When packages need to customize the configuration, they can install configuration snippets in /usr/lib/systemd/*.conf.d/ or /usr/local/lib/systemd/*.conf.d/. The main configuration file is read before any of the configuration directories, and has the lowest precedence; entries in a file in any configuration directory override entries in the single 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 with the lexicographically latest name takes precedence. For options which accept a list of values, entries are collected as they occur in files sorted lexicographically.
Files in /etc/ are reserved for the local administrator, who may use this logic to override the configuration files installed by vendor packages. 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.
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 "routing domains". All domains listed here (both search domains and routing 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, routing domains have no effect.
Use the construct "~." (which is composed from "~" to indicate a routing 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.
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 "allow-downgrade".
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 off.
Note that caching is turned off implicitly if the configured DNS server is on a host-local IP address (such as 127.0.0.1 or ::1), in order to avoid duplicate local caching.
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.
- RFC 4795
- RFC 6762
- IAB Statement