Try to be a little more clever at falling back to smaller DNS packet sizes.

src/config.h

@@ -26,6 +26,7 @@
 #define TIMEOUT 10 /* drop UDP queries after TIMEOUT seconds */
 #define FORWARD_TEST 50 /* try all servers every 50 queries */
 #define FORWARD_TIME 20 /* or 20 seconds */
+#define UDP_TEST_TIME 60 /* How often to reset our idea of max packet size. */
 #define SERVERS_LOGGED 30 /* Only log this many servers when logging state */
 #define LOCALS_LOGGED 8 /* Only log this many local addresses when logging state */
 #define RANDOM_SOCKS 64 /* max simultaneous random ports */

src/dnsmasq.h

@@ -520,6 +520,7 @@ struct server {
   struct serverfd *sfd; 
   char *domain; /* set if this server only handles a domain. */ 
   int flags, tcpfd, edns_pktsz;
+  time_t pktsz_reduced;
   unsigned int queries, failed_queries;
 #ifdef HAVE_LOOP
   u32 uid;

src/forward.c

@@ -272,14 +272,13 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,
 	  while (forward->blocking_query)
 	    forward = forward->blocking_query;
 	   
-	  forward->flags |= FREC_TEST_PKTSZ;
-	  
 	  blockdata_retrieve(forward->stash, forward->stash_len, (void *)header);
 	  plen = forward->stash_len;
 	  
+	  forward->flags |= FREC_TEST_PKTSZ;
 	  if (find_pseudoheader(header, plen, NULL, &pheader, &is_sign, NULL) && !is_sign)
 	    PUTSHORT(SAFE_PKTSZ, pheader);
-
+	  
 	  if (forward->sentto->addr.sa.sa_family == AF_INET) 
 	    log_query(F_NOEXTRA | F_DNSSEC | F_IPV4, "retry", (struct all_addr *)&forward->sentto->addr.in.sin_addr, "dnssec");
 #ifdef HAVE_IPV6
@@ -401,7 +400,8 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,
       int subnet, forwarded = 0;
       size_t edns0_len;
       unsigned char *oph = find_pseudoheader(header, plen, NULL, NULL, NULL, NULL);
-
+      unsigned char *pheader;
+      
       /* If a query is retried, use the log_id for the retry when logging the answer. */
       forward->log_id = daemon->log_id;
       
@@ -423,7 +423,7 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,
 	}
 #endif
 
-      if (find_pseudoheader(header, plen, &edns0_len, NULL, NULL, NULL))
+      if (find_pseudoheader(header, plen, &edns0_len, &pheader, NULL, NULL))
 	{
 	  /* If there wasn't a PH before, and there is now, we added it. */
 	  if (!oph)
@@ -432,6 +432,10 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,
 	  /* If we're sending an EDNS0 with any options, we can't recreate the query from a reply. */
 	  if (edns0_len > 11)
 	    forward->flags |= FREC_HAS_EXTRADATA;
+
+	  /* Reduce udp size on retransmits. */
+	  if (forward->flags & FREC_TEST_PKTSZ)
+	    PUTSHORT(SAFE_PKTSZ, pheader);
 	}
       
       while (1)
@@ -764,7 +768,11 @@ void reply_query(int fd, int family, time_t now)
   
   if (!server)
     return;
-  
+
+  /* If sufficient time has elapsed, try and expand UDP buffer size again. */
+  if (difftime(now, server->pktsz_reduced) > UDP_TEST_TIME)
+    server->edns_pktsz = daemon->edns_pktsz;
+
 #ifdef HAVE_DNSSEC
   hash = hash_questions(header, n, daemon->namebuff);
 #else
@@ -853,14 +861,16 @@ void reply_query(int fd, int family, time_t now)
   /* We tried resending to this server with a smaller maximum size and got an answer.
      Make that permanent. To avoid reduxing the packet size for a single dropped packet,
      only do this when we get a truncated answer, or one larger than the safe size. */
-  if (server && (forward->flags & FREC_TEST_PKTSZ) && 
+  if (server && server->edns_pktsz > SAFE_PKTSZ && (forward->flags & FREC_TEST_PKTSZ) && 
       ((header->hb3 & HB3_TC) || n >= SAFE_PKTSZ))
     {
       server->edns_pktsz = SAFE_PKTSZ;
+      server->pktsz_reduced = now;
       prettyprint_addr(&server->addr, daemon->addrbuff);
       my_syslog(LOG_WARNING, _("reducing DNS packet size for nameserver %s to %d"), daemon->addrbuff, SAFE_PKTSZ);
     }
-  
+
+    
   /* If the answer is an error, keep the forward record in place in case
      we get a good reply from another server. Kill it when we've
      had replies from all to avoid filling the forwarding table when