Improve connection handling when talking to TCP upsteam servers.

Specifically, be prepared to open a new connection when we
want to make multiple queries but the upstream server accepts
fewer queries per connection.

CHANGELOG

@@ -65,6 +65,11 @@ version 2.77
 	    Thanks to Kevin Darbyshire-Bryant and Eric Luehrsen
 	    for pushing this.
 
+            Improve connection handling when talking to TCP upsteam 
+	    servers. Specifically, be prepared to open a new TCP
+	    connection when we want to make multiple queries
+            but the upstream server accepts fewer queries per connection.
+
 	
 version 2.76
             Include 0.0.0.0/8 in DNS rebind checks. This range 

src/dnsmasq.h

@@ -485,6 +485,7 @@ union mysockaddr {
 #define SERV_FROM_FILE      4096  /* read from --servers-file */
 #define SERV_LOOP           8192  /* server causes forwarding loop */
 #define SERV_DO_DNSSEC     16384  /* Validate DNSSEC when using this server */
+#define SERV_GOT_TCP       32768  /* Got some data from the TCP connection */
 
 struct serverfd {
   int fd;

src/forward.c

@@ -1459,14 +1459,15 @@ static int tcp_key_recurse(time_t now, int status, struct dns_header *header, si
   unsigned char *payload = NULL;
   struct dns_header *new_header = NULL;
   u16 *length = NULL;
-
+ 
   while (1)
     {
       int type = SERV_DO_DNSSEC;
       char *domain;
       size_t m; 
       unsigned char c1, c2;
-            
+      struct server *firstsendto = NULL;
+      
       /* limit the amount of work we do, to avoid cycling forever on loops in the DNS */
       if (--(*keycount) == 0)
 	new_status = STAT_ABANDONED;
@@ -1504,81 +1505,86 @@ static int tcp_key_recurse(time_t now, int status, struct dns_header *header, si
       /* Find server to forward to. This will normally be the 
 	 same as for the original query, but may be another if
 	 servers for domains are involved. */		      
-      if (search_servers(now, NULL, F_QUERY, keyname, &type, &domain, NULL) == 0)
+      if (search_servers(now, NULL, F_QUERY, keyname, &type, &domain, NULL) != 0)
 	{
-	  struct server *start = server, *new_server = NULL;
-	  type &= ~SERV_DO_DNSSEC;
-			   
-	  while (1)
+	  new_status = STAT_ABANDONED;
+	  break;
+	}
+	
+      type &= ~SERV_DO_DNSSEC;
+      
+      while (1)
+	{
+	  if (!firstsendto)
+	    firstsendto = server;
+	  else
 	    {
-	      if (type == (start->flags & SERV_TYPE) &&
-		  (type != SERV_HAS_DOMAIN || hostname_isequal(domain, start->domain)) &&
-		  !(start->flags & (SERV_LITERAL_ADDRESS | SERV_LOOP)))
+	      if (!(server = server->next))
+		server = daemon->servers;
+	      if (server == firstsendto)
 		{
-		  new_server = start;
-		  if (server == start)
-		    {
-		      new_server = NULL;
-		      break;
-		    }
+		  /* can't find server to accept our query. */
+		  new_status = STAT_ABANDONED;
+		  break;
 		}
-			       
-	      if (!(start = start->next))
-		start = daemon->servers;
-	      if (start == server)
-		break;
 	    }
-       
-
-	  if (new_server)
+	  
+	  if (type != (server->flags & SERV_TYPE) ||
+	      (type == SERV_HAS_DOMAIN && !hostname_isequal(domain, server->domain)) ||
+	      (server->flags & (SERV_LITERAL_ADDRESS | SERV_LOOP)))
+	    continue;
+	  
+	retry:
+	  /* may need to make new connection. */
+	  if (server->tcpfd == -1)
 	    {
-	      server = new_server;
-	      /* may need to make new connection. */
-	      if (server->tcpfd == -1)
-		{
-		  if ((server->tcpfd = socket(server->addr.sa.sa_family, SOCK_STREAM, 0)) == -1)
-		    {
-		      new_status = STAT_ABANDONED;
-		      break;
-		    }
-
+	      if ((server->tcpfd = socket(server->addr.sa.sa_family, SOCK_STREAM, 0)) == -1)
+		continue; /* No good, next server */
+	      
 #ifdef HAVE_CONNTRACK
-		  /* Copy connection mark of incoming query to outgoing connection. */
-		  if (have_mark)
-		    setsockopt(server->tcpfd, SOL_SOCKET, SO_MARK, &mark, sizeof(unsigned int));
+	      /* Copy connection mark of incoming query to outgoing connection. */
+	      if (have_mark)
+		setsockopt(server->tcpfd, SOL_SOCKET, SO_MARK, &mark, sizeof(unsigned int));
 #endif	
-			  
-		  if (!local_bind(server->tcpfd,  &server->source_addr, server->interface, 1) ||
-		      connect(server->tcpfd, &server->addr.sa, sa_len(&server->addr)) == -1)
-		    {
-		      close(server->tcpfd);
-		      server->tcpfd = -1;
-		      new_status = STAT_ABANDONED;
-		      break;
-		    }
-
+	      
+	      if (!local_bind(server->tcpfd,  &server->source_addr, server->interface, 1) ||
+		  connect(server->tcpfd, &server->addr.sa, sa_len(&server->addr)) == -1)
+		{
+		  close(server->tcpfd);
+		  server->tcpfd = -1;
+		  continue; /* No good, next server */
 		}
-	    }
-	}
 
-      
-      if (!read_write(server->tcpfd, packet, m + sizeof(u16), 0) ||
-	  !read_write(server->tcpfd, &c1, 1, 1) ||
-	  !read_write(server->tcpfd, &c2, 1, 1) ||
-	  !read_write(server->tcpfd, payload, (c1 << 8) | c2, 1))
-	{
-	  new_status = STAT_ABANDONED;
+	      server->flags &= ~SERV_GOT_TCP;
+	    }
+	  
+	  if (!read_write(server->tcpfd, packet, m + sizeof(u16), 0) ||
+	      !read_write(server->tcpfd, &c1, 1, 1) ||
+	      !read_write(server->tcpfd, &c2, 1, 1) ||
+	      !read_write(server->tcpfd, payload, (c1 << 8) | c2, 1))
+	    {
+	      close(server->tcpfd);
+	      server->tcpfd = -1;
+	      /* We get data then EOF, reopen connection to same server,
+		 else try next. This avoids DoS from a server which accepts
+		 connections and then closes them. */
+	      if (server->flags & SERV_GOT_TCP)
+		goto retry;
+	      else
+		continue;
+	    }
+	  
+	  server->flags |= SERV_GOT_TCP;
+	  
+	  m = (c1 << 8) | c2;
+	  new_status = tcp_key_recurse(now, new_status, new_header, m, class, name, keyname, server, have_mark, mark, keycount);
 	  break;
 	}
-
-      m = (c1 << 8) | c2;
-      
-      new_status = tcp_key_recurse(now, new_status, new_header, m, class, name, keyname, server, have_mark, mark, keycount);
       
       if (new_status != STAT_OK)
 	break;
     }
-
+    
   if (packet)
     free(packet);
     
@@ -1820,7 +1826,8 @@ unsigned char *tcp_request(int confd, time_t now,
 			  (type == SERV_HAS_DOMAIN && !hostname_isequal(domain, last_server->domain)) ||
 			  (last_server->flags & (SERV_LITERAL_ADDRESS | SERV_LOOP)))
 			continue;
-		      
+
+		    retry:
 		      if (last_server->tcpfd == -1)
 			{
 			  if ((last_server->tcpfd = socket(last_server->addr.sa.sa_family, SOCK_STREAM, 0)) == -1)
@@ -1840,25 +1847,27 @@ unsigned char *tcp_request(int confd, time_t now,
 			      continue;
 			    }
 			  
+			  last_server->flags &= ~SERV_GOT_TCP;
+			}
+		      
 #ifdef HAVE_DNSSEC
-			  if (option_bool(OPT_DNSSEC_VALID) && (last_server->flags & SERV_DO_DNSSEC))
+		      if (option_bool(OPT_DNSSEC_VALID) && (last_server->flags & SERV_DO_DNSSEC))
+			{
+			  new_size = add_do_bit(header, size, ((unsigned char *) header) + 65536);
+			  
+			  if (size != new_size)
 			    {
-			      new_size = add_do_bit(header, size, ((unsigned char *) header) + 65536);
-			      
-			      if (size != new_size)
-				{
-				  added_pheader = 1;
-				  size = new_size;
-				}
-			      
-			      /* For debugging, set Checking Disabled, otherwise, have the upstream check too,
-				 this allows it to select auth servers when one is returning bad data. */
-			      if (option_bool(OPT_DNSSEC_DEBUG))
-				header->hb4 |= HB4_CD;
+			      added_pheader = 1;
+			      size = new_size;
 			    }
-#endif
+			  
+			  /* For debugging, set Checking Disabled, otherwise, have the upstream check too,
+			     this allows it to select auth servers when one is returning bad data. */
+			  if (option_bool(OPT_DNSSEC_DEBUG))
+			    header->hb4 |= HB4_CD;
 			}
-		      
+#endif
+					      
 		      *length = htons(size);
 
 		      /* get query name again for logging - may have been overwritten */
@@ -1872,9 +1881,17 @@ unsigned char *tcp_request(int confd, time_t now,
 			{
 			  close(last_server->tcpfd);
 			  last_server->tcpfd = -1;
-			  continue;
-			} 
+			  /* We get data then EOF, reopen connection to same server,
+			     else try next. This avoids DoS from a server which accepts
+			     connections and then closes them. */
+			  if (last_server->flags & SERV_GOT_TCP)
+			    goto retry;
+			  else
+			    continue;
+			}
 		      
+		      last_server->flags |= SERV_GOT_TCP;
+
 		      m = (c1 << 8) | c2;
 		      
 		      if (last_server->addr.sa.sa_family == AF_INET)