dnsproxy: Track cache hit statistics

For various decisions (which entries to prune, which entries to refresh)
and for debugging, it's useful to track how many hits a cache entry
gets.

This patch adds the basic counting for this; the only tricky aspect
is that we serve DNS entries out of the cache, even in case of a miss,
so we need to compensate for that by subtracting a hit first.

diff --git a/src/dnsproxy.c b/src/dnsproxy.c
index a30f4c4..d2fa07c 100644 (file)
--- a/src/dnsproxy.c
+++ b/src/dnsproxy.c
@@ -138,6 +138,7 @@ struct cache_data {
 
 struct cache_entry {
        char *key;
+       int hits;
        struct cache_data *ipv4;
        struct cache_data *ipv6;
 };
@@ -1085,6 +1086,13 @@ static int cache_update(struct server_data *srv, unsigned char *msg,
                        data->cache_until = entry->ipv4->cache_until;
                        memcpy(data->data, msg, msg_len);
                        entry->ipv6 = data;
+                       /*
+                        * we will get a "hit" when we serve the response
+                        * out of the cache
+                        */
+                       entry->hits--;
+                       if (entry->hits < 0)
+                               entry->hits = 0;
                        return 0;
                }
        }
@@ -1116,6 +1124,7 @@ static int cache_update(struct server_data *srv, unsigned char *msg,
 
                entry->key = g_strdup(question);
                entry->ipv4 = entry->ipv6 = NULL;
+               entry->hits = 0;
 
                if (type == 1)
                        entry->ipv4 = data;
@@ -1137,6 +1146,14 @@ static int cache_update(struct server_data *srv, unsigned char *msg,
                else
                        entry->ipv6 = data;
 
+               /*
+                * compensate for the hit we'll get for serving
+                * the response out of the cache
+                */
+               entry->hits--;
+               if (entry->hits < 0)
+                       entry->hits = 0;
+
                new_entry = FALSE;
        }
 
@@ -1208,8 +1225,10 @@ static int ns_resolv(struct server_data *server, struct request_data *req,
                else
                        data = entry->ipv6;
 
-               if (data)
+               if (data) {
                        ttl_left = data->valid_until - time(0);
+                       entry->hits++;
+               }
 
                if (data != NULL && req->protocol == IPPROTO_TCP) {
                        send_cached_response(req->client_sk, data->data,