Add client code to detect attempts to connect to 127.0.0.1 etc

We detect and reject said attempts if there is no chosen exit node or
circuit: connecting to a private addr via a randomly chosen exit node
will usually fail (if all exits reject private addresses), is always
ill-defined (you're not asking for any particular host or service),
and usually an error (you've configured all requests to go over Tor
when you really wanted to configure all _remote_ requests to go over
Tor).

This can also help detect forwarding loop requests.

Found as part of bug2279.

src/or/connection.c

@@ -1205,9 +1205,11 @@ connection_init_accepted_conn(connection_t *conn, uint8_t listener_type)
           conn->state = AP_CONN_STATE_SOCKS_WAIT;
           break;
         case CONN_TYPE_AP_TRANS_LISTENER:
+          TO_EDGE_CONN(conn)->is_transparent_ap = 1;
           conn->state = AP_CONN_STATE_CIRCUIT_WAIT;
           return connection_ap_process_transparent(TO_EDGE_CONN(conn));
         case CONN_TYPE_AP_NATD_LISTENER:
+          TO_EDGE_CONN(conn)->is_transparent_ap = 1;
           conn->state = AP_CONN_STATE_NATD_WAIT;
           break;
       }

src/or/connection_edge.c

@@ -1659,6 +1659,27 @@ connection_ap_handshake_rewrite_and_attach(edge_connection_t *conn,
         connection_mark_unattached_ap(conn, END_STREAM_REASON_TORPROTOCOL);
         return -1;
       }
+      if (!conn->use_begindir && !conn->chosen_exit_name && !circ) {
+        tor_addr_t addr;
+        if (tor_addr_from_str(&addr, socks->address) >= 0 &&
+            tor_addr_is_internal(&addr, 0)) {
+          /* If this is an explicit private address with no chosen exit node,
+           * then we really don't want to try to connect to it.  That's
+           * probably an error. */
+          if (conn->is_transparent_ap) {
+            log_warn(LD_NET,
+                     "Rejecting request for anonymous connection to private "
+                     "address %s on a TransPort or NatdPort.  Possible loop "
+                     "in your NAT rules?", safe_str_client(socks->address));
+          } else {
+            log_warn(LD_NET,
+                     "Rejecting SOCKS request for anonymous connection to "
+                     "private address %s", safe_str_client(socks->address));
+          }
+          connection_mark_unattached_ap(conn, END_STREAM_REASON_PRIVATE_ADDR);
+          return -1;
+        }
+      }
 
       if (!conn->use_begindir && !conn->chosen_exit_name && !circ) {
         /* see if we can find a suitable enclave exit */

src/or/or.h

@@ -583,6 +583,9 @@ typedef enum {
 /** This is a connection on the NATD port, and the destination IP:Port was
  * either ill-formed or out-of-range. */
 #define END_STREAM_REASON_INVALID_NATD_DEST 261
+/** The target address is in a private network (like 127.0.0.1 or 10.0.0.1);
+ * you don't want to do that over a randomly chosen exit */
+#define END_STREAM_REASON_PRIVATE_ADDR 262
 
 /** Bitwise-and this value with endreason to mask out all flags. */
 #define END_STREAM_REASON_MASK 511
@@ -1170,6 +1173,10 @@ typedef struct edge_connection_t {
    * zero, abandon the associated mapaddress. */
   unsigned int chosen_exit_retries:3;
 
+  /** True iff this is an AP connection that came from a transparent or
+   * NATd connection */
+  unsigned int is_transparent_ap:1;
+
   /** If this is a DNSPort connection, this field holds the pending DNS
    * request that we're going to try to answer.  */
   struct evdns_server_request *dns_server_request;

src/or/reasons.c

@@ -40,6 +40,8 @@ stream_end_reason_to_control_string(int reason)
     case END_STREAM_REASON_NET_UNREACHABLE: return "NET_UNREACHABLE";
     case END_STREAM_REASON_SOCKSPROTOCOL: return "SOCKS_PROTOCOL";
 
+    case END_STREAM_REASON_PRIVATE_ADDR: return "PRIVATE_ADDR";
+
     default: return NULL;
   }
 }
@@ -125,6 +127,9 @@ stream_end_reason_to_socks5_response(int reason)
       return SOCKS5_NET_UNREACHABLE;
     case END_STREAM_REASON_SOCKSPROTOCOL:
       return SOCKS5_GENERAL_ERROR;
+    case END_STREAM_REASON_PRIVATE_ADDR:
+      return SOCKS5_GENERAL_ERROR;
+
     default:
       log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
              "Reason for ending (%d) not recognized; "