PVE Guest crash detection

The Problem

Sometimes VMs/LXCs crash & there's no way to have Proxmox detect & reset them...

Possible Solution(s)

Have the VM/LXC itself run a watchdog & send a message regularly to Proxmox.

Then...

Have Proxmox watch for stale messages & reset any VMs/LXCs that are out of date by some specific time limit.

Questions

What messaging tools are built-in to Proxmox AND suitable?

One option is MQTT. But I'd rather keep this entirely contained to the PVE server itself.

Something using ncat might be best.

(That's ncat, not netcat... netcat doesn't work...)

• sudo apt install ncat

Thoughts

On the guests

Finding relevant info about guests

VMID of a container can be found in /etc/mtab via script:

• VMID=`cat /etc/mtab | grep '/dev/mapper/' | head -n1 | cut -d'-' -f4`
• PVEHOST=Schizo310.TinkerNet.ca
• TYPE=CT

For VMs, you'll need to put it into the SMBIOS Settings under Options (maybe there's a way to automate this?) & then read it in via script using dmidecode:

• VMID=`/usr/sbin/dmidecode -s system-serial-number`
• PVEHOST=`/usr/sbin/dmidecode -s system-product-name`
• TYPE=`/usr/sbin/dmidecode -s system-version`

(Has to be done as root...)

Sending the info to the host

A shell script (/root/bin/watchdog) with the above variable settings and:

• /usr/bin/echo -e $TYPE"\t"$VMID"\t\t"`/usr/bin/date +%s` | /usr/bin/ncat $PVEHOST 9999

a cron job sending something like:

• * * * * * /home/tinker/bin/watchdog

This cron line (in roots crontab) sends the guest type, the VMID, and the time expressed as seconds since the Epoch (1970-01-01 00:00 UTC) to port 9999 on the host.

On the PVE server

A similar cron task to that on the guests, but sending the server name instead of the guest VMID.

• VMID=Schizo310
• TYPE=PVE

A continuously running service based on:

• ncat -kl schizo310 9999

An example of the messages

root@schizo310:~# ncat -kl 192.168.1.2 9999
VM	11003		1715882341
PVE	Schizo310	1715882341
CT	11002		1715882341
CT	11002		1715882401
VM	11003		1715882401
PVE	Schizo310	1715882401

Now to just figure out how to check the guests times against the servers time...

Then, if a guests time differs from the hosts time by more than a minute, it's likely crashed. Reset it.

Ncat Caveats

Ncat, by default is a complete moron about DNS...

This appears to be some sort of bug in the GNU C compiler library NSS implementation.

If you start Ncat as a listener using the machines hostname, it has a good chance of binding to 127.0.1.1 instead of the machines actual IP address. This means it will never see incoming messages over the network. You can check if this is what it's doing by adding a -v option to the command line.

The solution when this happens is to edit /etc/nsswitch.conf and change the hosts line to remove files as an option (or, at least, move dns to the first position).