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Previously the calls for emitting DNS UDP packets were just called
dns_{transacion|scope}_emit(), but the one to establish a DNS TCP
connection was called dns_transaction_open_tcp(). Clean this up, and
rename them dns_{transaction|scope}_emit_udp() and
dns_transaction_open_tcp().
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This adds a mode that makes resolved automatically downgrade from DNSSEC
support to classic non-DNSSEC resolving if the configured DNS server is
not capable of DNSSEC. Enabling this mode increases compatibility with
crappy network equipment, but of course opens up the system to
downgrading attacks.
The new mode can be enabled by setting DNSSEC=downgrade-ok in
resolved.conf. DNSSEC=yes otoh remains a "strict" mode, where DNS
resolving rather fails then allow downgrading.
Downgrading is done:
- when the server does not support EDNS0+DO
- or when the server supports it but does not augment returned RRs with
RRSIGs. The latter is detected when requesting DS or SOA RRs for the
root domain (which is necessary to do proofs for unsigned data)
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The call already updates possible_features, it's pointless doing this in
the caller a second time.
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We previously set it once in the scope code and once in the stream code.
Remove it from the latter, as all other socket options are set in the
former.
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via TCP
Previously, if we couldn't reach a server via UDP we'd generate an
MAX_ATTEMPTS transaction result, but if we couldn't reach it via TCP
we'd generate a RESOURCES transaction result. While it is OK to generate
two different errors I think, "RESOURCES" is certainly a misnomer.
Introduce a new transaction result "CONNECTION_FAILURE" instead.
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Previously, we'd insist on an RRSIG for all DS/NSEC/NSEC3 RRs. With this
change we don't do that anymore, but also allow unsigned DS/NSEC/NSEC3
if we can prove that the zone they are located in is unsigned.
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This collects statistical data about transactions, dnssec verifications
and the cache, and exposes it over the bus. The systemd-resolve-host
tool learns new options to query these statistics and reset them.
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But keep track that the proof is not authenticated.
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Be stricter when searching suitable NSEC3 RRs for proof: generalize the
check we use to find suitable NSEC3 RRs, in nsec3_is_good(), and add
additional checks, such as checking whether all NSEC3 RRs use the same
parameters, have the same suffix and so on.
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When doing an NSEC3 proof, before detrmining whether a name is the
closest encloser we first need to figure out the longest common suffix
we have with any NSEC3 RR in the reply.
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Note that this is still not complete, one additional step is still
missing: when we verified that a wildcard RRset is properly signed, we
still need to do an NSEC/NSEC3 proof that no more specific RRset exists.
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Otherwise if we have an A lookup that failed DNSSEC validation, but an
AAAA lookup that succeeded, we might end up using the A data, but we
really should not.
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This might happen in some cases (empty non-terminals...) and we should
not choke on it.
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Let's simplify usage and memory management of DnsResourceRecord's
dns_resource_record_to_string() call: cache the formatted string as
part of the object, and return it on subsequent calls, freeing it when
the DnsResourceRecord itself is freed.
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If there are multiple SOA RRs, and we look for a suitable one covering
our request, then make sure to pick the one that is furthest away from
the root name, not just the first one we encounter.
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entries
We use ANY RR keys to store NXDOMAIN information, but we previously
didn't flush out old ANY RR items in the cache when adding new entries.
Fix that.
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Let's abstract which RRs shall honour CNAMEs, and which ones should not.
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Let's make sure we propagate the DNSSEC validation status from an
auxiliary DNSSEC transaction back to the originating transaction, to
improve the error messages we generate.
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the the bus client
It's useful to generate useful errors, so let's do that.
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We have many types of failure for a transaction, and
DNS_TRANSACTION_FAILURE was just one specific one of them, if the server
responded with a non-zero RCODE. Hence let's rename this, to indicate
which kind of failure this actually refers to.
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In most of the other call, we called similar functions that remove the
data structure link-ups to other objects "unlink", hence we should here,
too.
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Given that we already have dns_cache_remove_by_rr() this makes clearer
what the operation actually does.
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This is not used anywhere, but it's extremely useful when debugging.
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This adds a new DnsAnswer item flag "DNS_ANSWER_SHARED_OWNER" which is
set for mDNS RRs that lack the cache-flush bit. The cache-flush bit is
removed from the DnsResourceRecord object in favour of this.
This also splits out the code that removes previous entries when adding
new positive ones into a new separate call dns_cache_remove_previous().
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After all we want to allow NULL DnsAnswer objects as equivalent to empty
ones, hence we should use the right checks everywhere.
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Make sure the cache never altes the authenticated bit of RRs stored in
it, and drops it for RRs when passing it out again.
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OPT RRs after all use the class field for other purposes than actually
encoding a class, hence the cache flush bit doesn't apply really.
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Let's use dns_cache_remove() rather than
dns_cache_item_remove_and_free() to destroy the cache, since the former
requires far fewer hash table lookups.
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When we receieve a TTL=0 RR, then let's only flush that specific RR and
not the whole RRset.
On mDNS with RRsets that a shared-owner this is how specific RRs are
removed from the set, hence support this. And on non-mDNS the whole
RRset will already be removed much earlier in dns_cache_put() hence
there's no reason remove it again.
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We should never use the TTL of an unauthenticated SOA to cache an
authenticated RR.
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We call it anyway as one of the first calls in dns_cache_put(), hence
there's no reason to do this multiple times.
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Let's keep entries for longer and more of them. After all, due to the
DNSSEC hookup the amount of RRs we need to store is much higher now.
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Let's make DNS class helpers more like DNS type helpers, let's move them
from resolved-dns-rr.[ch] into dns-type.[ch].
This also adds two new calls dns_class_is_pseudo() and
dns_class_is_valid_rr() which operate similar to dns_type_is_pseudo()
and dns_type_is_valid_rr() but for classes instead of types.
This should hopefully make handling of DNS classes and DNS types more
alike.
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OK to be unsigned
This large patch adds a couple of mechanisms to ensure we get NSEC3 and
proof-of-unsigned support into place. Specifically:
- Each item in an DnsAnswer gets two bit flags now:
DNS_ANSWER_AUTHENTICATED and DNS_ANSWER_CACHEABLE. The former is
necessary since DNS responses might contain signed as well as unsigned
RRsets in one, and we need to remember which ones are signed and which
ones aren't. The latter is necessary, since not we need to keep track
which RRsets may be cached and which ones may not be, even while
manipulating DnsAnswer objects.
- The .n_answer_cachable of DnsTransaction is dropped now (it used to
store how many of the first DnsAnswer entries are cachable), and
replaced by the DNS_ANSWER_CACHABLE flag instead.
- NSEC3 proofs are implemented now (lacking support for the wildcard
part, to be added in a later commit).
- Support for the "AD" bit has been dropped. It's unsafe, and now that
we have end-to-end authentication we don't need it anymore.
- An auxiliary DnsTransaction of a DnsTransactions is now kept around as
least as long as the latter stays around. We no longer remove the
auxiliary DnsTransaction as soon as it completed. THis is necessary,
as we now are interested not only in the RRsets it acquired but also
in its authentication status.
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Let's be safe and explicitly avoid that we add an auxiliary transaction
dependency on ourselves.
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The DNS_ANSWER_FOREACH macros do this internally anyway, no need to
duplicate this.
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