resolved: cache - do negative caching only on the canonical name

Apart from dropping redundant information, this fixes an issue
where, due to broken DNS servers, we can only be certain of whether
an apparent NODATA response is in fact an NXDOMAIN response after
explicitly resolving the canonical name. This issue is outlined in
RFC2308. Moreover, by caching NXDOMAIN for an existing name, we
would mistakenly return NXDOMAIN for types which should not be
redirected. I.e., a query for AAAA on test-nx-1.jklm.no correctly
returns NXDOMAIN, but a query for CNAME should return the record
and a query for DNAME should return NODATA.

Note that this means we will not cache an NXDOMAIN response in the
presence of redirection, meaning one redundant roundtrip in case the
name is queried again.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/resolve/resolved-dns-answer.c b/src/resolve/resolved-dns-answer.c
index de8c4d9dd3..55e6ffbad7 100644
--- a/src/resolve/resolved-dns-answer.c
+++ b/src/resolve/resolved-dns-answer.c
@@ -249,6 +249,29 @@ int dns_answer_find_soa(DnsAnswer *a, const DnsResourceKey *key, DnsResourceReco
         return 0;
 }
 
+int dns_answer_find_cname_or_dname(DnsAnswer *a, const DnsResourceKey *key, DnsResourceRecord **ret) {
+        DnsResourceRecord *rr;
+
+        assert(key);
+
+        if (!a)
+                return 0;
+
+        /* For a {C,D}NAME record we can never find a matching {C,D}NAME record */
+        if (key->type == DNS_TYPE_CNAME || key->type == DNS_TYPE_DNAME)
+                return 0;
+
+        DNS_ANSWER_FOREACH(rr, a) {
+                if (dns_resource_key_match_cname_or_dname(key, rr->key, NULL)) {
+                        if (ret)
+                                *ret = rr;
+                        return 1;
+                }
+        }
+
+        return 0;
+}
+
 int dns_answer_merge(DnsAnswer *a, DnsAnswer *b, DnsAnswer **ret) {
         _cleanup_(dns_answer_unrefp) DnsAnswer *k = NULL;
         int r;