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

Provided by: systemd_229-4ubuntu21.31_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/. 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.

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 4794[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.

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.

NOTES