capable
of authenticating the server, so it is vulnerable to
"man-in-the-middle" attacks.
In addition to this global DNSOverTLS= setting
systemd-networkd.service(8) also maintains per-link
DNSOverTLS= settings. For system DNS servers (see above),
only the global DNSOverTLS= setting is in effect. For
per-link DNS servers the per-link setting is in effect,
unless it is unset in which case the global setting is used
instead.
Defaults to "no".
Added in version 239.
Cache=
Takes a boolean or "no-negative" as argument. If "yes" (the
default), resolving a domain name which already got queried
earlier will return the previous result as long as it is
still valid, and thus does not result in a new network
request. Be aware that turning off caching comes at a
performance penalty, which is particularly high when DNSSEC
is used. If "no-negative", only positive answers are cached.
Note that caching is turned off by default for host-local DNS
servers. See CacheFromLocalhost= for details.
Added in version 231.
CacheFromLocalhost=
Takes a boolean as argument. If "no" (the default), and
response cames from host-local IP address (such as 127.0.0.1
or ::1), the result wouldn't be cached in order to avoid
potential duplicate local caching.
Added in version 248.
DNSStubListener=
Takes a boolean argument or one of "udp" and "tcp". If "udp",
a DNS stub resolver will listen for UDP requests on addresses
127.0.0.53 and 127.0.0.54, port 53. If "tcp", the stub will
listen for TCP requests on the same addresses and port. If
"yes" (the default), the stub listens for both UDP and TCP
requests. If "no", the stub listener is disabled.
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.)
Note that the DNS stub listener is turned off implicitly when
its listening address and port are already in use.
Added in version 232.
DNSStubListenerExtra=
Takes an IPv4 or IPv6 address to listen on. The address may
be optionally prefixed with a protocol name ("udp" or "tcp")
separated with ":". If the protocol is not specified, the
service will listen on both UDP and TCP. It may be also
optionally suffixed by a numeric port number with separator
":". When an IPv6 address is specified with a port number,
then the address must be in the square brackets. If the port
is not specified, then the service uses port 53. Note that
this is independent of the primary DNS stub configured with
DNSStubListener=, and only configures additional sockets to
listen on. This option can be specified multiple times. If an
empty string is assigned, then the all previous assignments
are cleared. Defaults to unset.
Examples:
DNSStubListenerExtra=192.168.10.10
DNSStubListenerExtra=2001:db8:0:f102::10
DNSStubListenerExtra=192.168.10.11:9953
DNSStubListenerExtra=[2001:db8:0:f102::11]:9953
DNSStubListenerExtra=tcp:192.168.10.12
DNSStubListenerExtra=udp:2001:db8:0:f102::12
DNSStubListenerExtra=tcp:192.168.10.13:9953
DNSStubListenerExtra=udp:[2001:db8:0:f102::13]:9953
Added in version 247.
ReadEtcHosts=
Takes a boolean argument. If "yes" (the default),
systemd-resolved will read /etc/hosts, and try to resolve
hosts or address by using the entries in the file before
sending query to DNS servers.
Added in version 240.
ResolveUnicastSingleLabel=
Takes a boolean argument. When false (the default),
systemd-resolved will not resolve A and AAAA queries for
single-label names over classic DNS. Note that such names may
still be resolved if search domains are specified (see
Domains= above), or using other mechanisms, in particular via
LLMNR or from /etc/hosts. When true, queries for single-label
names will be forwarded to global DNS servers even if no
search domains are defined.
This option is provided for compatibility with configurations
where public DNS servers are not used. Forwarding
single-label names to servers not under your control is not
standard-conformant, see IAB Statement[4], and may create a
privacy and security risk.
Added in version 246.
StaleRetentionSec=SECONDS
Takes a duration value, which determines the length of time
DNS resource records can be retained in the cache beyond
their Time To Live (TTL). This allows these records to be
returned as stale records. By default, this value is set to
zero, meaning that DNS resource records are not stored in the
cache after their TTL expires.
This is useful when a DNS server failure occurs or becomes
unreachable. In such cases, systemd-resolved(8) continues to
use the stale records to answer DNS queries, particularly
when no valid response can be obtained from the upstream DNS
servers. However, this doesn't apply to NXDOMAIN responses,
as those are still perfectly valid responses. This feature
enhances resilience against DNS infrastructure failures and
outages.
systemd-resolved always attempts to reach the upstream DNS
servers first, before providing the client application with
any stale data. If this feature is enabled, cache will not be
flushed when changing servers.
Added in version 254.
systemd(1), systemd-resolved.service(8),
systemd-networkd.service(8), dnssec-trust-anchors.d(5),
resolv.conf(5)
1. 💣💥🧨💥💥💣 Please note that those configuration files must
be available at all times. If /usr/local/ is a separate
partition, it may not be available during early boot, and
must not be used for configuration.
2. RFC 4795
https://tools.ietf.org/html/rfc4795
3. RFC 6762
https://tools.ietf.org/html/rfc6762
4. IAB Statement
https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have
a bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
systemd 257~devel RESOLVED.CONF(5)
Pages that refer to this page: resolvectl(1), dnssec-trust-anchors.d(5), org.freedesktop.resolve1(5), systemd.network(5), systemd.directives(7), systemd.index(7), systemd-resolved.service(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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