The default configuration is defined during compilation, so a
configuration file is only needed when it is necessary to deviate
from those defaults. By default, the configuration file in
/etc/systemd/ contains commented out entries showing the defaults as
a guide to the administrator. This file can be edited to create local
When packages need to customize the configuration, they can install
configuration snippets in /usr/lib/systemd/*.conf.d/. Files in /etc/
are reserved for the local administrator, who may use this logic to
override the configuration files installed by vendor packages. The
main configuration file is read before any of the configuration
directories, and has the lowest precedence; entries in a file in any
configuration directory override entries in the single configuration
file. Files in the *.conf.d/ configuration subdirectories are sorted
by their filename in lexicographic order, regardless of which of the
subdirectories they reside in. If multiple files specify the same
option, the entry in the file with the lexicographically latest name
takes precedence. It is recommended to prefix all filenames in those
subdirectories with a two-digit number and a dash, to simplify the
ordering of the files.
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:
A space-separated list of IPv4 and IPv6 addresses to use as
system DNS servers. 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.
A space-separated list of IPv4 and IPv6 addresses to use as the
fallback DNS servers. 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.
A space-separated list of domains. These domains are used as
search suffixes when resolving single-label host names (domain
names which contain no dot), in order to qualify them into
fully-qualified domain names (FQDNs). Search domains are strictly
processed in the order they are specified, 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 are used instead, if that file exists and any
domains are configured in it. This setting defaults to the empty
Specified domain names may optionally be prefixed with "~". In
this case they do not define a search path, but preferably direct
DNS queries for the indicated domains to the DNS servers
configured with the system DNS= setting (see above), in case
additional, suitable per-link DNS servers are known. If no
per-link DNS servers are known using the "~" syntax has no
effect. Use the construct "~." (which is composed of "~" to
indicate a routing domain and "." to indicate the DNS root
domain that is the implied suffix of all DNS domains) to use the
system DNS server defined with DNS= preferably for all domains.
Takes a boolean argument or "resolve". Controls Link-Local
Multicast Name Resolution support (RFC 4794) 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.
Takes a boolean argument or "allow-downgrade". If 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 look-up 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 off.
Takes a boolean 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.
Note that caching is turned off implicitly if the configured DNS
server is on a host-local IP address (such as 127.0.0.1 or ::1),
in order to avoid duplicate local caching.
Takes a boolean argument or one of "udp" and "tcp". If "udp" (the
default), a DNS stub resolver will listen for UDP requests on
address 127.0.0.53 port 53. If "tcp", the stub will listen for
TCP requests on the same address and port. If "yes", the stub
listens for both UDP and TCP requests. If "no", the stub listener
Note that the DNS stub listener is turned off implicitly when its
listening address and port are already in use.
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
page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2017-03-13. If you dis‐
cover 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
systemd 233 RESOLVED.CONF(5)