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

Filename: 127-dirport-mirrors-downloads.txt
Title: Relaying dirport requests to Tor download site / website
Author: Roger Dingledine
Created: 2007-12-02
Status: Draft

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 banned. 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. Direct connections, 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?)

  Even if we don't do direct connections, 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 either a direct or 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 these connections will use the
  bridge's guards.

3. 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 we shouldn't demand to
  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 can remain
  scanning-resistant.

4. 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 signing keys 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: The mirrors should connect to the main distribution site
  via SSL. That way the exit relay can't influence anything.

  Answer #4: 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 #5: 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.

5. Linked connections: at what level should we proxy?

  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 can piggyback on
  this feature.

  We need to decide if we're going to be passing the bytes back and
  forth between the web browser and the main distribution site, or if
  we're going to be actually acting like a proxy (parsing out the file
  they want, fetching that file, and serving it back).

  Advantages of proxying without looking inside:
    - We don't need to build any sort of http support (including
      continues, partial fetches, etc etc).
  Disadvantages:
    - If the browser thinks it's speaking http, are there easy ways
      to pass the bytes to an https server and have everything work
      correctly? At the least, it would seem that the browser would
      complain about the cert. More generally, ssl wants to be negotiated
      before the URL and headers are sent, yet we need to read the URL
      and headers to know that this is a mirror request; so we have an
      ordering problem here.
    - Makes it harder to do caching later on, if we don't look at what
      we're relaying. (It might be useful down the road to cache the
      answers to popular requests, so we don't have to keep getting
      them again.)

6. Outstanding problems

  1) HTTP proxies already exist.  Why waste our time cloning one
  badly? When we clone existing stuff, we usually regret it.

  2) It's overbroad.  We only seem to need a secure get-a-tor feature,
  and instead we're contemplating building a locked-down HTTP proxy.

  3) It's going to add a fair bit of complexity to our code.  We do
  not currently implement HTTPS.  We'd need to refactor lots of the
  low-level connection stuff so that "SSL" and "Cell-based" were no
  longer synonymous.

  4) It's still unclear how effective this proposal would be in
  practice. You need to know that this feature exists, which means
  somebody needs to tell you about a bridge (mirror) address and tell
  you how to use it. And if they're doing that, they could (e.g.) tell
  you about a gmail autoresponder address just as easily, and then you'd
  get better authentication of the Tor program to boot.