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

Provided by: systemd-resolved_255.4-1ubuntu8.4_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 set during compilation, so configuration is only needed when
it is necessary to deviate from those defaults. The main configuration file is either in
/usr/lib/systemd/ or /etc/systemd/ and contains commented out entries showing the defaults
as a guide to the administrator. Local overrides can 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 in /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 of the files. This also defined a concept of drop-in
priority to allow distributions 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.

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. 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[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.

Added in version 216.

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.

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. In this mode, or when set to
"allow-downgrade" and the downgrade has happened, the resolver becomes
security-unaware and all forwarded queries have DNSSEC OK (DO) bit unset.

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 "no".

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", 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 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[3], 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.

NOTES