resolved: cache - rework which RR types we apply redirection to

The logic of dns_cache_get() is now:
 - look up the precise key;
 - look up NXDOMAIN item;
 - if an RR type that may be redirected
   (i.e., not CNAME, DNAME, RRSIG, NSEC, NSEC3, SIG, KEY, or
   NXT) look up a correpsonding CNAME or DNAME record;
 - look up a corresponding NSEC record;

Before this change we would give up before potentially finding
negative cache entries for DNAME, CNAME and NSEC records, we
would return NSEC records for aliases where we had DNAME or CNAME
records available and we would incorrectly try to redirect DNSSEC RRs.

1 files changed, 28 insertions, 20 deletions

diff --git a/src/resolve/resolved-dns-cache.c b/src/resolve/resolved-dns-cache.c
index a2cbdd8259..794d587d4d 100644
--- a/src/resolve/resolved-dns-cache.c
+++ b/src/resolve/resolved-dns-cache.c
@@ -548,7 +548,7 @@ static DnsCacheItem *dns_cache_get_by_key_follow_cname_dname_nsec(DnsCache *c, D
          * much, after all this is just a cache */
 
         i = hashmap_get(c->by_key, k);
-        if (i || IN_SET(k->type, DNS_TYPE_CNAME, DNS_TYPE_DNAME, DNS_TYPE_NSEC))
+        if (i)
                 return i;
 
         n = DNS_RESOURCE_KEY_NAME(k);
@@ -559,31 +559,39 @@ static DnsCacheItem *dns_cache_get_by_key_follow_cname_dname_nsec(DnsCache *c, D
         if (i && i->type == DNS_CACHE_NXDOMAIN)
                 return i;
 
-        /* Check if we have an NSEC record instead for the name. */
-        i = hashmap_get(c->by_key, &DNS_RESOURCE_KEY_CONST(k->class, DNS_TYPE_NSEC, n));
-        if (i)
-                return i;
+        /* The following record types should never be redirected. See
+         * <https://tools.ietf.org/html/rfc4035#section-2.5>. */
+        if (!IN_SET(k->type, DNS_TYPE_CNAME, DNS_TYPE_DNAME,
+                            DNS_TYPE_NSEC3, DNS_TYPE_NSEC, DNS_TYPE_RRSIG,
+                            DNS_TYPE_NXT, DNS_TYPE_SIG, DNS_TYPE_KEY)) {
+                /* Check if we have a CNAME record instead */
+                i = hashmap_get(c->by_key, &DNS_RESOURCE_KEY_CONST(k->class, DNS_TYPE_CNAME, n));
+                if (i)
+                        return i;
 
-        /* Check if we have a CNAME record instead */
-        i = hashmap_get(c->by_key, &DNS_RESOURCE_KEY_CONST(k->class, DNS_TYPE_CNAME, n));
-        if (i)
-                return i;
+                /* OK, let's look for cached DNAME records. */
+                for (;;) {
+                        char label[DNS_LABEL_MAX];
 
-        /* OK, let's look for cached DNAME records. */
-        for (;;) {
-                char label[DNS_LABEL_MAX];
+                        if (isempty(n))
+                                return NULL;
+
+                        i = hashmap_get(c->by_key, &DNS_RESOURCE_KEY_CONST(k->class, DNS_TYPE_DNAME, n));
+                        if (i)
+                                return i;
 
-                if (isempty(n))
-                        return NULL;
+                        /* Jump one label ahead */
+                        r = dns_label_unescape(&n, label, sizeof(label));
+                        if (r <= 0)
+                                return NULL;
+                }
+        }
 
-                i = hashmap_get(c->by_key, &DNS_RESOURCE_KEY_CONST(k->class, DNS_TYPE_DNAME, n));
+        if (k-> type != DNS_TYPE_NSEC) {
+                /* Check if we have an NSEC record instead for the name. */
+                i = hashmap_get(c->by_key, &DNS_RESOURCE_KEY_CONST(k->class, DNS_TYPE_NSEC, n));
                 if (i)
                         return i;
-
-                /* Jump one label ahead */
-                r = dns_label_unescape(&n, label, sizeof(label));
-                if (r <= 0)
-                        return NULL;
         }
 
         return NULL;