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

@@ -1466,6 +1466,7 @@ static int tcp_key_recurse(time_t now, int status, struct dns_header *header, si
       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)
@@ -1504,43 +1505,41 @@ 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;
+	  new_status = STAT_ABANDONED;
+	  break;
+	}
+	
       type &= ~SERV_DO_DNSSEC;
       
       while (1)
 	{
-	      if (type == (start->flags & SERV_TYPE) &&
-		  (type != SERV_HAS_DOMAIN || hostname_isequal(domain, start->domain)) &&
-		  !(start->flags & (SERV_LITERAL_ADDRESS | SERV_LOOP)))
+	  if (!firstsendto)
+	    firstsendto = server;
+	  else
 	    {
-		  new_server = start;
-		  if (server == start)
+	      if (!(server = server->next))
+		server = daemon->servers;
+	      if (server == firstsendto)
 		{
-		      new_server = NULL;
+		  /* 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 (type != (server->flags & SERV_TYPE) ||
+	      (type == SERV_HAS_DOMAIN && !hostname_isequal(domain, server->domain)) ||
+	      (server->flags & (SERV_LITERAL_ADDRESS | SERV_LOOP)))
+	    continue;
 	  
-	  if (new_server)
-	    {
-	      server = new_server;
+	retry:
 	  /* 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;
-		    }
+		continue; /* No good, next server */
 	      
 #ifdef HAVE_CONNTRACK
 	      /* Copy connection mark of incoming query to outgoing connection. */
@@ -1553,27 +1552,34 @@ static int tcp_key_recurse(time_t now, int status, struct dns_header *header, si
 		{
 		  close(server->tcpfd);
 		  server->tcpfd = -1;
-		      new_status = STAT_ABANDONED;
-		      break;
+		  continue; /* No good, next server */
 		}
 
+	      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))
 	    {
-	  new_status = STAT_ABANDONED;
-	  break;
+	      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;
 	    }
 	  
-      m = (c1 << 8) | c2;
+	  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;
+	}
       
       if (new_status != STAT_OK)
 	break;
@@ -1821,6 +1827,7 @@ unsigned char *tcp_request(int confd, time_t now,
 			  (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)
@@ -1840,6 +1847,9 @@ 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))
 			{
@@ -1857,7 +1867,6 @@ unsigned char *tcp_request(int confd, time_t now,
 			    header->hb4 |= HB4_CD;
 			}
 #endif
-			}
 					      
 		      *length = htons(size);
 
@@ -1872,9 +1881,17 @@ unsigned char *tcp_request(int confd, time_t now,
 			{
 			  close(last_server->tcpfd);
 			  last_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 (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)