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.
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.
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 "routing domains".
All domains listed here (both search domains and routing
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, routing domains have no effect.
Use the construct "~." (which is composed from "~" to
indicate a routing 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.
LLMNR=
Takes a boolean argument or "resolve". Controls Link-Local
Multicast Name Resolution support (RFC 4795
MulticastDNS=
Takes a boolean argument or "resolve". Controls Multicast DNS
support (RFC 6762
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. If the hostname was specified in DNS= by
using the format 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 off.
Cache=
Takes a boolean or "no-negative" as 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. If "no-negative", only positive answers are cached.
Note that caching is turned off by default for host-local DNS
servers. See CacheFromLocalhost= for details.
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.
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.
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
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.
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
