From 5643c00afe29eae4b2e3575277038e60e6967d09 Mon Sep 17 00:00:00 2001
From: Tom Gundersen <teg@jklm.no>
Date: Mon, 17 Aug 2015 07:56:57 +0200
Subject: 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.
---
 src/resolve/resolved-bus.c       |  2 --
 src/resolve/resolved-dns-cache.c | 25 ++++++++++++++++++++++++-
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/src/resolve/resolved-bus.c b/src/resolve/resolved-bus.c
index 19db781ac4..bf1b7c8ab4 100644
--- a/src/resolve/resolved-bus.c
+++ b/src/resolve/resolved-bus.c
@@ -220,8 +220,6 @@ static void bus_method_resolve_hostname_complete(DnsQuery *q) {
                         added++;
                 }
 
-                // what about the cache?
-
                 /* If we didn't find anything, then let's restart the
                  * query, this time with the cname */
                 if (added <= 0) {
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 */
 
-- 
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