tor/doc/spec/proposals/127-dirport-mirrors-downloads.txt

Filename: 127-dirport-mirrors-downloads.txt
Title: Relaying dirport requests to Tor download site
Version: $Revision$
Last-Modified: $Date$
Author: Roger Dingledine
Created: 2007-12-02
Status: Needs-Research

1. Overview

  Some countries and networks block connections to the Tor website. As
  time goes by, this will remain a problem and it may even become worse.

  We have a big pile of mirrors (google for "Tor mirrors"), but few of
  our users think to try a search like that. Also, many of these mirrors
  might be automatically blocked since their pages contain words that
  might cause them to get blocked. And lastly, we can imagine a future
  where the blockers are aware of the mirror list too.

  Here we describe a new set of URLs for Tor's DirPort that will relay
  connections from users to the official Tor download site. Rather than
  trying to cache a bunch of new Tor packages (which is a hassle in terms
  of keeping them up to date, and a hassle in terms of drive space used),
  we instead just proxy the requests directly to Tor's /dist page.

  Specifically, we should support

    GET /tor/dist/$1

  and

    GET /tor/website/$1

2. Linked connections

  Check out the connection_ap_make_link() function, as called from
  directory.c. Tor clients use this to create a "fake" socks connection
  back to themselves, and then they attach a directory request to it,
  so they can launch directory fetches via Tor. We could piggyback on
  this feature.

3. One-hop circuits or three-hop circuits?

  We could relay the connections directly to the download site -- but
  this produces recognizable outgoing traffic on the bridge or cache's
  network, which will probably surprise our nice volunteers. (Is this
  a good enough reason to discard the direct connection idea?)

  But we still have a choice: should we do a one-hop begindir-style
  connection to the mirror site (make a one-hop circuit to it, then send a
  'begindir' cell down the circuit), or should we do a normal three-hop
  anonymized connection?

  If these mirrors are mainly bridges, doing a one-hop connection creates
  another way to enumerate bridges. That would argue for three-hop. On
  the other hand, downloading a 10+ megabyte installer through a normal
  Tor circuit can't be fun. But if you're already getting throttled a
  lot because you're in the "relayed traffic" bucket, you're going to
  have to accept a slow transfer anyway. So three-hop it is.

  Speaking of which, we would want to label this connection
  as "relay" traffic for the purposes of rate limiting; see
  connection_counts_as_relayed_traffic() and or_conn->client_used. This
  will be a bit tricky though, because it uses the bridge's guards.

4. Scanning resistance

  One other goal we'd like to achieve, or at least not hinder, is making
  it hard to scan large swaths of the Internet to look for responses
  that indicate a bridge.

  In general this is a really hard problem, so it's not critical that
  we solve it here. But we can note that some bridges should open their
  DirPort (and offer this functionality), and others shouldn't. Then some
  bridges provide a download mirror while others are scanning-resistant.

5. Integrity checking

  If we serve this stuff in plaintext from the bridge, anybody in between
  the user and the bridge can intercept and modify it. The bridge can too.

  If we do an anonymized three-hop connection, the exit node can also
  intercept and modify the exe it sends back.

  Are we setting ourselves up for rogue exit relays, or rogue bridges,
  that trojan our users?

  Answer #1: Users need to do pgp signature checking. Not a very good
  answer, a) because it's complex, and b) because they don't know the
  right signatures in the first place.

  Answer #2: The mirrors could exit from a specific Tor relay, using the
  '.exit' notation. This would make connections a bit more brittle, but
  would resolve the rogue exit relay issue. We could even round-robin
  among several, and the list could be dynamic -- for example, all the
  relays with an Authority flag that allow exits to the Tor website.

  Answer #3: We could suggest that users only use trusted bridges for
  fetching a copy of Tor. Hopefully they heard about the bridge from a
  trusted source rather than from the adversary.

  Answer #4: What if the adversary is trawling for Tor downloads by
  network signature -- either by looking for known bytes in the binary,
  or by looking for "GET /tor/dist/"? It would be nice to encrypt the
  connection from the bridge user to the bridge. And we can! The bridge
  already supports TLS.  Rather than initiating a TLS renegotiation after
  connecting to the ORPort, the user should actually request a URL. Then
  the ORPort can either pass the connection off as a linked conn to the
  dirport, or renegotiate and become a Tor connection, depending on how
  the client behaves.