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, 1 insertions, 1 deletions

diff --git a/src/resolve/resolved-dns-rr.h b/src/resolve/resolved-dns-rr.h
index 4c0f72eea3..632ee59994 100644
--- a/src/resolve/resolved-dns-rr.h
+++ b/src/resolve/resolved-dns-rr.h
@@ -246,7 +246,7 @@ DnsResourceKey* dns_resource_key_unref(DnsResourceKey *key);
 bool dns_resource_key_is_address(const DnsResourceKey *key);
 int dns_resource_key_equal(const DnsResourceKey *a, const DnsResourceKey *b);
 int dns_resource_key_match_rr(const DnsResourceKey *key, const DnsResourceRecord *rr, const char *search_domain);
-int dns_resource_key_match_cname(const DnsResourceKey *key, const DnsResourceRecord *rr, const char *search_domain);
+int dns_resource_key_match_cname_or_dname(const DnsResourceKey *key, const DnsResourceKey *cname, const char *search_domain);
 int dns_resource_key_match_soa(const DnsResourceKey *key, const DnsResourceKey *soa);
 int dns_resource_key_to_string(const DnsResourceKey *key, char **ret);
 DEFINE_TRIVIAL_CLEANUP_FUNC(DnsResourceKey*, dns_resource_key_unref);