Building a Terminal Server from a Pi4

Sometimes in the world of networking, you just need console access to a device. Most of the time, it’s fine to connect in-band, over the network, but other times? You need to do stuff that takes that same network out of service, so out-of-band, or OOB is a must-have. To that end, most network devices offer serial console ports. Some use old-school DB9 connectors, others use an RJ45 jack, and many newer devices use USB-based console ports.

On the first 2 cases, you typically need some sort of USB serial adapter connected to your computer to make the connection. A couple of the most common chipsets used are the Prolific PL2303 family of chipsets, and the Silicon Labs CP210x family of chipsets. Interestingly, the USB-based console devices move that chipset out of an adapter and inside the network device. Hook up a USB-A (or -C) to Mini or Micro-USB cable, and you’re ready to connect to the device using the serial console app of your choice. Many of the latest devices have even shifted to USB-C for these onboard ports (and there was much rejoicing!)

So, my requirement? I’ve got 5 things in the rack in my home office that have serial console ports. All but 1 of them offers the USB console option, all of which use the Mini-USB connector on the device. So, off to the IOT junk box I keep, scavenging for parts. I found a pre-COVID supply chain disaster Raspberry Pi 4 board with power supply and a USB 3.0 hub. Why the hub? Well, the Pi only has a small number of USB ports, and I need more devices connected, so the hub solves that issue. I decided to beef things up a bit with the Argon ONE M.2 case, so I could run the Pi from an M.2 SSD rather than an SD card. I tossed an M.2 SATA SSD in the basement of the case and went to work. Note – this case doesn’t support NVMe, so make sure you’re not trying to use it here. I installed the latest Ubuntu LTS release on the Pi (22.04 LTS) on an SD Card, transferred the system over to the SSD, changed the bootloader order, and removed the SD Card. All ready.

Next? Just a couple of packages. First up, ser2net. It’s exactly what it sounds like – it lets you bridge a serial port to the network. Most commonly, you expose the serial port so that you telnet to a special port number and boom, you’re connected. Being more security minded, I bind to the loopback and use ssh. More on that in a bit.

One thing that you do need to think about is predictable serial port device names. Linux turns up usb serial ports in the order they’re connected, as /dev/ttyUSB0, ttyUSB1, etc. The hitch here is that things don’t always register as connected in the same order. In other words, you can plug 2 ports in, and they can flip positions across reboots. So what do you do? The udev daemon comes to your rescue here. I found a great guide with procedures on finding all the appropriate parameters. In the end, you’re going to create a udev rules file to map your USB serial ports to persistent names. Here’s my /etc/udev/rules.d/99-usb-serial.rules file:

# switches - internal serial
SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", ATTRS{serial}=="01373013", SYMLINK+="con-sw0-shire"
SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", ATTRS{serial}=="01373118", SYMLINK+="con-sw1-shire"

# prod and lab firewalls - internal serial
SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="8470", ATTRS{serial}=="04350063E4F5", SYMLINK+="con-fw-rivendell"
SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="8470", ATTRS{serial}=="0435005004C4", SYMLINK+="con-lab-fangorn"

# lab router - dongle
SUBSYSTEM=="tty", ATTRS{idVendor}=="067b", ATTRS{idProduct}=="2303", SYMLINK+="con-lab-isengard"

Once you’ve got that file in place, run the following command to cause udevd to recognize the new config and put the symlinks in-place: sudo udevadm control --reload-rules && sudo udevadm trigger.

Got your persistent device names in-place? Ok, it’s time to configure ser2net. Here’s my /etc/ser2net.yaml.

%YAML 1.1
define: &banner \r\n\o [\d]\r\n\r\n

connection: &rivendell
    accepter: telnet(rfc2217),tcp,,7000
    connector: serialdev,/dev/con-fw-rivendell,9600n81,local
      banner: *banner

connection: &switch0
    accepter: telnet(rfc2217),tcp,,7001
    connector: serialdev,/dev/con-sw0,9600n81,local
      banner: *banner

connection: &switch1
    accepter: telnet(rfc2217),tcp,,7002
    connector: serialdev,/dev/con-sw1,9600n81,local
      banner: *banner

connection: &fangorn
    accepter: telnet(rfc2217),tcp,,7003
    connector: serialdev,/dev/con-lab-fangorn,9600n81,local
      banner: *banner

connection: &isengard
    accepter: telnet(rfc2217),tcp,,7004
    connector: serialdev,/dev/con-lab-isengard,115200n81,local
      banner: *banner

The next piece of the puzzle? Access to those consoles from across the network. I’m handling this with some additional sshd instances. This requires 2 bits of additional config to get going. First, additional config files in /etc/ssh, 1 per additional instance. These instances are configured to telnet to the appropriate localhost-bound console port upon successful connect. As a matter of course, I also turn off PasswordAuthentication, which means no tunneled cleartext passwords, and enable Challenge-Response auth. Naturally, authenticating with certificates is enabled.

Include /etc/ssh/sshd_config.d/*.conf

Port 4000
PasswordAuthentication no
#PermitEmptyPasswords no
ChallengeResponseAuthentication yes

UsePAM yes
PrintMotd no
PidFile /run/

AcceptEnv LANG LC_*

ForceCommand telnet localhost 7000

The last piece, which is completely optional? Setup a WiFi AP on the Pi. I’ve not written up that piece here, as there are plenty of guides on doing that. Be aware of one point though – hostapd and the networkd configuration renderer are incompatible at this time. The solution is to either define your interface in /etc/network/interfaces.d/wlan0, or make sure your netplan config is using NetworkManager as the renderer.