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This adds support for searching single-label hostnames in a set of
configured search domains.
A new object DnsQueryCandidate is added that links queries to scopes.
It keeps track of the search domain last used for a query on a specific
link. Whenever a host name was unsuccessfuly resolved on a scope all its
transactions are flushed out and replaced by a new set, with the next
search domain appended.
This also adds a new flag SD_RESOLVED_NO_SEARCH to disable search domain
behaviour. The "systemd-resolve-host" tool is updated to make this
configurable via --search=.
Fixes #1697
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This also adds client-side support for this to systemd-resolve-host.
Note that the ResolveService() API can deal both with DNS-SD service
(consisting of service name, type and domain), as well as classic SRV
services (consisting just of a type and a domain), all exposed in the
same call.
This patch also reworks CNAME handling in order to reuse it between
hostname, RR and service lookups.
In contrast to Avahi and Bonjour, this new API will actually reolve the
A/AAAA RRs the SRV RRs point to in one go (unless this is explicitly
disabled). This normally comes for free, as these RRs are sent along
the SRV responses anyway, hence let's make use of that. This makes the
API considerably easier to use, as a single ResolveService() invocation
will return all necessary data to pick a server and connect() to it.
Note that this only implements the DNS-SD resolving step, it does not
implement DNS-SD browsing, as that makes sense primarily on mDNS, due to
its continuous nature.
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When a NXDATA or a NODATA response is received for an alias it may
include CNAME records from the redirect chain. We should cache the
response for each of these names to avoid needless roundtrips in
the future.
It is not sufficient to do the negative caching only for the
canonical name, as the included redirection chain is not guaranteed
to be complete. In fact, only the final CNAME record from the chain
is guaranteed to be included.
We take care not to cache entries that redirects outside the current
zone, as the SOA will then not be valid.
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With this change we'll now also generate synthesized RRs for the local
LLMNR hostname (first label of system hostname), the local mDNS hostname
(first label of system hostname suffixed with .local), the "gateway"
hostname and all the reverse PTRs. This hence takes over part of what
nss-myhostname already implemented.
Local hostnames resolve to the set of local IP addresses. Since the
addresses are possibly on different interfaces it is necessary to change
the internal DnsAnswer object to track per-RR interface indexes, and to
change the bus API to always return the interface per-address rather than
per-reply. This change also patches the existing clients for resolved
accordingly (nss-resolve + systemd-resolve-host).
This also changes the routing logic for queries slightly: we now ensure
that the local hostname is never resolved via LLMNR, thus making it
trustable on the local system.
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negative caching
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addresses are link-local or not
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hence let's not generate that
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different clients
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