[tor-bugs] #15937 [Tor]: Clients fail on the 7th rapid SOCKS request to the same HS

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#15937: Clients fail on the 7th rapid SOCKS request to the same HS
----------------------+------------------------------------
 Reporter:  teor      |          Owner:  dgoulet
     Type:  defect    |         Status:  reopened
 Priority:  Low       |      Milestone:  Tor: 0.2.8.x-final
Component:  Tor       |        Version:
 Severity:  Normal    |     Resolution:
 Keywords:  tor-hs    |  Actual Points:
Parent ID:            |         Points:  small/medium
  Sponsor:  SponsorR  |
----------------------+------------------------------------

Comment (by dgoulet):

 Replying to [comment:17 teor]:
 > Replying to [comment:12 dgoulet]:
 > > Hrm ok it's a fun puzzle but I think I figured it out and the solution
 could be simple.
 > >
 > > In `rend_client_refetch_v2_renddesc()`, at the bottom if the fetch
 request fails by not finding a new HSDir because all 6 have been queried
 already, we call `rend_client_desc_trynow()` which closes all
 `RENDDESC_WAIT` connections for a .onion thus losing all previous 6.
 > >
 > > I can't figure out why we do that if we can't find an HSDir... so can
 we simply remove this?:
 > > {{{
 > >   if (ret <= 0) {
 > >     /* Close pending connections on error or if no hsdir can be found.
 */
 > >     rend_client_desc_trynow(rend_query->onion_address);
 > >   }
 > > }}}
 > >
 > > When the fetch succeeds (in `connection_dir_client_reached_eof()`), we
 already call that function to move to the next stage for HS connection so
 I propose we remove the above.
 >
 > Makes sense to me, but do we limit the number of pending connections to
 one per HSDir?

 One socks connection can query all 6 HSDirs.

 > And do we retry when the connections actually fail?

 There is a "cache" of last queried HSDir that is we don't query an HSDir
 more than once for the same request. Once the descriptor arrives or the
 SOCKS connection ends, the cache is purge for the requested onion.

 >
 > (I don't know this code very well. I am concerned that we could end up
 asking a HSDir multiple times, or end up stalling if the network is slow
 when we first try all 6, but speeds up later.)

 See `last_hid_serv_requests` in rendclient.c for more info.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15937#comment:18>
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