SSH Over Serial Port

What started as a rant on Mastadon has turned into an actual solution. The problem I need to solve is an out-of-band (non-network) connection between two machines. The problem also was I needed a super secure mechanism to connect to a local Linux box which was VPN’d into a corporate network. (I didn’t want a second network interface because I’m not smart enough to set up security to properly protect a dual-homed machine. The most secure connection is no connection.)

I started thinking: what if I could run a terminal aware protocol such as telnet carried over a serial port. Rather than have a raw character connection over the RS232, let’s carry something that has a terminal control plane. My first thought was hacking telnetd/telnet code to route over serial port. But a colleague (hi, KyleS!) suggested socat.

I’ve used socat before. It’s an amazing tool that can connect disparate socket/file descriptors together.

I prototyped the idea using my Fedora Linux laptop on one side and a Raspberry-Pi on the other. The two machines are connected via USB FTDI serial and a null modem adapter. I am using the R-Pi as the ssh server side. Rather than worry about telnet, I used the already running ssh.

On the Raspberry-Pi (server side):

 socat GOPEN:/dev/ttyUSB0,cfmakeraw,b115200 TCP:localhost:22,reuseaddr

On the Fedora Linux (client side):

 socat TCP-LISTEN:2222,bind=localhost,reuseaddr GOPEN:/dev/ttyUSB0,cfmakeraw,b115200

And then the connection from the client to the server is as simple as:

ssh -p 2222 pi@localhost

And now my serial connection can run Vim with full glorious syntax highlighting. The terminal resize SIGWINCH works. The connection is at 115200 so slightly slower than even 10Mbps Ethernet. But I have an out of band (non-Ethernet) connection between two machines so if (when) I bork the network config (often), I can recover the remote. I can edit code through the serial connection (edit my borked network scripts).

Next logical step is to set up the same socat/Dropbear trick with router firmware.

UPDATE 20230207. The initial connection can be very quirky if there is anything leftover in the serial port. Unlike a file handle or a network socket, closing the serial port will leave detritus available to read. If the ssh connection fails for some reason (in my case, hitting problems with known_hosts and ‘localhost’), the trash on the serial port will confuse the ssh at each ends.

I’m tinkering with some solutions. I’m hoping calls to ‘stty -F /dev/ttyUSB0 sane’ will help. Will update this post when I’ve found a reliable solution.

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