RESOLVED.CONF(5) resolved.conf RESOLVED.CONF(5)
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.
SEE ALSO
systemd(1), systemd-resolved.service(8), systemd-networkd.service(8),
dnssec-trust-anchors.d(5), resolv.conf(5)
NOTES
1. RFC 4795
https://tools.ietf.org/html/rfc4795
2. RFC 6762
https://tools.ietf.org/html/rfc6762
3. IAB Statement
https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/
systemd 255 RESOLVED.CONF(5)
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