resolved: explicitly handle case when the trust anchor is empty

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.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/resolve/resolved-bus.c b/src/resolve/resolved-bus.c
index 2c79204f0f..db180a51a3 100644
--- a/src/resolve/resolved-bus.c
+++ b/src/resolve/resolved-bus.c
@@ -67,6 +67,9 @@ static int reply_query_state(DnsQuery *q) {
                 return sd_bus_reply_method_errorf(q->request, BUS_ERROR_DNSSEC_FAILED, "DNSSEC validation failed: %s",
                                                   dnssec_result_to_string(q->answer_dnssec_result));
 
+        case DNS_TRANSACTION_NO_TRUST_ANCHOR:
+                return sd_bus_reply_method_errorf(q->request, BUS_ERROR_NO_TRUST_ANCHOR, "No suitable trust anchor known");
+
         case DNS_TRANSACTION_RCODE_FAILURE: {
                 _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;