NAME
resolved.conf, resolved.conf.d - Network Name Resolution configuration filesSYNOPSIS
/etc/systemd/resolved.conf /etc/systemd/resolved.conf.d/*.conf /run/systemd/resolved.conf.d/*.conf /usr/lib/systemd/resolved.conf.d/*.confDESCRIPTION
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. Initially, the main configuration file in /etc/systemd/ contains commented out entries showing the defaults as a guide to the administrator. Local overrides can be created by editing this file or by creating drop-ins, as described below. 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. 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.
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 "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.
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 "no".
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".
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 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.
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:
ReadEtcHosts=
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
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[3],
and may create a privacy and security risk.
SEE ALSO
systemd(1), systemd-resolved.service(8), systemd-networkd.service(8), dnssec-trust-anchors.d(5), resolv.conf(5)NOTES
- 1.
- RFC 4795
- 2.
- RFC 6762
- 3.
- IAB Statement
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