Since we honour RFC5011 revoked keys it might happen we end up with an
empty trust anchor, or one where there's no entry for the root left.
With this patch the logic is changed what to do in this case.
Before this patch we'd end up requesting the root DS, which returns with
NODATA but a signed NSEC we cannot verify, since the trust anchor is
empty after all. Thus we'd return a DNSSEC result of "missing-key", as
we lack a verified version of the key.
With this patch in place, look-ups for the root DS are explicitly
recognized, and not passed on to the DNS servers. Instead, if
downgrade-ok mode is on an unsigned NODATA response is synthesized, so
that the validator code continues under the assumption the root zone was
unsigned. If downgrade-ok mode is off a new transaction failure is
generated, that makes this case recognizable.
We already try hard not to create cyclic transaction dependencies, where
a transaction requires another one for DNSSEC validation purposes, which
in turn (possibly indirectly) pulls in the original transaction again,
thus resulting in a cyclic dependency and ultimately a deadlock since
each transaction waits for another one forever.
So far we wanted to avoid such cyclic dependencies by only going "up the
tree" when requesting auxiliary RRs and only going from one RR type to
another, but never back. However this turned out to be insufficient.
Consider a domain that publishes one or more DNSKEY but which has no DS
for it. A request for the domain's DNSKEY triggers a request for the
domain's DS, which will then fail, but return an NSEC, signed by the
DNSKEY. To validate that we'd request the DNSKEY again. Thus a DNSKEY
request results in a DS request which results in the original DNSKEY
request again. If the original lookup had been a DS lookup we'd end up
in the same cyclic dependency, hence we cannot statically break one of
them, since both requests are of course fully valid. Hence, do full
cyclic dependency checking: each time we are about to add a dependency
to a transaction, check if the transaction is already a dependency of
the dependency (recursively down the tree).
If any of the transactions started by
dns_transaction_request_dnssec_keys() finishes promptly without
requiring asynchronous operation this is reported back to the issuing
transaction from the same stackframe. This might ultimately result in
this transaction to be freed while we are still in its
_request_dnssec_keys() stack frame. To avoid memory corruption block the
transaction GC while in the call, and manually issue a GC after it
returned.
With this patch resolved will properly handle revoked keys, but not
augment the locally configured trust anchor database with newly learned
keys.
Specifically, resolved now refuses validating RRsets with
revoked keys, and it will remove revoked keys from the configured trust
anchors (only until reboot).
This patch does not add logic for adding new keys to the set of trust
anchors. This is a deliberate decision as this only can work with
persistent disk storage, and would result in a different update logic
for stateful and stateless systems. Since we have to support stateless
systems anyway, and don't want to encourage two independent upgrade
paths we focus on upgrading the trust anchor database via the usual OS
upgrade logic.
Whenever a trust anchor entry is found revoked and removed from the
trust anchor a recognizable log message is written, encouraging the user
to update the trust anchor or update his operating system.
When applying canonical DNSSEC ordering for an RRset only order by the
wire format of the RRs' RDATA, not by the full wire formatting. The RFC
isn't particularly clear about this, but this is apparently how it is
done. This fixes validation of pentagon.gov's DS RRset.
This adds negative trust anchor support and allows reading trust anchor
data from disk, from files
/etc/systemd/dnssec-trust-anchors.d/*.positive and
/etc/systemd/dnssec-trust-anchros.d/*.negative, as well as the matching
counterparts in /usr/lib and /run.
The positive trust anchor files are more or less compatible to normal
DNS zone files containing DNSKEY and DS RRs. The negative trust anchor
files contain only new-line separated hostnames for which to require no
signing.
By default no trust anchor files are installed, in which case the
compiled-in root domain DS RR is used, as before. As soon as at least
one positive root anchor for the root is defined via trust anchor files
this buil-in DS RR is not added though.
For NXDOMAIN, it is not sufficient to prove that the next-closest
enclosure does not exist, we must also prove that there is no
wildcard domain directly below the closest enclosure which would
synthesise the name that has been requested.
For positive responses, in addition to exact matches, we should
accept wildcard ones. In that case we must first prove that
there is no precise match (i.e., that the closest encounter
is not the record itself) and secondly that the source of
synthesis exists.
Previously, we'd use the same set of identifiers for both, but that's
actually incorrect. It didn't matter much since the only NSEC3 hash
algorithm defined (SHA-1) is mapped to code 1 which is also what it is
encoded as in DS digests, but we really should make sure to use two
distinct enumerations.
We don't implement it, and we have no intention to, but at least mention
that it exists.
(This also adds a couple of other algorithms to the algorithm string
list, where these strings were missing previously.)
