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Re: [tor-talk] Advice on XMPP as a hidden service



On Mar 6, 2014, at 3:44, Mike Cardwell <tor@xxxxxxxxxxxxxxxxxx> wrote:

* on the Thu, Mar 06, 2014 at 02:02:50AM -0600, Cypher wrote:


1. Let's say my hidden service is xxxcf.onion. What would the users
final JID be? Would they still be user@xxxxxxxxxxxxx or would the
onion address come into play?

Depends. When somebody adds "user@xxxxxxxxxxxxx" into their XMPP client
it will do a DNS SRV lookup of "_xmpp-server._tcp.chat.cpunk.us" and
currently will receive "chat.cpunk.us" as the response, and so connect
to the host "chat.cpunk.us". I think a lot of clients fall back to
connecting directly to the A/AAAA record if the SRV record lookup fails.
So you *could* just add an additional higher priority SRV record to
chat.cpunk.us containing your onion address. I assume in this situation most clients would try to connect to the .onion address, fail immediately because they're not using Tor, and then fall back to the 2nd SRV record
"chat.cpunk.us"

However, there are probably many badly written clients out there which
will fail in lots of exotic ways. Allowing people to sign up with
"user@xxxxxxxxxxxxx", would help the service work with clients that don't support SRV records. People using Tor wouldn't be able to do SRV lookups anyway as they're not supported by the Tor resolver. It would also prevent DNS spoofing. "user@xxxxxxxxxxxxx" should also help avoid various leaks
that clients might have.

Hmm, good points. Though I wonder if force might me something needed in this case: use a properly functioning client or go elsewhere. My reasoning is that I have to wonder if a developer who isn't capable of writing DNS related code could be trusted to implement proper crypto. Since our server tries to focus on secure communication, perhaps this could be a teachable moment and help the Tor network at the same time.

2. Is it necessary to actually configure a hidden service at all?
Can't users just point their SOCKS proxy capable XMPP client to the
server or does going through an onion address provide something else
in this case that I'm not aware of?

Hidden services offer several benefits. If you're not using a hidden
service, your client could accidentally connect to the server outside
of Tor. The client might do something "helpful" like fall back to making a direct connection when it can't connect to the configured socks proxy.
It prevents DNS spoofing. It prevents malicious exit nodes attempting
to discover information about the traffic they're exiting, attempting
to perform SSL stripping attacks etc.

3. even though we run a Jingle node and act as a media relay, I assume
users still will not be able to do voice and video while connected to
our server over Tor.

If any of this relies on UDP, then no. Even if it's entirely TCP, the
latency added by onion routing will probably be too much in most cases.
Test it.

Is that correct? Is there any way to safely offer voice and video to
Tor connected users?

I don't know.

--
Mike Cardwell  https://grepular.com/     http://cardwellit.com/
OpenPGP Key    35BC AF1D 3AA2 1F84 3DC3  B0CF 70A5 F512 0018 461F
XMPP OTR Key   8924 B06A 7917 AAF3 DBB1  BF1B 295C 3C78 3EF1 46B4
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