resolved.conf, resolved.conf.d - Network Name Resolution configuration files
These configuration files control local DNS and LLMNR name resolution.
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.
The following options are available in the [Resolve] section:
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.
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".
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".
Note that caching is turned off by default for host-local DNS servers. See CacheFromLocalhost= for details.
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=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
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.
This is useful when a DNS server failure occurs or becomes unreachable. In such cases, systemd-resolved 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.
- RFC 4795
- RFC 6762
- IAB Statement