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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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CNAME records are special in the way they are treated by DNS servers,
and our cache should mimic that behavior: In case a domain name has an
alias, its CNAME record is returned in place of any other.
Our cache was not doing this despite caching the CNAME records, this
entailed needless lookups to re-resolve the CNAME.
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Only one key is allowed per transaction now, so let's simplify things and only allow putting
one question key into the cache at a time.
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Patch via coccinelle.
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Currently, dns_cache_put() does a number of things:
1) It unconditionally removes all keys contained in the passed
question before adding keys from the newly arrived answers.
2) It puts positive entries into the cache for all RRs contained
in the answer.
3) It creates negative entries in the cache for all keys in the
question that are not answered.
Allow passing q = NULL in the parameters and skip 1) and 3), so
we can use that function for mDNS responses. In this case, the
question is irrelevant, we are interested in all answers we got.
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Let's simplify things and only maintain a single RR key per transaction
object, instead of a full DnsQuestion. Unicast DNS and LLMNR don't
support multiple questions per packet anway, and Multicast DNS suggests
coalescing questions beyond a single dns query, across the whole system.
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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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We cannot rely on CLOCK_BOOTTIME being supported by the kernel, so fallack
to CLOCK_MONOTONIC if the former is not supported.
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Reported by Cristian Rodríguez
http://lists.freedesktop.org/archives/systemd-devel/2015-May/031626.html
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It is redundant to store 'hash' and 'compare' function pointers in
struct Hashmap separately. The functions always comprise a pair.
Store a single pointer to struct hash_ops instead.
systemd keeps hundreds of hashmaps, so this saves a little bit of
memory.
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and timeing out transactions
That way the cache doens't get confused when the system is suspended.
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responses
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Name defending is still missing.
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different clients
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