resolved: cache - handle CNAME redirection

CNAME records are special in the way they are treated by DNS servers,
and our cache should mimic that behavior: In case a domain name has an
alias, its CNAME record is returned in place of any other.

Our cache was not doing this despite caching the CNAME records, this
entailed needless lookups to re-resolve the CNAME.

1 files changed, 24 insertions, 1 deletions

diff --git a/src/resolve/resolved-dns-cache.c b/src/resolve/resolved-dns-cache.c
index 478ad29bbf..93b98f3727 100644
--- a/src/resolve/resolved-dns-cache.c
+++ b/src/resolve/resolved-dns-cache.c
@@ -493,6 +493,29 @@ fail:
         return r;
 }
 
+static DnsCacheItem *dns_cache_get_by_key_follow_cname(DnsCache *c, DnsResourceKey *k) {
+        _cleanup_(dns_resource_key_unrefp) DnsResourceKey *cname_key = NULL;
+        DnsCacheItem *i, *j;
+
+        assert(c);
+        assert(k);
+
+        i = hashmap_get(c->by_key, k);
+        if (i || k->type == DNS_TYPE_CNAME)
+                return i;
+
+        /* check if we have a CNAME record instead */
+        cname_key = dns_resource_key_new_cname(k);
+        if (!cname_key)
+                return NULL;
+
+        j = hashmap_get(c->by_key, cname_key);
+        if (j)
+                return j;
+
+        return i;
+}
+
 int dns_cache_lookup(DnsCache *c, DnsResourceKey *key, int *rcode, DnsAnswer **ret) {
         _cleanup_(dns_answer_unrefp) DnsAnswer *answer = NULL;
         unsigned n = 0;
@@ -522,7 +545,7 @@ int dns_cache_lookup(DnsCache *c, DnsResourceKey *key, int *rcode, DnsAnswer **r
                 return 0;
         }
 
-        first = hashmap_get(c->by_key, key);
+        first = dns_cache_get_by_key_follow_cname(c, key);
         if (!first) {
                 /* If one question cannot be answered we need to refresh */