Fix failure to add client MAC address to queries in TCP mode.

The options which cause dnsmasq to decorate a DNS query with the MAC
address on the originating client can fail when the query is sent
using TCP.

In TCP mode, dnsmasq spawns a new process to handle each TCP connection.
These child processes do not have an open netlink socket, which is
needed to read the kernel ARP table, so the process of adding the
client's MAC address as an EDNS0 option silently fails.

This is fixed by this patch by updating dnsmasq's ARP cache in the
main process just before forking the TCP-handler child process.
This ensures that the copy of the ARP cache inherited by the
TCP-handler contains the information required without the need to
read the kernel ARP table.

Thanks to Bruno Ravara for spotting and characterising this bug.

src/dnsmasq.c

@@ -2033,9 +2033,24 @@ static void do_tcp_connection(struct listener *listener, time_t now, int slot)
   
   if (!option_bool(OPT_DEBUG))
     {
+      /* The code in edns0.c qthat decorates queries with the source MAC address depends
+	 on the code in arp.c, which populates a cache with the contents of the ARP table
+	 using netlink. Since the child process can't use netlink, we pre-populate
+	 the cache with the ARP table entry for our source here, including a negative entry
+	 if there is nothing for our address in the ARP table.
+	 
+	 When the edns0 code calls find_mac() in the child process, it will
+	 get the correct answer from the cache inherited from the parent
+	 without having to use netlink to consult the kernel ARP table.
+
+	 edns0_needs_mac() simply calls find_mac if any EDNS0 options
+	 which need a MAC address are enabled. */
+      
+      edns0_needs_mac(&tcp_addr, now);
+
       if (pipe(pipefd) == -1)
 	goto closeconandreturn; /* pipe failed */
-            
+
       if ((p = fork()) == -1)
 	{
 	  /* fork failed */