On 15/04/2020 09:29, Tom Hughes via devel wrote:
> I'm not sure what happens if there are multiple interfaces with
> no specific routing but I think it may try them all?
Found the documentation now - it does try them all. Full details
from systemd-resolved(8) are:
Lookup requests are routed to the available DNS servers, LLMNR and
MulticastDNS interfaces according to the following rules:
· Lookups for the special hostname "localhost" are never routed to
the network. (A few other, special domains are handled the same
way.)
· Single-label names are routed to all local interfaces capable of IP
multicasting, using the LLMNR protocol. Lookups for IPv4 addresses
are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are
only sent via LLMNR on IPv6. Lookups for the locally configured
host name and the "_gateway" host name are never routed to LLMNR.
· Multi-label names with the domain suffix ".local" are routed to all
local interfaces capable of IP multicasting, using the MulticastDNS
protocol. As with LLMNR IPv4 address lookups are sent via IPv4 and
IPv6 address lookups are sent via IPv6.
· Other multi-label names are routed to all local interfaces that
have a DNS server configured, plus the globally configured DNS
server if there is one. Address lookups from the link-local address
range are never routed to DNS. Note that by default lookups for
domains with the ".local" suffix are not routed to DNS servers,
unless the domain is specified explicitly as routing or search
domain for the DNS server and interface. This means that on
networks where the ".local" domain is defined in a site-specific
DNS server, explicit search or routing domains need to be
configured to make lookups within this DNS domain work. Note that
today it's generally recommended to avoid defining ".local" in a
DNS server, as RFC6762[3] reserves this domain for exclusive
MulticastDNS use.
If lookups are routed to multiple interfaces, the first successful
response is returned (thus effectively merging the lookup zones on all
matching interfaces). If the lookup failed on all interfaces, the last
failing response is returned.
Routing of lookups may be influenced by configuring per-interface
domain names and other settings. See systemd.network(5) and
resolvectl(1) for details. The following query routing logic applies
for unicast DNS traffic:
· If a name to look up matches (that is: is equal to or has as
suffix) any of the configured search or route-only domains of any
link (or the globally configured DNS settings), the "best matching"
search/route-only domain is determined: the matching one with the
most labels. The query is then sent to all DNS servers of any links
or the globally configured DNS servers associated with this "best
matching" search/route-only domain. (Note that more than one link
might have this same "best matching" search/route-only domain
configured, in which case the query is sent to all of them in
parallel).
· If a query does not match any configured search/route-only domain
(neither per-link nor global), it is sent to all DNS servers that
are configured on links with the "DNS default route" option set, as
well as the globally configured DNS server.
· If there is no link configured as "DNS default route" and no global
DNS server configured, the compiled-in fallback DNS server is used.
· Otherwise the query is failed as no suitable DNS servers could be
determined.
The "DNS default route" option is a boolean setting configurable with
resolvectl or in .network files. If not set, it is implicitly
determined based on the configured DNS domains for a link: if there's
any route-only domain (not matching "~.") it defaults to false,
otherwise to true.
Effectively this means: in order to preferably route all DNS queries
not explicitly matched by search/route-only domain configuration to a
specific link, configure a "~." route-only domain on it. This will
ensure that other links will not be considered for the queries (unless
they too carry such a route-only domain). In order to route all such
DNS queries to a specific link only in case no other link is
preferable, then set the "DNS default route" option for the link to
true, and do not configure a "~." route-only domain on it. Finally, in
order to ensure that a specific link never receives any DNS traffic not
matching any of its configured search/route-only domains, set the "DNS
default route" option for it to false.
Tom
--
Tom Hughes (tom@xxxxxxxxxx)
http://compton.nu/
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