Implement proposal 204: ignore subdomains in hidden service addresses

The implementation is pretty straightforward: parse_extended_hostname() is
modified to drop any leading components from an address like
'foo.aaaaaaaaaaaaaaaa.onion'.

changes/proposal204

@@ -0,0 +1,5 @@
+  o Minor features:
+    - Ignore sub-domain components of a .onion address. This makes HTTP "virtual"
+      hosting possible: http://foo.aaaaaaaaaaaaaaaa.onion/ and
+      http//bar.aaaaaaaaaaaaaaaa.onion/ can be two different websites hosted at
+      the same location. Implements proposal 204.

src/or/connection_edge.c

@@ -2813,6 +2813,9 @@ connection_ap_can_use_exit(const entry_connection_t *conn, const node_t *exit)
 /** If address is of the form "y.onion" with a well-formed handle y:
  *     Put a NUL after y, lower-case it, and return ONION_HOSTNAME.
  *
+ *  If address is of the form "x.y.onion" with a well-formed handle x:
+ *     Drop "x.", put a NUL after y, lower-case it, and return ONION_HOSTNAME.
+ *
  * If address is of the form "y.onion" with a badly-formed handle y:
  *     Return BAD_HOSTNAME and log a message.
  *
@@ -2826,6 +2829,7 @@ hostname_type_t
 parse_extended_hostname(char *address)
 {
     char *s;
+    char *q;
     char query[REND_SERVICE_ID_LEN_BASE32+1];
 
     s = strrchr(address,'.');
@@ -2840,9 +2844,18 @@ parse_extended_hostname(char *address)
 
     /* so it is .onion */
     *s = 0; /* NUL-terminate it */
-    if (strlcpy(query, address, REND_SERVICE_ID_LEN_BASE32+1) >=
+    /* locate a 'sub-domain' component, in order to remove it */
+    q = strrchr(address, '.');
+    if (q == address) {
+      goto failed; /* reject sub-domain, as DNS does */
+    }
+    q = (NULL == q) ? address : q + 1;
+    if (strlcpy(query, q, REND_SERVICE_ID_LEN_BASE32+1) >=
         REND_SERVICE_ID_LEN_BASE32+1)
       goto failed;
+    if (q != address) {
+      memmove(address, q, strlen(q) + 1 /* also get \0 */);
+    }
     if (rend_valid_service_id(query)) {
       return ONION_HOSTNAME; /* success */
     }

src/test/test.c

@@ -1412,11 +1412,20 @@ test_rend_fns(void)
   char address2[] = "aaaaaaaaaaaaaaaa.onion";
   char address3[] = "fooaddress.exit";
   char address4[] = "www.torproject.org";
+  char address5[] = "foo.abcdefghijklmnop.onion";
+  char address6[] = "foo.bar.abcdefghijklmnop.onion";
+  char address7[] = ".abcdefghijklmnop.onion";
 
   test_assert(BAD_HOSTNAME == parse_extended_hostname(address1));
   test_assert(ONION_HOSTNAME == parse_extended_hostname(address2));
+  test_streq(address2, "aaaaaaaaaaaaaaaa");
   test_assert(EXIT_HOSTNAME == parse_extended_hostname(address3));
   test_assert(NORMAL_HOSTNAME == parse_extended_hostname(address4));
+  test_assert(ONION_HOSTNAME == parse_extended_hostname(address5));
+  test_streq(address5, "abcdefghijklmnop");
+  test_assert(ONION_HOSTNAME == parse_extended_hostname(address6));
+  test_streq(address6, "abcdefghijklmnop");
+  test_assert(BAD_HOSTNAME == parse_extended_hostname(address7));
 
   pk1 = pk_generate(0);
   pk2 = pk_generate(1);