Fix DNS server fd garbage collection.

If we're talking to upstream servers from a fixed port, specified by query-port
we create the fds to do this once, before dropping root, so that ports <1024 can be used.

But we call check_servers() before reading /etc/resolv.conf, so if the only servers
are in resolv.conf, at that point there will be no servers, and the fds get garbage
collected away, only to be recreated (but without root) after we read /etc/resolv.conf

Make pre-allocated server fds immortal, to avoid this problem.

src/dnsmasq.h

@@ -506,7 +506,7 @@ struct serverfd {
   int fd;
   union mysockaddr source_addr;
   char interface[IF_NAMESIZE+1];
-  unsigned int ifindex, used;
+  unsigned int ifindex, used, preallocated;
   struct serverfd *next;
 };
 

src/network.c

@@ -1290,6 +1290,7 @@ static struct serverfd *allocate_sfd(union mysockaddr *addr, char *intname)
   sfd->source_addr = *addr;
   sfd->next = daemon->sfds;
   sfd->ifindex = ifindex;
+  sfd->preallocated = 0;
   daemon->sfds = sfd;
 
   return sfd; 
@@ -1300,6 +1301,7 @@ static struct serverfd *allocate_sfd(union mysockaddr *addr, char *intname)
 void pre_allocate_sfds(void)
 {
   struct server *srv;
+  struct serverfd *sfd;
   
   if (daemon->query_port != 0)
     {
@@ -1311,7 +1313,8 @@ void pre_allocate_sfds(void)
 #ifdef HAVE_SOCKADDR_SA_LEN
       addr.in.sin_len = sizeof(struct sockaddr_in);
 #endif
-      allocate_sfd(&addr, "");
+      if ((sfd = allocate_sfd(&addr, "")))
+	sfd->preallocated = 1;
 #ifdef HAVE_IPV6
       memset(&addr, 0, sizeof(addr));
       addr.in6.sin6_family = AF_INET6;
@@ -1320,7 +1323,8 @@ void pre_allocate_sfds(void)
 #ifdef HAVE_SOCKADDR_SA_LEN
       addr.in6.sin6_len = sizeof(struct sockaddr_in6);
 #endif
-      allocate_sfd(&addr, "");
+      if ((sfd = allocate_sfd(&addr, "")))
+	sfd->preallocated = 1;
 #endif
     }
   
@@ -1473,9 +1477,10 @@ void check_servers(void)
   /* interface may be new since startup */
   if (!option_bool(OPT_NOWILD))
     enumerate_interfaces(0);
-  
+
+  /* don't garbage collect pre-allocated sfds. */
   for (sfd = daemon->sfds; sfd; sfd = sfd->next)
-    sfd->used = 0;
+    sfd->used = sfd->preallocated;
 
   for (count = 0, serv = daemon->servers; serv; serv = serv->next)
     {