Enter running machine as systemd service

[MrFoxPro]

Is it possible to connect terminal stdin/stdout to deployed machine, to inspect what's going on there?

[luochen1990]

Having same issue, maybe a sshd service might work, but is there any easier way ( like a tty directly ) ?

[astro]

If you look into git history, this existed as bin/microvm-console before, using a pty instance, just for qemu and cloud-hypervisor. I dropped it because I wasn't too happy with it.

I am happy to have consoles/serials configurable but with sensible defaults. I cannot give an ETA when I'll have time for that.

Also, I am delighted that @Mic92 has updated https://github.com/Mic92/vmsh -- please play with that!

[luochen1990]

Thanks, but vmsh seems not working for me , not sure why :(

[Mic92]

I think I would need more time to fix some issues with VMSH. But here are some thoughts about serial/console support in microvm.nix itself:

• Serial devices do not set TERM or terminal size correctly. Here is an serial nixos module that works around that: https://github.com/numtide/srvos/blob/main/nixos/common/serial.nix
• virtio console would be ideal because it knows about the terminal size and can also update it dynamically. At least on the protocol level, I don't know what the state of the art hypervisors are doing.
• Another option some vsock based daemon that works like ssh but doesn't require any special network configuration.
• Or simply use ssh, this is what I am doing just now:

I allocate an tap interface called "management" to each vm (on the host I use mgt-$name). And allow ssh traffic from it:

{
    # Only allow ssh on internal tap devices
    networking.firewall.interfaces.management.allowedTCPPorts = [ 22 ];
    services.openssh.openFirewall = false;
}

Than I set the link-local ipv6 address to "fe80::1" on the host and "fe80::2" in the VM.

I can than use this ssh wrapper to access my machine:

{
  
  environment.systemPackages = [
    (pkgs.writeScriptBin "ssh-vm" ''
      #!/usr/bin/env bash
      if [[ "$#" -ne 1 ]]; then
        echo "Usage: $0 <vm-name>"
        exit 1
      fi
      vm=$1
      shift
      # we can disable host key checking because we use ipv6 link local addresses and no other vm can spoof them on this interface
      ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no "$@" root@fe80::2%mgt-$vm
    '')
  ];
}

This allows me to login by using the VM name:

$ ssh-vm foo

[Mic92]

Systemd also now parses terminal name and size from the kernel command line, but this would be mainly useful for the initial terminal at boot time and not for ad-hoc ones: https://github.com/systemd/systemd/blob/6ac299e3cedb1d9eb8df88010c4994a90aa12a9a/NEWS#L144

[bouk]

A future version of systemd will make it easy to connect to a running VM over VSOCK: https://github.com/systemd/systemd/pull/30777 which this project can use!

[jim3692]

I just came across this issue on my configuration. For me, @Mic92's solution, with SSH over IPv6, did not work. I instead changed the Qemu parameters to forward /dev/ttyS0 to a unix sock. This allows me to at least access my VMs using socat.

I have these changes in this branch: https://github.com/jim3692/microvm.nix/tree/console-in-unix-sock

I have also implemented the microvm -s <name> command, which runs socat in raw mode, inside screen. I could not find an easier way to be able to leave the socat session.

EDIT: My VM's IPv6 is fe80::ff:fe00:1 and not fe80::2. I am not sure how link local works, but the VM's MAC is 02:00:00:00:00:01. I managed to successfully SSH using the correct IP.

[astro]

I prefer waiting for ssh over vsock rather than bringing back what we had before with microvm-console for only a few hypervisors.

BTW, find your machine's link-local addresses by pinging ff02::1%$interface (that's my favourite IPv6 address).

[tomfitzhenry]

I prefer waiting for ssh over vsock

This is doable today.

In your host:

microvm.my-vm.vsock.cid = 1337;

In your guest:

services.openssh = {
  enable = true;
  startWhenNeeded = true;
};
systemd.sockets.sshd = {
  socketConfig = {
    ListenStream = [
      "vsock:1337:22"
    ];
  };
};

Then, to connect to your guest from your host:

$ ssh -o "ProxyCommand socat - VSOCK-CONNECT:1337:22" root@localhost