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This is often needed for proper DNSSEC support, and even to handle AAAA records
without falling back to TCP.
If the path between the client and server is fully compliant, this should always
work, however, that is not the case, and overlarge packets will get mysteriously
lost in some cases.
For that reason, we use a similar fallback mechanism as we do for palin EDNS0,
EDNS0+DO, etc.:
The large UDP size feature is different from the other supported feature, as we
cannot simply verify that it works based on receiving a reply (as the server
will usually send us much smaller packets than what we claim to support, so
simply receiving a reply does not mean much).
For that reason, we keep track of the largest UDP packet we ever received, as this
is the smallest known good size (defaulting to the standard 512 bytes). If
announcing the default large size of 4096 fails (in the same way as the other
features), we fall back to the known good size. The same logic of retrying after a
grace-period applies.
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This indicates that we can handle DNSSEC records (per RFC3225), even if
all we do is silently drop them. This feature requires EDNS0 support.
As we do not yet support larger UDP packets, this feature increases the
risk of getting truncated packets.
Similarly to how we fall back to plain UDP if EDNS0 fails, we will fall
back to plain EDNS0 if EDNS0+DO fails (with the same logic of remembering
success and retrying after a grace period after failure).
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This is a minimal implementation of RFC6891. Only default values
are used, so in reality this will be a noop.
EDNS0 support is dependent on the current server's feature level,
so appending the OPT pseudo RR is done when the packet is emitted,
rather than when it is assembled. To handle different feature
levels on retransmission, we strip off the OPT RR again after
sending the packet.
Similarly, to how we fall back to TCP if UDP fails, we fall back
to plain UDP if EDNS0 fails (but if EDNS0 ever succeeded we never
fall back again, and after a timeout we will retry EDNS0).
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Needed for EDNS0.
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After all, this is likely a local DNS forwarder that caches anyway,
hence there's no point in caching twice.
Fixes #2038.
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RFC 6763 is very clear that TXT RRs should allow arbitrary binary
content, hence let's actually accept that. This also means accepting NUL
bytes in the middle of strings.
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This is a continuation of the previous include sort patch, which
only sorted for .c files.
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Some flags are defined differently on unicast DNS and LLMNR, let's
document this in the DNS_PACKET_MAKE_FLAGS() macro.
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With more protocols to come, switch repetitive if-else blocks with a
switch-case statements.
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We explicitly need to turn off name compression when marshalling or
demarshalling RRs for bus transfer, since they otherwise refer to packet
offsets that reference packets that are not transmitted themselves.
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Needed for DNSSEC.
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Currently we only make sure our links can handle the size of the payload witohut
taking the headers into account.
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The C and T bits in the DNS packet header definitions are specific to LLMNR.
In regular DNS, they are called AA and RD instead. Reflect that by calling
the macros accordingly, and alias LLMNR specific macros.
While at it, define RA, AD and CD getters as well.
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This patch removes includes that are not used. The removals were found with
include-what-you-use which checks if any of the symbols from a header is
in use.
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something up
Also, return on which protocol/family/interface we found something.
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Name defending is still missing.
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different clients
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Let's settle on a single type for all address family values, even if
UNIX is very inconsitent on the precise type otherwise. Given that
socket() is the primary entrypoint for the sockets API, and that uses
"int", and "int" is relatively simple and generic, we settle on "int"
for this.
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Let's turn resolved into a something truly useful: a fully asynchronous
DNS stub resolver that subscribes to network changes.
(More to come: caching, LLMNR, mDNS/DNS-SD, DNSSEC, IDN, NSS module)
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