Force combinationation of --socks-proxy and --proto UDP to use IPv4.

Our current socks.c code does not handle IPv6 + UDP mode (socket
negotiated with server is IPv4-only, addresses passed in the
packets are IPv4-only).  If this combination is specified, print
an explanatory message and force IPv4-only.

While at it, extend socks.c code to print address+port of auxiliary
UDP connection to SOCKS server (helps debugging).

Trac: #1221

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20191020150039.21516-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18952.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

src/openvpn/options.c

@@ -2825,6 +2825,24 @@ options_postprocess_mutate_ce(struct options *o, struct connection_entry *ce)
 #endif
     }
 
+    /* our socks code is not fully IPv6 enabled yet (TCP works, UDP not)
+     * so fall back to IPv4-only (trac #1221)
+     */
+    if (ce->socks_proxy_server && proto_is_udp(ce->proto) && ce->af != AF_INET)
+    {
+        if (ce->af == AF_INET6)
+        {
+            msg(M_INFO, "WARNING: '--proto udp6' is not compatible with "
+                "'--socks-proxy' today.  Forcing IPv4 mode." );
+        }
+        else
+        {
+            msg(M_INFO, "NOTICE: dual-stack mode for '--proto udp' does not "
+                "work correctly with '--socks-proxy' today.  Forcing IPv4." );
+        }
+        ce->af = AF_INET;
+    }
+
     /*
      * Set MTU defaults
      */

src/openvpn/socks.c

@@ -414,6 +414,10 @@ recv_socks_reply(socket_descriptor_t sd,
     {
         memcpy(&addr->addr.in4.sin_addr, buf + 4, sizeof(addr->addr.in4.sin_addr));
         memcpy(&addr->addr.in4.sin_port, buf + 8, sizeof(addr->addr.in4.sin_port));
+        struct gc_arena gc = gc_new();
+        msg(M_INFO, "SOCKS proxy wants us to send UDP to %s",
+            print_sockaddr(addr, &gc));
+        gc_free(&gc);
     }