I often spend time away from home and want to be able to reach my home lab, both hardware and virtual. I use a Wireguard VPN, running within Home Assistant on a RPi 3 for the remote access, which means the network side is sorted. However I often find I’m juggling a few projects and might need to rebuild a hardware controller or an AP… that essentially needs console access. Whilst I could use a dedicated DC style console server they’re expensive, awkward, and overkill so I use Ser2Net on another RPi. There are plenty of tutorials on how to setup Ser2Net which are probably better than this one, but everything I found is based on an older version. Since then the config has changed to YAML and I found the defaults didn’t behave as I expected… so here we are.

Ser2Net “provides a way for a user to connect from a network connection to a serial port” – so says the project author Corey Minyard. You define a TTY interface and how you would like to connect to it. By default Ser2Net forwards raw data over TCP via a specified port to and from the TTY interface. Again, by default, you can only access a TTY over the network from localhost.

I threw my console server together using the latest version of Ubuntu for Raspberry Pi, Ubuntu 23.04 at the time of writing, running on a Pi 2b. Any old RPi is a good choice for the low power consumption and very light requirements.

Ubuntu 23.04 repositories contain ser2net version 4.3.11 which differs from previous version in that it uses YAML for the config. This is found in /etc/ser2net.yaml.

I’ve used three different types of USB Console – a couple of cheap FTDI cables from Amazon, USB console interface on an Aruba 7005 controller, and the Aruba TTL to serial cable for an AP. All were recognised by the OS.

How to set it up:

Build your Pi (or whatever machine you’re using) with Ubuntu 23.04 (or later)
Run sudo apt update && upgrade (just because we always should)
Install ser2net with apt install ser2net
Connect the USB serial interfaces and issue the command: sudo dmesg | grep ttyUSB
This will show you the USB to serial interfaces that have been recognised by the OS. It will look something like this:

You can now add these connections to the YAML file.

A quick note on security. Ser2Net doesn’t have any authentication. You can restrict the listener to localhost, as distinct from the host IP, and that means everything is protected by the strength of your ser2net host logon. I just want a telnet port forwarded to the TTY so it’s easy. This is not a good idea for any production environment without having other layers of security. In this case it’s a lab, and it’s only accessible either in person or via my VPN… so it’s good enough for me, but might not be for you.

The YAML for my console interfaces looks like this:

connection: &con0096
    accepter: telnet,192.168.26.3,2000
    enable: on
    options:
      banner: *banner
      kickolduser: true
      telnet-brk-on-sync: true
    connector: serialdev,
              /dev/ttyUSB0,
              9600n81,local
 

You need to increment the connection number, the port (2000 in this example) and the connector (/dev/ttyUSB0) in this example. If you have duplicates it won’t work properly though I believe you can use the same device in multiple connections to allow different port settings. The IP address of the accepter is where it listens for a connection. The default config has this set to tcp,localhost,xxxx which passes raw data over TCP (use something like nc) and is only available from the local machine. It probably goes without saying, but I’ll say it anyway, under the connector be sure to check the serial port settings are correct.

After changing the config file restart the service with sudo systemctl restart ser2net

I can now telnet to my ser2net host of 192.168.26.3 and depending on the port I get connected to a different machine. As shown in the screenshot above I have four interfaces connected and could use a USB hub or multi-port serial interface to access more machines.