Ubuntu Manpage: resolved.conf, resolved.conf.d - Network Name Resolution configuration files

Provided by: systemd_245.4-4ubuntu3.24_amd64

NAME

       resolved.conf, resolved.conf.d - Network Name Resolution configuration files

SYNOPSIS

       /etc/systemd/resolved.conf

       /etc/systemd/resolved.conf.d/*.conf

       /run/systemd/resolved.conf.d/*.conf

       /usr/lib/systemd/resolved.conf.d/*.conf

DESCRIPTION

       These configuration files control local DNS and LLMNR name resolution.

CONFIGURATION DIRECTORIES AND PRECEDENCE

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/ or /usr/local/lib/systemd/*.conf.d/. 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 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 with the lexicographically latest name takes
precedence. For options which accept a list of values, entries are collected as they occur
in files sorted lexicographically.

Files in /etc/ are reserved for the local administrator, who may use this logic to
override the configuration files installed by vendor packages. 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.

OPTIONS

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 4795[1]) 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.

MulticastDNS=
Takes a boolean argument or "resolve". Controls Multicast DNS support (RFC 6762[2]) 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.

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 "allow-downgrade"

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 for it'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.

Server Name Indication (SNI) can be used when opening a TLS connection. Entries in
DNS= should be in format "address#server_name".

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 off.

Cache=
Takes a boolean or "no-negative" as argument. If "yes", 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" (the default), only positive answers are cached.

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.

DNSStubListener=
Takes a boolean argument or one of "udp" and "tcp". If "udp", 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 default), the stub
listens for both UDP and TCP requests. If "no", the stub listener is disabled.

Note that the DNS stub listener is turned off implicitly when its listening address
and port are already in use.

ReadEtcHosts=
Takes a boolean argument. If "yes" (the default), the DNS stub resolver will read
/etc/hosts, and try to resolve hosts or address by using the entries in the file
before sending query to DNS servers.

NOTES