I have a rack-mounted KVM now, and it’s great, but I’m working on building out a new Proxmox cluster which might not have a graphics output at all on some of the nodes. So, I need a new remote access solution for them.

The new nodes I’m planning on building will all be based on used consumer hardware, so I’m limited by what would be available on normal mATX boards. This doesn’t include IPMI, and if I go with AMD-based CPUs, doesn’t include an iGPU either. So I either need to get cheap GPUs to add to the second PCIe slot (which I then can’t use for things like HBAs), or at least put a GPU in for troubleshooting. However, adding a GPU often moves PCIe devices, leading to issues where the network card on enp4 is now enp5 when the GPU is attahed, for example. So trouleshooting for real would be a nightmare with this approach.

But! There is an alternative. Despite being ancient, many boards still have a COM header connected to the chipset, which provides a basic serial port header (‘COM1’ in Windows) and is completely usable. So, my goal is to connect this COM header to a female RJ45 port on the back of the computer, so I can plug in a ‘Cisco-style’ terminal cable. These sorts of cables (RJ45 serial to USB) are readily available cheaply on the internet, and I can open a terminal emulator on my laptop to diagnose the worst issues that lock up the system.

Since some systems are rumored to not POST without a graphics card, I’m testing this approach today.




Building the Cable

The Cisco/Juniper style RJ45 pinout is fairly simple if we exclude all of the flow control lines, so we just need 3 wires. I bought a pack of 2x5 IDC headers and a panel mount female RJ45 extension cable (which can be mounted in the case), cut the male end off, and crimped the resulting wires directly in the IDC header. Then I used a cheap USB to RJ45 serial cable and all worked great.

Here’s the pinout drawing for reference: Pinout

Proxmox Setup

Setup is really two steps - enabling kernel console via serial, and enabling a tty on that serial port. One of these requires editing the kernel cmdline (Which can vary a bit - see Proxmox’s docs for more info), and the other requires using systemd.

First, we need to add a new console argument to the kernel cmdline. Default for this is tty0, and we can add multiple arguments, so we should make sure both are specified. On my system I am using systemd-boot, so I edit the file /etc/kernel/cmdline and add:

console=tty0 console=ttyS0,115200n8

(ttyS0 should be ‘COM1’ in Windows, and should be the COM port on the motherboard. You can also use USB Serial here if you need to, or any other serial ports you have available)

Now, we just need to enable getty on our serial port so we have a proper login terminal:

systemctl enable serial-getty@ttyS0.service

And reboot, watching boot messages on the console happily

Of course, since Proxmox is Debian (with Ubuntu kernels), this should work on any of your other systems also Debian-based, as long as you are aware of the right place for the cmdline config (GRUB or Systemdboot).


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