resolved.conf, resolved.conf.d — Network Name Resolution configuration files
/etc/systemd/resolved.conf
/etc/systemd/resolved.conf.d/*.conf
/run/systemd/resolved.conf.d/*.conf
/usr/lib/systemd/resolved.conf.d/*.conf
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 overrides.
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:
DNS=
¶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.
FallbackDNS=
¶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.
Domains=
¶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 list.
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.
LLMNR=
¶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.
DNSSEC=
¶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.