[tor-dev] Idea which may or may not of been discussed

[Nathaniel Suchy]

Currently tor traffic uses an TLS handshake hostname like the following:

$ sudo tcpdump -An "tcp" | grep "www"
listening on pktap, link-type PKTAP (Apple DLT_PKTAP), capture size 262144 bytes
.............".
...www.odezz26nvv7jeqz1xghzs.com......................#.!...www.bxbko3qi7vacgwyk4ggulh.com..........6....m.....>...:.........|../* Z....W....X=..6...C../....................................0...0..0.......'....F./0.. *.H........0%1#0!..U....www.b6zazzahl3h3faf4x2.com0...160402000000Z..170317000000Z0'1%0#..U....www.tm3ddrghe22wgqna5u8g.net0..0..

A network observer could run a DNS lookup on the hostnames and see if they
are real or not. So my idea would be to register a set of random hostnames
which are legitimate and point the IPs somewhere to avoid looking for an NX
Domain response and dropping the stream. You could even give each relay a
unique subdomain and rotate these every few weeks. This may be expensive to
implement but could make blocking Tor traffic with this method harder.
Thoughts?

Cordially,
Nathaniel Suchy

[Matt Traudt]

Post by Nathaniel Suchy
$ sudo tcpdump -An "tcp" | grep "www"
listening on pktap, link-type PKTAP (Apple DLT_PKTAP), capture size 262144 bytes
.............". ...www.odezz26nvv7jeqz1xghzs.com.........
.............#.!...www.bxbko3qi7vacgwyk4ggulh.com.........
.6....m.....>...:.........|../*
Z....W....X=..6...C../....................................0...0..0.......'....F./0..
*.H........0%1#0!..U....www.b6zazzahl3h3faf4x2.com0...160402000000Z..170317000000Z0'1%0#..U....www.tm3ddrghe22wgqna5u8g.net0..0..
A network observer could run a DNS lookup on the hostnames and see if
they are real or not. So my idea would be to register a set of random
hostnames which are legitimate and point the IPs somewhere to avoid
looking for an NX Domain response and dropping the stream. You could
even give each relay a unique subdomain and rotate these every few
weeks. This may be expensive to implement but could make blocking Tor
traffic with this method harder. Thoughts?

Why wouldn't it be just as easy for censors to identify the small set of
registered domains that Tor relays use and block TLS connections that
involve them?

I don't see how changing the domain a relay uses from aaaaaa.foo.com to
bbbbbb.foo.com helps. The censor would just notice 'foo.com' and block it.

In fact, I think this would make censorship easier.

Matt

[David Fifield]

Post by Matt Traudt
Why wouldn't it be just as easy for censors to identify the small set of
registered domains that Tor relays use and block TLS connections that
involve them?

And in general, IMO pluggable transports are the right layer to address
this, not the Tor TLS layer. The way Tor uses TLS is already way more
complicated than it needs to be, partly because of past attempts to
build obfuscation into the core protocol rather than handling it as a
separate layer.
https://trac.torproject.org/projects/tor/wiki/org/projects/Tor/TLSHistory

The certificate server name is a pretty easy distinguishing feature--but
it's not the only one. But there are other ways in which the Tor TLS
handshake stands out, even if you use real server names with legit
certs. It's not easy to hack OpenSSL into perfectly imitating e.g., a
Firefox TLS fingerprint. That's why meek uses an instance of Firefox to
do its TLS, and why https://github.com/refraction-networking/utls
exists.