Further optimisation of --port-limit.

No longer try and fail to open every port when the port range
is in complete use; go straight to re-using an existing socket.

Die at startup if port range is smaller than --port-limit, since
the code behaves badly in this case.

src/config.h

@@ -24,6 +24,7 @@
 #define KEYBLOCK_LEN 40 /* choose to minimise fragmentation when storing DNSSEC keys */
 #define DNSSEC_WORK 50 /* Max number of queries to validate one question */
 #define TIMEOUT 10     /* drop UDP queries after TIMEOUT seconds */
+#define SMALL_PORT_RANGE 30 /* If DNS port range is smaller than this, use different allocation. */
 #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. */

src/dnsmasq.c

@@ -266,6 +266,10 @@ int main (int argc, char **argv)
   if (daemon->max_port < daemon->min_port)
     die(_("max_port cannot be smaller than min_port"), NULL, EC_BADCONF);
 
+  if (daemon->max_port != 0 &&
+      daemon->max_port - daemon->min_port + 1 < daemon->randport_limit)
+    die(_("port_limit must not be larger than available port range"), NULL, EC_BADCONF);
+  
   now = dnsmasq_time();
 
   if (daemon->auth_zones)

src/forward.c

@@ -2435,6 +2435,21 @@ static int random_sock(struct server *s)
 
   if ((fd = socket(s->source_addr.sa.sa_family, SOCK_DGRAM, 0)) != -1)
     {
+      /* We need to set IPV6ONLY so we can use the same ports
+	 for IPv4 and IPV6, otherwise, in restriced port situations,
+	 we can end up with all our available ports in use for 
+	 one address family, and the other address family cannot be used. */
+      if (s->source_addr.sa.sa_family == AF_INET6)
+	{
+	  int opt = 1;
+
+	  if (setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, &opt, sizeof(opt)) == -1)
+	    {
+	      close(fd);
+	      return -1;
+	    }
+	}
+      
       if (local_bind(fd, &s->source_addr, s->interface, s->ifindex, 0))
 	return fd;
 
@@ -2479,9 +2494,22 @@ int allocate_rfd(struct randfd_list **fdlp, struct server *serv)
 {
   static int finger = 0;
   int i, j = 0;
+  int ports_full = 0;
   struct randfd_list **up, *rfl, *found, **found_link;
   struct randfd *rfd = NULL;
   int fd = 0;
+  int ports_avail = 0;
+  
+  /* We can't have more randomsocks for this AF available than ports in  our port range,
+     so check that here, to avoid trying and failing to bind every port
+     in local_bind(), called from random_sock(). The actual check is below when 
+     ports_avail != 0 */
+  if (daemon->max_port != 0)
+    {
+      ports_avail = daemon->max_port - daemon->min_port + 1;
+      if (ports_avail >= SMALL_PORT_RANGE)
+	ports_avail = 0;
+    }
   
   /* If server has a pre-allocated fd, use that. */
   if (serv->sfd)
@@ -2508,8 +2536,24 @@ int allocate_rfd(struct randfd_list **fdlp, struct server *serv)
       return found->rfd->fd;
     }
 
+  /* check for all available ports in use. */
+  if (ports_avail != 0)
+    {
+      int ports_inuse;
+
+      for (ports_inuse = 0, i = 0; i < daemon->numrrand; i++)
+	if (daemon->randomsocks[i].refcount != 0 &&
+	    daemon->randomsocks[i].serv->source_addr.sa.sa_family == serv->source_addr.sa.sa_family &&
+	    ++ports_inuse >= ports_avail)
+	  {
+	    ports_full = 1;
+	    break;
+	  }
+    }
+  
   /* limit the number of sockets we have open to avoid starvation of 
      (eg) TFTP. Once we have a reasonable number, randomness should be OK */
+  if (!ports_full)
     for (i = 0; i < daemon->numrrand; i++)
       if (daemon->randomsocks[i].refcount == 0)
 	{

src/network.c

@@ -1371,7 +1371,7 @@ int local_bind(int fd, union mysockaddr *addr, char *intname, unsigned int ifind
 	 or both are set. Otherwise use the OS's random ephemeral port allocation by
 	 leaving port == 0 and tries == 1 */
       ports_avail = daemon->max_port - daemon->min_port + 1;
-      tries =  (ports_avail < 30) ? ports_avail : 100;
+      tries =  (ports_avail < SMALL_PORT_RANGE) ? ports_avail : 100;
       port = htons(daemon->min_port + (rand16() % ports_avail));
     }
   
@@ -1401,7 +1401,7 @@ int local_bind(int fd, union mysockaddr *addr, char *intname, unsigned int ifind
 	return 0;
 
       /* For small ranges, do a systematic search, not a random one. */
-      if (ports_avail < 30)
+      if (ports_avail < SMALL_PORT_RANGE)
 	{
 	  unsigned short hport = ntohs(port);
 	  if (hport++ == daemon->max_port)