resolved: add support NSEC3 proofs, as well as proofs for domains that are 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.

1 files changed, 11 insertions, 2 deletions

diff --git a/src/resolve/resolved-dns-transaction.h b/src/resolve/resolved-dns-transaction.h
index 5f6a699454..f6ec8e5ead 100644
--- a/src/resolve/resolved-dns-transaction.h
+++ b/src/resolve/resolved-dns-transaction.h
@@ -75,12 +75,21 @@ struct DnsTransaction {
         DnsPacket *sent, *received;
 
         DnsAnswer *answer;
-        unsigned n_answer_cacheable; /* Specifies how many RRs of the answer shall be cached, from the beginning */
         int answer_rcode;
         DnsTransactionSource answer_source;
+
+        /* Indicates whether the primary answer is authenticated,
+         * i.e. whether the RRs from answer which directly match the
+         * question are authenticated, or, if there are none, whether
+         * the NODATA or NXDOMAIN case is. It says nothing about
+         * additional RRs listed in the answer, however they have
+         * their own DNS_ANSWER_AUTHORIZED FLAGS. Note that this bit
+         * is defined different than the AD bit in DNS packets, as
+         * that covers more than just the actual primary answer. */
         bool answer_authenticated;
 
-        /* Contains DS and DNSKEY RRs we already verified and need to authenticate this reply */
+        /* Contains DNSKEY, DS, SOA RRs we already verified and need
+         * to authenticate this reply */
         DnsAnswer *validated_keys;
 
         usec_t start_usec;