[tor-bugs] #6761 [Tor Relay]: PDS_NO_EXISTING_SERVERDESC_FETCH is somewhat archaic

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#6761: PDS_NO_EXISTING_SERVERDESC_FETCH is somewhat archaic
-----------------------+----------------------------------------------------
 Reporter:  arma       |          Owner:                    
     Type:  defect     |         Status:  new               
 Priority:  normal     |      Milestone:  Tor: 0.2.4.x-final
Component:  Tor Relay  |        Version:                    
 Keywords:             |         Parent:                    
   Points:             |   Actualpoints:                    
-----------------------+----------------------------------------------------
 In bug #366 we made it so Tor won't open a second dir fetch to an
 authority if it has one already. Great.
 {{{
         rs = router_pick_trusteddirserver(type, pds_flags);
         if (rs == NULL && (pds_flags & (PDS_NO_EXISTING_SERVERDESC_FETCH|
                                         PDS_NO_EXISTING_MICRODESC_FETCH)))
 {
 [...]
             log_debug(LD_DIR, "Deferring serverdesc fetch: all authorities
 "
                       "are in use.");
 }}}

 But we didn't update it to look for begindir conns, so it only applies to
 direct dir fetches. Ok.
 {{{
       if (no_microdesc_fetching) {
         if (connection_get_by_type_addr_port_purpose(
              CONN_TYPE_DIR, &addr, d->dir_port,
 DIR_PURPOSE_FETCH_MICRODESC)) {
           ++n_busy;
           continue;
 }}}

 So it doesn't apply to clients, only relays. That makes sense, because
 relays are the ones who typically would contact authorities anyway.
 {{{
   int prefer_authority = directory_fetches_from_authorities(options);
 }}}

 But when a relay starts up and gets a consensus, it has a line like this
 nowadays:
 {{{
 Sep 04 05:47:40.000 [info] launch_descriptor_downloads(): Launching 33
 requests for 3114 routers, 96 at a time
 }}}

 33 requests! Surely that's way more than the 8 or so authorities we have.
 And relays don't use begindir to talk to authorities, since it slows them
 down too much:
 {{{
   int use_begindir = supports_begindir &&
                      directory_command_should_use_begindir(options, _addr,
                        or_port, router_purpose, anonymized_connection);
 }}}

 Doesn't that mean we hit the "one per authority" limit and drop the rest
 of those requests?

 It turns out that directory_fetches_from_authorities() is false for most
 relays when they start up:
 {{{
   if (server_mode(options) && router_pick_published_address(options,
 &addr)<0)
     return 1; /* we don't know our IP address; ask an authority. */
   refuseunknown = ! router_my_exit_policy_is_reject_star() &&
     should_refuse_unknown_exits(options);
   if (options->DirPort == NULL && !refuseunknown)
     return 0;
   if (!server_mode(options) || !advertised_server_mode())
     return 0;
   me = router_get_my_routerinfo();
   if (!me || (!me->dir_port && !refuseunknown))
     return 0; /* if dirport not advertised, return 0 too */
   return 1;
 }}}

 So these relays end up asking arbitrary other relays they found in the
 consensus! Cue Nick's circus music here. Not the best way to get fresh
 info.

 In my case here (and I expect it's a common case), my relay failed the
 "!advertised_server_mode" check, since it hadn't done its reachability
 test yet so it hadn't published a descriptor yet.

 Maybe this is actually a feature that just-starting-up relays don't fetch
 descriptors from authorities. It probably doesn't hurt much, and probably
 helps authority load a bit.

 But I don't think it's a feature that we allow multiple descriptor-
 fetching dir requests in parallel to an authority iff they're begindir
 requests.

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/6761>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online
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