resolved.conf, resolved.conf.d — Network Name Resolution configuration files
/etc/systemd/resolved.conf
/etc/systemd/resolved.conf.d/*.conf
/run/systemd/resolved.conf.d/*.conf
/usr/lib/systemd/resolved.conf.d/*.conf
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.
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.
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.
LLMNR=¶Takes a boolean argument or
"resolve". Controls Link-Local Multicast Name
Resolution support (RFC 4795) 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) 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 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 "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 "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:
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, and may create a privacy and security risk.
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.