Scale the DNS random scket pool on the value of dns-forward-max.

Previously, without min-port or max-port configured, dnsmasq would
default to the compiled in defaults for those, which are 1024 and
65535. Now, when neither are configured, it defaults instead to
the kernel's ephemeral port range, which is typically
32768 to 60999 on Linux systems. This change eliminates the
possibility that dnsmasq may be using a registered port > 1024
when a long-running daemon starts up and wishes to claim it.

This change does likely slighly reduce the number of random ports
and therefore the protection from reply spoofing. The older
behaviour can be restored using the min-port and max-port config
switches should that be a concern.

CHANGELOG

@@ -49,7 +49,23 @@ version 2.85
 	source address errors from fatal to run-time. The error will be
 	logged and communiction with the server not possible.
 
+	Change the method of allocation of random source ports for DNS.
+	Previously, without min-port or max-port configured, dnsmasq would
+	default to the compiled in defaults for those, which are 1024 and
+	65535. Now, when neither are configured, it defaults instead to
+	the kernel's ephemeral port range, which is typically
+	32768 to 60999 on Linux systems. This change eliminates the
+	possibility that dnsmasq may be using a registered port > 1024
+	when a long-running daemon starts up and wishes to claim it.
+	This change does likely slighly reduce the number of random ports
+	and therefore the protection from reply spoofing. The older
+	behaviour can be restored using the min-port and max-port config
+	switches should that be a concern.
 
+	Scale the size of the DNS random-port pool based on the
+	value of the --dns-forward-max configuration.
+
+	
 version 2.84
 	Fix a problem, introduced in 2.83, which could see DNS replies
 	being sent via the wrong socket. On machines running both

src/config.h

@@ -29,7 +29,6 @@
 #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 */
 #define LEASE_RETRY 60 /* on error, retry writing leasefile after LEASE_RETRY seconds */
 #define CACHESIZ 150 /* default cache size */
 #define TTL_FLOOR_LIMIT 3600 /* don't allow --min-cache-ttl to raise TTL above this under any circumstances */

src/dnsmasq.c

@@ -237,9 +237,16 @@ int main (int argc, char **argv)
     die(_("Ubus not available: set HAVE_UBUS in src/config.h"), NULL, EC_BADCONF);
 #endif
   
+  /* Handle only one of min_port/max_port being set. */
+  if (daemon->min_port != 0 && daemon->max_port == 0)
+    daemon->max_port = MAX_PORT;
+  
+  if (daemon->max_port != 0 && daemon->min_port == 0)
+    daemon->min_port = MIN_PORT;
+   
   if (daemon->max_port < daemon->min_port)
     die(_("max_port cannot be smaller than min_port"), NULL, EC_BADCONF);
-
+  
   now = dnsmasq_time();
 
   if (daemon->auth_zones)
@@ -392,6 +399,14 @@ int main (int argc, char **argv)
       cache_init();
       blockdata_init();
       hash_questions_init();
+
+      /* Scale random socket pool by ftabsize, but
+	 limit it based on available fds. */
+      daemon->numrrand = daemon->ftabsize/2;
+      if (daemon->numrrand > max_fd/3)
+	daemon->numrrand = max_fd/3;
+      /* safe_malloc returns zero'd memory */
+      daemon->randomsocks = safe_malloc(daemon->numrrand * sizeof(struct randfd));
     }
 
 #ifdef HAVE_INOTIFY
@@ -980,7 +995,7 @@ int main (int argc, char **argv)
 	 a single file will be sent to may clients (the file only needs
 	 one fd). */
 
-      max_fd -= 30; /* use other than TFTP */
+      max_fd -= 30 + daemon->numrrand; /* use other than TFTP */
       
       if (max_fd < 0)
 	max_fd = 5;
@@ -1693,7 +1708,7 @@ static int set_dns_listeners(time_t now)
   for (serverfdp = daemon->sfds; serverfdp; serverfdp = serverfdp->next)
     poll_listen(serverfdp->fd, POLLIN);
     
-  for (i = 0; i < RANDOM_SOCKS; i++)
+  for (i = 0; i < daemon->numrrand; i++)
     if (daemon->randomsocks[i].refcount != 0)
       poll_listen(daemon->randomsocks[i].fd, POLLIN);
 
@@ -1745,7 +1760,7 @@ static void check_dns_listeners(time_t now)
     if (poll_check(serverfdp->fd, POLLIN))
       reply_query(serverfdp->fd, now);
   
-  for (i = 0; i < RANDOM_SOCKS; i++)
+  for (i = 0; i < daemon->numrrand; i++)
     if (daemon->randomsocks[i].refcount != 0 && 
 	poll_check(daemon->randomsocks[i].fd, POLLIN))
       reply_query(daemon->randomsocks[i].fd, now);

src/dnsmasq.h

@@ -1130,7 +1130,8 @@ extern struct daemon {
   pid_t tcp_pids[MAX_PROCS];
   int tcp_pipes[MAX_PROCS];
   int pipe_to_parent;
-  struct randfd randomsocks[RANDOM_SOCKS];
+  int numrrand;
+  struct randfd *randomsocks;
   struct randfd_list *rfl_spare, *rfl_poll;
   int v6pktinfo; 
   struct addrlist *interface_addrs; /* list of all addresses/prefix lengths associated with all local interfaces */

src/forward.c

@@ -245,6 +245,53 @@ static unsigned int search_servers(time_t now, union all_addr **addrpp, unsigned
   return  flags;
 }
 
+#ifdef HAVE_CONNTRACK
+static void set_outgoing_mark(struct frec *forward, int fd)
+{
+  /* Copy connection mark of incoming query to outgoing connection. */
+  unsigned int mark;
+  if (get_incoming_mark(&forward->frec_src.source, &forward->frec_src.dest, 0, &mark))
+    setsockopt(fd, SOL_SOCKET, SO_MARK, &mark, sizeof(unsigned int));
+}
+#endif
+
+static void log_query_mysockaddr(unsigned int flags, char *name, union mysockaddr *addr, char *arg)
+{
+  if (addr->sa.sa_family == AF_INET)
+    log_query(flags | F_IPV4, name, (union all_addr *)&addr->in.sin_addr, arg);
+  else
+    log_query(flags | F_IPV6, name, (union all_addr *)&addr->in6.sin6_addr, arg);
+}
+
+static void server_send(struct server *server, int fd,
+			const void *header, size_t plen, int flags)
+{
+  while (retry_send(sendto(fd, header, plen, flags,
+			   &server->addr.sa,
+			   sa_len(&server->addr))));
+}
+
+#ifdef HAVE_DNSSEC
+static void server_send_log(struct server *server, int fd,
+			const void *header, size_t plen, int dumpflags,
+			unsigned int logflags, char *name, char *arg)
+{
+#ifdef HAVE_DUMPFILE
+	  dump_packet(dumpflags, (void *)header, (size_t)plen, NULL, &server->addr);
+#endif
+	  log_query_mysockaddr(logflags, name, &server->addr, arg);
+	  server_send(server, fd, header, plen, 0);
+}
+#endif
+
+static int server_test_type(const struct server *server,
+			    const char *domain, int type, int extratype)
+{
+  return (type == (server->flags & (SERV_TYPE | extratype)) &&
+      (type != SERV_HAS_DOMAIN || hostname_isequal(domain, server->domain)) &&
+      !(server->flags & (SERV_LITERAL_ADDRESS | SERV_LOOP)));
+}
+
 static int forward_query(int udpfd, union mysockaddr *udpaddr,
 			 union all_addr *dst_addr, unsigned int dst_iface,
 			 struct dns_header *header, size_t plen, time_t now, 
@@ -345,17 +392,10 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,
 	    PUTSHORT(SAFE_PKTSZ, pheader);
 	  
 	  if ((fd = allocate_rfd(&forward->rfds, forward->sentto)) != -1)
-	    {
-	      if (forward->sentto->addr.sa.sa_family == AF_INET) 
-		log_query(F_NOEXTRA | F_DNSSEC | F_IPV4, "retry", (union all_addr *)&forward->sentto->addr.in.sin_addr, "dnssec");
-	      else
-		log_query(F_NOEXTRA | F_DNSSEC | F_IPV6, "retry", (union all_addr *)&forward->sentto->addr.in6.sin6_addr, "dnssec");
-	      
-	      while (retry_send(sendto(fd, (char *)header, plen, 0,
-				       &forward->sentto->addr.sa,
-				       sa_len(&forward->sentto->addr))));
-	    }
-	  
+	    server_send_log(forward->sentto, fd, header, plen,
+			    DUMP_SEC_QUERY,
+			    F_NOEXTRA | F_DNSSEC, "retry", "dnssec");
+
 	  return 1;
 	}
 #endif
@@ -506,20 +546,14 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,
 	     domain may be NULL, in which case server->domain 
 	     must be NULL also. */
 	  
-	  if (type == (start->flags & SERV_TYPE) &&
-	      (type != SERV_HAS_DOMAIN || hostname_isequal(domain, start->domain)) &&
-	      !(start->flags & (SERV_LITERAL_ADDRESS | SERV_LOOP)) &&
+	  if (server_test_type(start, domain, type, 0) &&
 	      ((fd = allocate_rfd(&forward->rfds, start)) != -1))
 	    {
 	      
 #ifdef HAVE_CONNTRACK
 	      /* Copy connection mark of incoming query to outgoing connection. */
 	      if (option_bool(OPT_CONNTRACK))
-		{
-		  unsigned int mark;
-		  if (get_incoming_mark(&forward->frec_src.source, &forward->frec_src.dest, 0, &mark))
-		    setsockopt(fd, SOL_SOCKET, SO_MARK, &mark, sizeof(unsigned int));
-		}
+		set_outgoing_mark(forward, fd);
 #endif
 	      
 #ifdef HAVE_DNSSEC
@@ -556,12 +590,8 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,
 		  
 		  if (!gotname)
 		    strcpy(daemon->namebuff, "query");
-		  if (start->addr.sa.sa_family == AF_INET)
-		    log_query(F_SERVER | F_IPV4 | F_FORWARD, daemon->namebuff, 
-			      (union all_addr *)&start->addr.in.sin_addr, NULL); 
-		  else
-		    log_query(F_SERVER | F_IPV6 | F_FORWARD, daemon->namebuff, 
-			      (union  all_addr *)&start->addr.in6.sin6_addr, NULL);
+		  log_query_mysockaddr(F_SERVER | F_FORWARD, daemon->namebuff,
+				       &start->addr, NULL);
 		  start->queries++;
 		  forwarded = 1;
 		  forward->sentto = start;
@@ -878,22 +908,10 @@ void reply_query(int fd, time_t now)
 	      }
 	    
 	  
-	  if ((fd = allocate_rfd(&forward->rfds, start)) == -1)
-	    return;
-	  
-#ifdef HAVE_DUMPFILE
-	  dump_packet(DUMP_SEC_QUERY, (void *)header, (size_t)plen, NULL, &start->addr);
-#endif
-
-	  while (retry_send(sendto(fd, (char *)header, plen, 0,
-				   &start->addr.sa,
-				   sa_len(&start->addr))));
-	  
-	  if (start->addr.sa.sa_family == AF_INET) 
-	    log_query(F_NOEXTRA | F_DNSSEC | F_IPV4, "retry", (union all_addr *)&start->addr.in.sin_addr, "dnssec");
-	  else
-	    log_query(F_NOEXTRA | F_DNSSEC | F_IPV6, "retry", (union all_addr *)&start->addr.in6.sin6_addr, "dnssec");
-	  
+	  if ((fd = allocate_rfd(&forward->rfds, start)) != -1)
+	    server_send_log(start, fd, header, plen,
+			    DUMP_SEC_QUERY,
+			    F_NOEXTRA | F_DNSSEC, "retry", "dnssec");
 	  return;
 	}
 #endif
@@ -1064,9 +1082,7 @@ void reply_query(int fd, time_t now)
 			  
 			  while (1)
 			    {
-			      if (type == (start->flags & (SERV_TYPE | SERV_DO_DNSSEC)) &&
-				  ((type & SERV_TYPE) != SERV_HAS_DOMAIN || hostname_isequal(domain, start->domain)) &&
-				  !(start->flags & (SERV_LITERAL_ADDRESS | SERV_LOOP)))
+			      if (server_test_type(start, domain, type, 0))
 				{
 				  new_server = start;
 				  if (server == start)
@@ -1109,13 +1125,6 @@ void reply_query(int fd, time_t now)
 		      nn = dnssec_generate_query(header,((unsigned char *) header) + server->edns_pktsz,
 						 daemon->keyname, forward->class, querytype, server->edns_pktsz);
 
-		      if (server->addr.sa.sa_family == AF_INET) 
-			log_query(F_NOEXTRA | F_DNSSEC | F_IPV4, daemon->keyname, (union all_addr *)&(server->addr.in.sin_addr),
-				  querystr("dnssec-query", querytype));
-		      else
-			log_query(F_NOEXTRA | F_DNSSEC | F_IPV6, daemon->keyname, (union all_addr *)&(server->addr.in6.sin6_addr),
-				  querystr("dnssec-query", querytype));
-  
 		      memcpy(new->hash, hash_questions(header, nn, daemon->namebuff), HASH_SIZE);
 		      new->new_id = get_id();
 		      header->id = htons(new->new_id);
@@ -1129,22 +1138,12 @@ void reply_query(int fd, time_t now)
 		      if ((fd = allocate_rfd(&new->rfds, server)) != -1)
 			{
 #ifdef HAVE_CONNTRACK
-			  /* Copy connection mark of incoming query to outgoing connection. */
 			  if (option_bool(OPT_CONNTRACK))
-			    {
-			      unsigned int mark;
-			      if (get_incoming_mark(&orig->frec_src.source, &orig->frec_src.dest, 0, &mark))
-				setsockopt(fd, SOL_SOCKET, SO_MARK, &mark, sizeof(unsigned int));
-			    }
+			    set_outgoing_mark(orig, fd);
 #endif
-			  
-#ifdef HAVE_DUMPFILE
-			  dump_packet(DUMP_SEC_QUERY, (void *)header, (size_t)nn, NULL, &server->addr);
-#endif
-			  
-			  while (retry_send(sendto(fd, (char *)header, nn, 0, 
-						   &server->addr.sa, 
-						   sa_len(&server->addr)))); 
+			  server_send_log(server, fd, header, nn, DUMP_SEC_QUERY,
+					  F_NOEXTRA | F_DNSSEC, daemon->keyname,
+					  querystr("dnssec-query", querytype));
 			  server->queries++;
 			}
 		    }		  
@@ -1503,13 +1502,9 @@ void receive_query(struct listener *listen, time_t now)
       struct auth_zone *zone;
 #endif
       char *types = querystr(auth_dns ? "auth" : "query", type);
-      
-      if (family == AF_INET) 
-	log_query(F_QUERY | F_IPV4 | F_FORWARD, daemon->namebuff, 
-		  (union all_addr *)&source_addr.in.sin_addr, types);
-      else
-	log_query(F_QUERY | F_IPV6 | F_FORWARD, daemon->namebuff, 
-		  (union all_addr *)&source_addr.in6.sin6_addr, types);
+
+      log_query_mysockaddr(F_QUERY | F_FORWARD, daemon->namebuff,
+			   &source_addr, types);
 
 #ifdef HAVE_AUTH
       /* find queries for zones we're authoritative for, and answer them directly */
@@ -1671,9 +1666,7 @@ static int tcp_key_recurse(time_t now, int status, struct dns_header *header, si
 		}
 	    }
 	  
-	  if (type != (server->flags & (SERV_TYPE | SERV_DO_DNSSEC)) ||
-	      (type == SERV_HAS_DOMAIN && !hostname_isequal(domain, server->domain)) ||
-	      (server->flags & (SERV_LITERAL_ADDRESS | SERV_LOOP)))
+	  if (!server_test_type(server, domain, type, SERV_DO_DNSSEC))
 	    continue;
 
 	retry:
@@ -1697,9 +1690,8 @@ static int tcp_key_recurse(time_t now, int status, struct dns_header *header, si
 		}
 	      
 #ifdef MSG_FASTOPEN
-	      while(retry_send(sendto(server->tcpfd, packet, m + sizeof(u16),
-				      MSG_FASTOPEN, &server->addr.sa, sa_len(&server->addr))));
-	      
+	      server_send(server, server->tcpfd, packet, m + sizeof(u16), MSG_FASTOPEN);
+
 	      if (errno == 0)
 		data_sent = 1;
 #endif
@@ -1730,14 +1722,9 @@ static int tcp_key_recurse(time_t now, int status, struct dns_header *header, si
 		continue;
 	    }
 
+	  log_query_mysockaddr(F_NOEXTRA | F_DNSSEC, keyname, &server->addr,
+		      querystr("dnssec-query", new_status == STAT_NEED_KEY ? T_DNSKEY : T_DS));
 
-	  if (server->addr.sa.sa_family == AF_INET) 
-	    log_query(F_NOEXTRA | F_DNSSEC | F_IPV4, keyname, (union all_addr *)&(server->addr.in.sin_addr),
-		      querystr("dnssec-query", new_status == STAT_NEED_KEY ? T_DNSKEY : T_DS));
-	  else
-	    log_query(F_NOEXTRA | F_DNSSEC | F_IPV6, keyname, (union all_addr *)&(server->addr.in6.sin6_addr),
-		      querystr("dnssec-query", new_status == STAT_NEED_KEY ? T_DNSKEY : T_DS));
-	  
 	  server->flags |= SERV_GOT_TCP;
 	  
 	  m = (c1 << 8) | c2;
@@ -1875,12 +1862,8 @@ unsigned char *tcp_request(int confd, time_t now,
 #endif
 	  char *types = querystr(auth_dns ? "auth" : "query", qtype);
 	  
-	  if (peer_addr.sa.sa_family == AF_INET) 
-	    log_query(F_QUERY | F_IPV4 | F_FORWARD, daemon->namebuff, 
-		      (union all_addr *)&peer_addr.in.sin_addr, types);
-	  else
-	    log_query(F_QUERY | F_IPV6 | F_FORWARD, daemon->namebuff, 
-		      (union all_addr *)&peer_addr.in6.sin6_addr, types);
+	  log_query_mysockaddr(F_QUERY | F_FORWARD, daemon->namebuff,
+			       &peer_addr, types);
 	  
 #ifdef HAVE_AUTH
 	  /* find queries for zones we're authoritative for, and answer them directly */
@@ -1995,9 +1978,7 @@ unsigned char *tcp_request(int confd, time_t now,
 			}
 		      
 		      /* server for wrong domain */
-		      if (type != (last_server->flags & SERV_TYPE) ||
-			  (type == SERV_HAS_DOMAIN && !hostname_isequal(domain, last_server->domain)) ||
-			  (last_server->flags & (SERV_LITERAL_ADDRESS | SERV_LOOP)))
+		      if (!server_test_type(last_server, domain, type, 0))
 			continue;
 
 		    retry:
@@ -2022,9 +2003,8 @@ unsigned char *tcp_request(int confd, time_t now,
 			    }
 			  
 #ifdef MSG_FASTOPEN
-			    while(retry_send(sendto(last_server->tcpfd, packet, size + sizeof(u16),
-						    MSG_FASTOPEN, &last_server->addr.sa, sa_len(&last_server->addr))));
-			    
+			    server_send(last_server, last_server->tcpfd, packet, size + sizeof(u16), MSG_FASTOPEN);
+
 			    if (errno == 0)
 			      data_sent = 1;
 #endif
@@ -2062,13 +2042,9 @@ unsigned char *tcp_request(int confd, time_t now,
 		      last_server->flags |= SERV_GOT_TCP;
 
 		      m = (c1 << 8) | c2;
-		      
-		      if (last_server->addr.sa.sa_family == AF_INET)
-			log_query(F_SERVER | F_IPV4 | F_FORWARD, daemon->namebuff, 
-				  (union all_addr *)&last_server->addr.in.sin_addr, NULL); 
-		      else
-			log_query(F_SERVER | F_IPV6 | F_FORWARD, daemon->namebuff, 
-				  (union all_addr *)&last_server->addr.in6.sin6_addr, NULL);
+
+		      log_query_mysockaddr(F_SERVER | F_FORWARD, daemon->namebuff,
+					   &last_server->addr, NULL);
 
 #ifdef HAVE_DNSSEC
 		      if (option_bool(OPT_DNSSEC_VALID) && !checking_disabled && (last_server->flags & SERV_DO_DNSSEC))
@@ -2242,7 +2218,7 @@ int allocate_rfd(struct randfd_list **fdlp, struct server *serv)
    
   /* limit the number of sockets we have open to avoid starvation of 
      (eg) TFTP. Once we have a reasonable number, randomness should be OK */
-  for (i = 0; i < RANDOM_SOCKS; i++)
+  for (i = 0; i < daemon->numrrand; i++)
     if (daemon->randomsocks[i].refcount == 0)
       {
 	if ((fd = random_sock(serv)) != -1)
@@ -2257,9 +2233,9 @@ int allocate_rfd(struct randfd_list **fdlp, struct server *serv)
   
   /* No free ones or cannot get new socket, grab an existing one */
   if (!rfd)
-    for (j = 0; j < RANDOM_SOCKS; j++)
+    for (j = 0; j < daemon->numrrand; j++)
       {
-	i = (j + finger) % RANDOM_SOCKS;
+	i = (j + finger) % daemon->numrrand;
 	if (daemon->randomsocks[i].refcount != 0 &&
 	    server_isequal(serv, daemon->randomsocks[i].serv) &&
 	    daemon->randomsocks[i].refcount != 0xfffe)
@@ -2271,7 +2247,7 @@ int allocate_rfd(struct randfd_list **fdlp, struct server *serv)
 	  }
       }
 
-  if (j == RANDOM_SOCKS)
+  if (j == daemon->numrrand)
     {
       struct randfd_list *rfl_poll;
 
@@ -2546,9 +2522,8 @@ static struct frec *lookup_frec_by_query(void *hash, unsigned int flags)
 void resend_query()
 {
   if (daemon->srv_save)
-    while(retry_send(sendto(daemon->fd_save, daemon->packet, daemon->packet_len, 0,
-			    &daemon->srv_save->addr.sa, 
-			    sa_len(&daemon->srv_save->addr)))); 
+    server_send(daemon->srv_save, daemon->fd_save,
+		daemon->packet, daemon->packet_len, 0);
 }
 
 /* A server record is going away, remove references to it */
@@ -2563,7 +2538,7 @@ void server_gone(struct server *server)
 
   /* If any random socket refers to this server, NULL the reference.
      No more references to the socket will be created in the future. */
-  for (i = 0; i < RANDOM_SOCKS; i++)
+  for (i = 0; i < daemon->numrrand; i++)
     if (daemon->randomsocks[i].refcount != 0 && daemon->randomsocks[i].serv == server)
       daemon->randomsocks[i].serv = NULL;
   

src/network.c

@@ -720,13 +720,17 @@ int enumerate_interfaces(int reset)
      to a server is specified by an interface, so cache them.
      Update the cache here. */
   for (serv = daemon->servers; serv; serv = serv->next)
-    if (strlen(serv->interface) != 0)
+    if (serv->interface[0] != 0)
       {
-	 struct ifreq ifr;
-
-	 safe_strncpy(ifr.ifr_name, serv->interface, IF_NAMESIZE);
-	 if (ioctl(param.fd, SIOCGIFINDEX, &ifr) != -1) 
-	   serv->ifindex = ifr.ifr_ifindex;
+#ifdef HAVE_LINUX_NETWORK
+	struct ifreq ifr;
+	
+	safe_strncpy(ifr.ifr_name, serv->interface, IF_NAMESIZE);
+	if (ioctl(param.fd, SIOCGIFINDEX, &ifr) != -1) 
+	  serv->ifindex = ifr.ifr_ifindex;
+#else
+	serv->ifindex = if_nametoindex(serv->interface);
+#endif
       }
     
 again:
@@ -1306,9 +1310,9 @@ int local_bind(int fd, union mysockaddr *addr, char *intname, unsigned int ifind
 {
   union mysockaddr addr_copy = *addr;
   unsigned short port;
-  int tries = 1, done = 0;
-  unsigned int ports_avail = ((unsigned short)daemon->max_port - (unsigned short)daemon->min_port) + 1;
- 
+  int tries = 1;
+  unsigned short ports_avail = 1;
+
   if (addr_copy.sa.sa_family == AF_INET)
     port = addr_copy.in.sin_port;
   else
@@ -1317,34 +1321,43 @@ int local_bind(int fd, union mysockaddr *addr, char *intname, unsigned int ifind
   /* cannot set source _port_ for TCP connections. */
   if (is_tcp)
     port = 0;
-  else if (port == 0)
+  else if (port == 0 && daemon->max_port != 0)
     {
-      /* Bind a random port within the range given by min-port and max-port */
+      /* Bind a random port within the range given by min-port and max-port if either
+	 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 ? 3 * ports_avail : 100;
-      port = htons(daemon->min_port + (rand16() % ((unsigned short)ports_avail)));
+      port = htons(daemon->min_port + (rand16() % ports_avail));
     }
   
-  while (tries--)
+  while (1)
     {
+      /* elide bind() call if it's to port 0, address 0 */
       if (addr_copy.sa.sa_family == AF_INET)
-	addr_copy.in.sin_port = port;
-      else
-	addr_copy.in6.sin6_port = port;
-
-      if (bind(fd, (struct sockaddr *)&addr_copy, sa_len(&addr_copy)) != -1)
 	{
-	  done = 1;
-	  break;
+	  if (port == 0 && addr_copy.in.sin_addr.s_addr == 0)
+	    break;
+	  addr_copy.in.sin_port = port;
+	}
+      else
+	{
+	  if (port == 0 && IN6_IS_ADDR_UNSPECIFIED(&addr_copy.in6.sin6_addr))
+	    break;
+	  addr_copy.in6.sin6_port = port;
 	}
       
-      if (errno != EADDRINUSE && errno != EACCES)
-	return 0;
+      if (bind(fd, (struct sockaddr *)&addr_copy, sa_len(&addr_copy)) != -1)
+	break;
       
-      port = htons(daemon->min_port + (rand16() % ((unsigned short)ports_avail)));
-    }
+       if (errno != EADDRINUSE && errno != EACCES) 
+	 return 0;
 
-  if (!done)
-    return 0;
+      if (--tries == 0)
+	return 0;
+
+      port = htons(daemon->min_port + (rand16() % ports_avail));
+    }
 
   if (!is_tcp && ifindex > 0)
     {

src/option.c

@@ -5076,9 +5076,7 @@ void read_opts(int argc, char **argv, char *compile_opts)
   daemon->soa_refresh = SOA_REFRESH;
   daemon->soa_retry = SOA_RETRY;
   daemon->soa_expiry = SOA_EXPIRY;
-  daemon->max_port = MAX_PORT;
-  daemon->min_port = MIN_PORT;
-
+  
 #ifndef NO_ID
   add_txt("version.bind", "dnsmasq-" VERSION, 0 );
   add_txt("authors.bind", "Simon Kelley", 0);