When binding on the IPv6 any address bind for IPv4 too

On some Linux systems this is the default. On most other operating
systems accepting IPv4 connections on a IPv6 socket only happens when
explicitly requested. Even on Linux it depends on the sysctl config.

This change makes it independent from the system's defaults whether IPv4
connections will be accepted too through the same socket.

src/network/network.cc

@@ -41,6 +41,7 @@
 #include <netinet/in.h>
 #include <assert.h>
 #include <errno.h>
+#include <string.h>
 #include <unistd.h>
 
 #include "dos_assert.h"
@@ -326,6 +327,14 @@ bool Connection::try_bind( const char *addr, int port_low, int port_high )
       throw NetworkException( "Unknown address family", 0 );
     }
 
+    if ( local_addr.sa.sa_family == AF_INET6
+      && memcmp(&local_addr.sin6.sin6_addr, &in6addr_any, sizeof(in6addr_any)) == 0 ) {
+      const int off = 0;
+      if ( setsockopt( sock(), IPPROTO_IPV6, IPV6_V6ONLY, &off, sizeof(off) ) ) {
+        perror( "setsockopt( IPV6_V6ONLY, off )" );
+      }
+    }
+
     if ( bind( sock(), &local_addr.sa, local_addr_len ) == 0 ) {
       set_MTU( local_addr.sa.sa_family );
       return true;