Scenario:
Many embedded devices (running Linux) out in the fields, behind routers so NAT'd and we can't make connections to them.
We need for a support person to be able to initiate a terminal/shell session on any of the devices.
Their local terminal will also be NAT'd behind a corporate firewall, so we need some central "meeting point" that both they and the device can connect to.
If necessary, we could require the support person to log into some sort of terminal server, but I'd prefer a solution that just popped up a terminal window on their desktop.
We can (through other means) tell the device to execute some arbitary script or application to start up the session.
Without the NAT, it's just SSH to the device and away we go. But what are my options in this NAT'd environment?
We're OK to develop code at either end or at the meeting point server if required, but obviously if there are apps out there so we don't have to write stuff, even better.
Pointers to other questions I may have missed (although I have looked) or to applications that I should consider for the central "meeting point" server welcomed
SSH is an adequate tool for this. You will, as you say, need a middle-man server. But it would be very easy to set up, assuming that your 'other means of executing a script' are remote and can be executed from your office.
So, fire up a new server on a global IP (an Amazon AWS micro node is free for a year and would do the job just fine), and install an ssh deamon. Say it has the hostname middleman.example.org.
The script to put onto your embedded devices would look like;
#!/bin/bash
ssh -i ./middle_id.pem -R 22:localhost:2222 middleuser@middle.example.org
(The private key authentication would be a way of making the login non-interactive)
The script to put onto your desktop machines would look like; (assuming the argument $1 is the IP of the embedded device, and that prod_remote_device.sh executes the above script on the chosen embedded device.)
#!/bin/bash
./prod_remote_device.sh $1
ssh -i ./device_id.pem deviceuser@middle.example.org:2222
And that should forward your connection to the embedded device.