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.

7 files changed, 47 insertions, 14 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;
diff --git a/src/resolve/resolved-dns-answer.h b/src/resolve/resolved-dns-answer.h
index 8d95131dbe..56b462ed7e 100644
--- a/src/resolve/resolved-dns-answer.h
+++ b/src/resolve/resolved-dns-answer.h
@@ -57,6 +57,7 @@ int dns_answer_match_key(DnsAnswer *a, const DnsResourceKey *key);
 int dns_answer_contains_rr(DnsAnswer *a, DnsResourceRecord *rr);
 
 int dns_answer_find_soa(DnsAnswer *a, const DnsResourceKey *key, DnsResourceRecord **ret);
+int dns_answer_find_cname_or_dname(DnsAnswer *a, const DnsResourceKey *key, DnsResourceRecord **ret);
 
 int dns_answer_merge(DnsAnswer *a, DnsAnswer *b, DnsAnswer **ret);
 int dns_answer_extend(DnsAnswer **a, DnsAnswer *b);
diff --git a/src/resolve/resolved-dns-cache.c b/src/resolve/resolved-dns-cache.c
index 4aacc268e2..008277ab09 100644
--- a/src/resolve/resolved-dns-cache.c
+++ b/src/resolve/resolved-dns-cache.c
@@ -479,6 +479,15 @@ int dns_cache_put(
         if (r > 0)
                 return 0;
 
+        /* But not if it has a matching CNAME/DNAME (the negative
+         * caching will be done on the canonical name, not on the
+         * alias) */
+        r = dns_answer_find_cname_or_dname(answer, key, NULL);
+        if (r < 0)
+                goto fail;
+        if (r > 0)
+                return 0;
+
         /* See https://tools.ietf.org/html/rfc2308, which say that a
          * matching SOA record in the packet is used to to enable
          * negative caching. */
diff --git a/src/resolve/resolved-dns-question.c b/src/resolve/resolved-dns-question.c
index 3249448d3b..4ed7434d3c 100644
--- a/src/resolve/resolved-dns-question.c
+++ b/src/resolve/resolved-dns-question.c
@@ -117,7 +117,7 @@ int dns_question_matches_cname(DnsQuestion *q, DnsResourceRecord *rr, const char
                 return 0;
 
         for (i = 0; i < q->n_keys; i++) {
-                r = dns_resource_key_match_cname(q->keys[i], rr, search_domain);
+                r = dns_resource_key_match_cname_or_dname(q->keys[i], rr->key, search_domain);
                 if (r != 0)
                         return r;
         }
diff --git a/src/resolve/resolved-dns-rr.c b/src/resolve/resolved-dns-rr.c
index 55e85eec2b..74c9d87319 100644
--- a/src/resolve/resolved-dns-rr.c
+++ b/src/resolve/resolved-dns-rr.c
@@ -220,19 +220,19 @@ int dns_resource_key_match_rr(const DnsResourceKey *key, const DnsResourceRecord
         return 0;
 }
 
-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 r;
 
         assert(key);
-        assert(rr);
+        assert(cname);
 
-        if (rr->key->class != key->class && key->class != DNS_CLASS_ANY)
+        if (cname->class != key->class && key->class != DNS_CLASS_ANY)
                 return 0;
 
-        if (rr->key->type == DNS_TYPE_CNAME)
-                r = dns_name_equal(DNS_RESOURCE_KEY_NAME(key), DNS_RESOURCE_KEY_NAME(rr->key));
-        else if (rr->key->type == DNS_TYPE_DNAME)
-                r = dns_name_endswith(DNS_RESOURCE_KEY_NAME(key), DNS_RESOURCE_KEY_NAME(rr->key));
+        if (cname->type == DNS_TYPE_CNAME)
+                r = dns_name_equal(DNS_RESOURCE_KEY_NAME(key), DNS_RESOURCE_KEY_NAME(cname));
+        else if (cname->type == DNS_TYPE_DNAME)
+                r = dns_name_endswith(DNS_RESOURCE_KEY_NAME(key), DNS_RESOURCE_KEY_NAME(cname));
         else
                 return 0;
 
@@ -246,10 +246,10 @@ int dns_resource_key_match_cname(const DnsResourceKey *key, const DnsResourceRec
                 if (r < 0)
                         return r;
 
-                if (rr->key->type == DNS_TYPE_CNAME)
-                        return dns_name_equal(joined, DNS_RESOURCE_KEY_NAME(rr->key));
-                else if (rr->key->type == DNS_TYPE_DNAME)
-                        return dns_name_endswith(joined, DNS_RESOURCE_KEY_NAME(rr->key));
+                if (cname->type == DNS_TYPE_CNAME)
+                        return dns_name_equal(joined, DNS_RESOURCE_KEY_NAME(cname));
+                else if (cname->type == DNS_TYPE_DNAME)
+                        return dns_name_endswith(joined, DNS_RESOURCE_KEY_NAME(cname));
         }
 
         return 0;
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);
diff --git a/src/resolve/resolved-dns-transaction.c b/src/resolve/resolved-dns-transaction.c
index efed761001..61be38a6cd 100644
--- a/src/resolve/resolved-dns-transaction.c
+++ b/src/resolve/resolved-dns-transaction.c
@@ -1390,7 +1390,7 @@ int dns_transaction_validate_dnssec(DnsTransaction *t) {
                                 }
 
                                 /* Is this a CNAME for a record we were looking for? If so, it's also fatal for the whole transaction */
-                                r = dns_resource_key_match_cname(t->key, rr, NULL);
+                                r = dns_resource_key_match_cname_or_dname(t->key, rr->key, NULL);
                                 if (r < 0)
                                         return r;
                                 if (r > 0) {