| RESOLVED.CONF(5) | resolved.conf | RESOLVED.CONF(5) |
.SH "NAME" resolved.conf, resolved.conf.d - Network Name Resolution
configuration files
.SH "SYNOPSIS"
.PP
.SH "DESCRIPTION"
.PP These configuration files control local DNS and LLMNR name
resolution.
.SH "CONFIGURATION DIRECTORIES AND PRECEDENCE"
.PP 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 loaded from one of the listed directories in order of
priority, only the first file found is used: /etc/systemd/, /run/systemd/,
/usr/local/lib/systemd/ [1], /usr/lib/systemd/. The vendor version of the
file contains commented out entries showing the defaults as a guide to the
administrator. Local overrides can also 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 is shipped under /usr/), however using
drop-ins for local configuration is recommended over modifications to the
main configuration file.
.PP 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.
.PP 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. This also defines a concept of
drop-in priorities to allow OS vendors 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. It is
recommended to use the range 10-40 for drop-ins in /usr/ and the range 60-90
for drop-ins in /etc/ and /run/, to make sure that local and transient
drop-ins take priority over drop-ins shipped by the OS vendor.
.PP 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.
.SH "OPTIONS"
.PP The following options are available in the [Resolve] section:
.PP 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.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP LLMNR=
Takes a boolean argument or resolve. Controls Link-Local Multicast Name
Resolution support (RFC 4795[2]) 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.
.RE
.PP MulticastDNS=
Takes a boolean argument or resolve. Controls Multicast DNS support (RFC
6762[3]) 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.
.RE
.PP 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.
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.
.RE
.PP 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.
.RE
.PP 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.
Added in version 231.
.RE
.PP 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 would not be
cached in order to avoid potential duplicate local caching.
Added in version 248.
.RE
.PP DNSCacheSize=, MulticastDNSCacheSize=,
LLMNRCacheSize=
Takes a non-negative integer. Configures the maximum number of DNS resource
record entries that may be stored in the per-scope cache for unicast DNS,
Multicast DNS (mDNS), and Link-Local Multicast Name Resolution (LLMNR)
respectively. Each defaults to 4096. The maximum allowed value is 16777216
(2^24). Setting any of these to 0 effectively disables caching for the
respective protocol. These settings are only effective when Cache= is
set to yes or no-negative. If Cache=no, caching is fully disabled
regardless of these values.
Note that Multicast DNS relies heavily on caching for request suppression and
efficient operation. It is recommended to keep MulticastDNSCacheSize=
at a reasonably high value even when reducing DNSCacheSize=.
Note that systemd-resolved automatically flushes all caches on system
memory pressure, thus in most cases manual cache size configuration should
not be necessary.
Note that caching is turned off by default for host-local DNS servers. See
CacheFromLocalhost= for details.
Added in version 261.
.RE
.PP 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.
.RE
.PP RefuseRecordTypes=
Takes a list of DNS record types separated by space. Specified record types
in each query will be refused. This option can be specified multiple times.
If an empty string is specified, then all previous assignments are
cleared.
Examples:
RefuseRecordTypes=AAAA SRV TXT
Note that to refuse AAAA record (unlike SRV, TXT, etc.) completely, please
also disable IPv6 stack in kernel with "sysctl -w
net.ipv6.conf.all.disable_ipv6=1".
Added in version 258.
.RE
.PP 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.
.RE
.PP ReadEtcHosts=
Takes a boolean argument. If yes (the default), systemd-resolved will
read /etc/hosts, and try to resolve hosts or addresses by using the entries
in the file before sending query to DNS servers.
Added in version 240.
.RE
.PP ReadStaticRecords=
Takes a boolean argument. If yes (the default), systemd-resolved will
read /etc/systemd/resolve/static.d/*.rr, /run/systemd/resolve/static.d/*.rr,
/usr/local/lib/systemd/resolve/static.d/*.rr,
/usr/lib/systemd/resolve/static.d/*.rr, and try to resolve lookups by using
the entries in these files before sending query to DNS servers. This
functionality is very similar to the one controlled by ReadEtcHosts=,
but allows more flexible control of DNS resource records fields beyond just
A/AAAA/PTR. See systemd.rr(5) for details.
If both this option and ReadEtcHosts= are enabled then this mechanism
takes precedence: any records discovered via static resource records will
take precedence over records under the same name from /etc/hosts.
Added in version 261.
.RE
.PP 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[4],
and may create a privacy and security risk.
Added in version 246.
.RE
.PP 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 does not 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.
.RE
.SH "SEE ALSO"
.PP systemd(1), systemd-resolved.service(8),
systemd-networkd.service(8), dnssec-trust-anchors.d(5),
systemd.rr(5), resolv.conf(5)
NOTES
- 1.
-
💣💥🧨💥💥💣 Please note that those configuration files must be available at all times. If /usr/local/ is a separate partition, it may not be available during early boot, and must not be used for configuration. - 2.
- RFC 4795
- 3.
- RFC 6762
- 4.
- IAB Statement
| systemd 262.2 |