Mosh over Tor (Except Not Really)

This post first appeared on my old blog in June 2017. It is preserved, but maybe not updated, here.

I'm in the process of setting up a new server and I'm trying to be super ultra mega secure about it. It's running FreeBSD with some fancy security options enabled, blah blah blah, oh and I made SSH over Tor the only way to remotely access it for administration. It's a private onion service, which is super cool in itself, but since I don't mind leaking the location of this server, it is also a single-onion service. This does seem to have a positive impact on the speed and latency to the machine, but after a few weeks of managing the machine completely over Tor, I determined I wanted more usability.

My main complaint is the lack of immediate local echoing of what I type. Mosh does that, but mosh uses UDP, which doesn't work over Tor. There's two ways I could approach this. The first would actually be called "Mosh over Tor," but I ultimately went for the second as it would actually allow me to roam (another great feature of mosh).

  1. I could use socat to tunnel UDP over Tor. Create the tunnel and then mosh to localhost:some-port.

  2. Or I could authenticate over SSH over Tor and then create the actual UDP connection over the regular Internet.

So I now present to you the script I use to (not really) use mosh over Tor. It's a healthy mixture of things specific to me and hardcoded values that need changing for every use case. But it is a starting point if you would like to try your hand at (2) above too.

#!/usr/bin/env bash
SUCCESS_LINE=$(ssh $SSH_HOSTNAME "mosh-server new -i $MOSH_IP" | grep 'MOSH CONNECT')
[[ "$SUCCESS_LINE" == "" ]] && echo "failed to connect :(" && exit 1
MOSH_PORT=$(echo $SUCCESS_LINE | cut -d ' ' -f 3)

export MOSH_KEY="$(echo $SUCCESS_LINE | cut -d ' ' -f 4)"
mosh-client $MOSH_IP $MOSH_PORT

At its core, mosh first authenticates you over SSH. It then runs mosh-server on the server and passes some information back to your local computer. It then uses that information to tell mosh-client how to connect to the server. All I've done here is do all that manually (in what is probably a bad, easily-broken, and I-am-lucky-it-works way).

So if you save the above script as moshfoo, make it executable, put it in your PATH, and fix the MOSH_IP variable to be the public IP of the server foo, then you can run


and assuming ssh can figure out how to connect and authenticate you, everything will work out wonderfully.

Do you have a better way to accomplish local echo when SSHing into an onion service? Do you want to call me stupid? Or perhaps inform me that I'm blind for missing an obvious solution or glaring problem? Please let me know!

Matt Traudt (pastly)

Tech, Pets, and Vettes