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Status: Obsolete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