Issues with router_get_by_nickname()

https://trac.torproject.org/projects/tor/ticket/1859

There are two problems in this bug:

1. When an OP makes a .exit request specifying itself as the exit, and the exit
   is not yet listed, Tor gets all the routerinfos needed for the circuit but
   discovers in circuit_is_acceptable() that its own routerinfo is not in the
   routerdigest list and cannot be used. Tor then gets locked in a cycle of
   repeating these two steps. When gathering the routerinfos for a circuit,
   specifically when the exit has been chosen by .exit notation, Tor needs to
   apply the same rules it uses later on when deciding if it can build a
   circuit with those routerinfos.

2. A different bug arises in the above situation when the Tor instance's
   routerinfo *is* listed in the routerlist, it shares its nickname with a
   number of other Tor nodes, and it does not have 'Named' rights to its
   nickname.
   So for example, if (i) there are five nodes named Bob in the network, (ii) I
   am running one of them but am flagged as 'Unnamed' because someone else
   claimed the 'Bob' nickname first, and (iii) I run my Tor as both client
   and exit the following can happen to me:
     - I go to www.evil.com
     - I click on a link www.evil.com.bob.exit
     - My request will exit through my own Tor node rather than the 'Named'
       node Bob or any of the others.
     - www.evil.com now knows I am actually browsing from the same computer
       that is running my 'Bob' node

So to solve both issues we need to ensure:

- When fulfilling a .exit request we only choose a routerinfo if it exists in
  the routerlist, even when that routerinfo is ours.
- When getting a router by nickname we only return our own router information
  if it is not going to be used for building a circuit.

We ensure this by removing the special treatment afforded our own router in
router_get_by_nickname(). This means the function will only return the
routerinfo of our own router if it is in the routerlist built from authority
info and has a unique nickname or is bound to a non-unique nickname.

There are some uses of router_get_by_nickname() where we are looking for the
router by name because of a configuration directive, specifically local
declaration of NodeFamilies and EntryNodes and other routers' declaration of
MyFamily. In these cases it is not at first clear if we need to continue
returning our own routerinfo even if our router is not listed and/or has a
non-unique nickname with the Unnamed flag.

The patch treats each of these cases as follows:

Other Routers' Declaration of MyFamily
 This happens in routerlist_add_family(). If another router declares our router
 in its family and our router has the Unnamed flag or is not in the routerlist
 yet, should we take advantage of the fact that we know our own routerinfo to
 add us in anyway? This patch says 'no, treat our own router just like any
 other'. This is a safe choice because it ensures our client has the same view
 of the network as other clients. We also have no good way of knowing if our
 router is Named or not independently of the authorities, so we have to rely on
 them in this.

Local declaration of NodeFamilies
 Again, we have no way of knowing if the declaration 'NodeFamilies
 Bob,Alice,Ringo' refers to our router Bob or the Named router Bob, so we have
to defer to the authorities and treat our own router like any other.

Local declaration of NodeFamilies
 Again, same as above. There's also no good reason we would want our client to
 choose it's own router as an entry guard if it does not meet the requirements
 expected of any other router on the network.

In order to reduce the possibility of error, the patch also replaces two
instances where we were using router_get_by_nickname() with calls to
router_get_by_hexdigest() where the identity digest of the router
is available.

src/or/connection_edge.c

@@ -600,7 +600,7 @@ circuit_discard_optional_exit_enclaves(extend_info_t *info)
         !edge_conn->chosen_exit_retries)
       continue;
     r1 = router_get_by_nickname(edge_conn->chosen_exit_name, 0);
-    r2 = router_get_by_nickname(info->nickname, 0);
+    r2 = router_get_by_hexdigest(info->identity_digest);
     if (!r1 || !r2 || r1 != r2)
       continue;
     tor_assert(edge_conn->socks_request);

src/or/rendclient.c

@@ -755,7 +755,7 @@ rend_client_get_random_intro(const rend_data_t *rend_query)
   intro = smartlist_get(entry->parsed->intro_nodes, i);
   /* Do we need to look up the router or is the extend info complete? */
   if (!intro->extend_info->onion_key) {
-    router = router_get_by_nickname(intro->extend_info->nickname, 0);
+    router = router_get_by_hexdigest(intro->extend_info->identity_digest);
     if (!router) {
       log_info(LD_REND, "Unknown router with nickname '%s'; trying another.",
                intro->extend_info->nickname);

src/or/routerlist.c

@@ -2244,9 +2244,6 @@ router_get_by_nickname(const char *nickname, int warn_if_unnamed)
     return router_get_by_hexdigest(nickname);
   if (!strcasecmp(nickname, UNNAMED_ROUTER_NICKNAME))
     return NULL;
-  if (server_mode(get_options()) &&
-      !strcasecmp(nickname, get_options()->Nickname))
-    return router_get_my_routerinfo();
 
   maybedigest = (strlen(nickname) >= HEX_DIGEST_LEN) &&
     (base16_decode(digest,DIGEST_LEN,nickname,HEX_DIGEST_LEN) == 0);