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Proxmox cheatsheet example

Erstellt am 21.10.2025 um 16:08:05 Uhr | muellda

Aktualisiert am 21.10.2025 um 16:08:05 Uhr | muellda

#Switching Proxmox from static IP to DHCP

When you first install Proxmox, it will set whatever initial IP you give it to be that IP forever (wow is that annoying). If you SSH into the Proxmox box and have a look at /etc/network/interfaces it probably looks something like this:

auto lo
iface lo inet loopback

iface eno1 inet manual

auto vmbr0
iface vmbr0 inet static
        address 10.0.20.210/24
        gateway 10.0.20.1
        bridge-ports eno1
        bridge-stp off
        bridge-fd 0

iface wlp0s20f3 inet manual


source /etc/network/interfaces.d/*

What you’ll want to do is set the iface vmbr0 inet static to be iface vmbr0 net dhcp and then comment out a few lines (see below) that give the box the static IP:

auto lo
iface lo inet loopback

iface eno1 inet manual

auto vmbr0
iface vmbr0 inet dhcp
#       address 10.0.20.207/24
#       gateway 10.0.20.1
        bridge-ports eno1
#       bridge-stp off
#       bridge-fd 0

iface wlp0s20f3 inet manual

Then reboot and DHCP should get picked up this time!

I just learned that the new version of Proxmox (9.0 and greater) seems to have „broken“ the ability to set DHCP the way I describe above. I became aware of this when I ran ifreload -a I got this error:

error: vmbr0; cmd '/sbin/dhclient -pf /run/dhclient.vmbr0.pid -lf /var/lib/dhcp/dhclient.vmbr0.leases vmbr0' failed ([Error 2] no such file or directory: '/sbin/dhclient')

After trying a lot of things that didn’t work, I was able to resolve this by installing dhclient:

apt update
apt install isc-dhcp-client -y

#Install QEMU guest agent (Windows)

1. Mount the virtio-win.iso

and install D:\guest-agent\qemu-ga-x86_64.msiShutdown the system.On the Proxmox system, head to Options > QEMU Guest Agent > Use QEMU Guest Agent (tick the box)Power up the VMOpen devmgmt.msc – look for PCI Simple Communications Controller and right-click it, then click Update DriverSelect Browse my computer for drivers and feed it the path of D:\vioserial\YOUR-WINDOWS-VERSION\amd64\Per Proxmox docs

1. , check if the service is running in PowerShell:

Get-Service QEMU-GA

8. Additionally, from Proxmox command line you can run qm agent xxx ping to make sure an empty prompt is returned back (if you get QEMU guest agent is not running start at step 1 and double check everything).

#Install QEMU guest agent (Linux)

Check out these instructions

but basically:

sudo apt-get install qemu-guest-agent -y
sudo systemctl start qemu-guest-agent
sudo systemctl enable qemu-guest-agent

#Network connectivity issues for Linux VMs tagged with VLANs

I had a super frustrating problem where after moving Linux VMs from one host to another, they weren’t pulling DHCP or not able to route traffic or both. This post

and ChatGPT nudged me towards making a /etc/netplan/01-netcfg.yaml file that looks something like this:

network:
  ethernets:
    ens18:
      dhcp4: true
    ens19:
      dhcp4: true
  version: 2

Once I did that and then sudo netplan apply the DHCP addresses got pulled and routing worked!

#QM command cheat sheet

#Unlock a system (that might be locked from snapshot)

qm unlock XXX

#List machines

qm list

#Check status on if a particular machine is booted

qm status xxx

#Get full info on one VM – like if autoboot is set, hardware info, etc:

qm config xxx

#Enable autoboot (if onboot does not = 1 from `qm config X‘)

qm set X -onboot 1

#Change boot order

qm set X --startup order=2,up=60

#Stop a VM

qm stop X

#Take a snapshot

qm snapshot xxx NAME-OF-SNAP --description "Description of my snapshot"

#List snapshots

qm listsnapshot xxx

#Restore a snapshot

qm rollback xxx NAME-OF-SNAP

#Start a VM

qm start X

#Bring network link up or down

qm set VMID -net0 virtio=THE:MAC:ADDRESS:OF:THE:PC,bridge=vmbr1,link_down=1,tag=10

#Run a powershell command directly against a specific VM

qm guest exec xxx -- cmd /c "dir c:\users\administrator\desktop"

#Another example of downloading and then running a PowerShell script

qm guest exec 100 -- cmd /c "powershell invoke-webrequest https://somesite/script.ps1 -outfile c:\users\administrator\desktop\script.ps1"
qm guest exec 100 -- cmd /c "powershell.exe -ExecutionPolicy Bypass -File "C:\\Users\\administrator\\Desktop\\script.ps1""

#Yet another rexample of installing NIC driver on a fresh Windows build

qm guest exec 100 -- cmd /c "pnputil /add-driver d:\NetKVM\w11\amd64\netkvm.inf /install"

#Yet ANOTHER example of installing Chrome

qm guest exec 100 -- cmd /c "powershell Invoke-WebRequest 'https://dl.google.com/chrome/install/chrome_installer.exe' -OutFile c:\users\ttadmin\desktop\chrome_installer.exe && c:\users\ttadmin\desktop\chrome_installer.exe /silent /install"

#Then tailing the last 10 lines of a log file

qm guest exec 100 -- cmd /c "powershell.exe -ExecutionPolicy Bypass -Command Get-Content -Path 'C:\\some\\path\\install.log' -Tail 10"

#Check if qmagent is running

qm agent xxx ping

#Delete/destroy a VM

qm destroy xxx

#Backup a VM and move to another Proxmox node in a cluster

vzdump <vmid> --storage <storage_name> --mode snapshot

Then move to another node:

scp /path/to/backup/vzdump-qemu-<vmid>.vma root@<destination_node_ip>:/var/lib/vz/dump/

Then restore:

qmrestore /var/lib/vz/dump/backup.vma <vmid> --storage <target-storage>

#Example

This will dump a snapshot of VM 121 to a file path like /var/lib/vz/dump/vzdump-qemu-121-2024_09_13-10_25_00.vma

vzdump 121 --storage local --mode snapshot

Now move to another node:

scp /var/lib/vz/dump/vzdump-qemu-121-2024_09_13-10_25_00.vma root@target.host.for.VM:/var/lib/vz/dump/

Now restore:

qmrestore /var/lib/vz/dump/vzdump-qemu-121-2024_09_13-10_25_00.vma 123 --storage local-lvm

In the example above, 123 is the VM ID you want to assign the imported host, and local-lvm is the storage pool to restore to.

#Resize a disk

Shutdown the affected VM, then check the details on the VM you want to resize:

qm config xxx

This will tell you what kind of hard drive type (like virtio0) you have

To do the resize and grow the disk by 20G:

qm resize 100 virtio0 +20G

#Resize a disk at Linux command line

Once the resizing is done, you can do the extending portion from the Linux command line. Using the latest Ubuntu OS as an example, here’s what I did. First find the partion that needs growing:

sudo fdisk -l

Then grow it (in this example the target is dev/vda1:

sudo growpart /dev/vda 1

Finish the resizing:

sudo resize2fs /dev/vda1

Done!

#Add memory (RAM) to a system

For example, upgrade RAM to 8192:

# Check how much memory is free
free -h

# Stop the VM that needs a RAM facelift
qm stop (VMID)

# Set to 8 gigs
qm set (VMID) -memory 8192

# Set to 16 gigs
qm set (VMID) -memory 16384

#Reset a Windows VM password

qm guest passwd <vmid> <username>

So for example:

qm guest passwd 100 administrator

#Troubleshooting

Here are some issues/fixes I’ve seen in general when working with

#Network issues

I have now run into several instances where my pentest boxes suddenly appear to go offline – both VMs show down at the exact same date/timestamp. My assumption was that there was potentially a hardware issue that was creeping up. But on a recent pentest I remoted back into the Windows VM after the suspected crash, and found that event viewer showed the VM humming away but throwing many errors about DNS lookup and connectivity failures.

Fast-forward through a lot of Proxmox forum searching and ChatGPT chatting and I found out that my NUC and hardware and driver version seem to have a common issue where the card seizes up and basically the whole NUC needs a reboot to get back online.

This is how I figured out there was a problem:

# journalctl logs around the time in question
journalctl --since "2025-09-23 17:15:00" --until "2025-09-23 17:30:00" -p 3..6

Below are a temporary and permanent fix that might help:

#Temporary fix

Disable offloading features (runtime only; survives until reboot)

ethtool -K eno1 tso off gso off gro off

Make sure the change „stuck“

ethtool -k eno1 | egrep 'tso|gso|gro'

Your output will look something like:

tx-gso-robust: off [fixed]
tx-gso-partial: off [fixed]
tx-gso-list: off [fixed]
rx-gro-hw: off [fixed]
rx-gro-list: off
rx-udp-gro-forwarding: off

Resume using your VMs as normal, and then you can also watch the logs „live“ on the NUC to see if issues pop up again:

journalctl -kf | grep -i e1000e

#Permanent fix

At the time of this writing I have not tried this yet, but ChatGPT thinks if the temporary fix does it for you, you can edit your /etc/network/interfaces to include the tso command fix. For example:

auto lo
iface lo inet loopback

iface eno1 inet manual
    post-up /sbin/ethtool -K $IFACE tso off gso off gro off
    post-down /sbin/ethtool -K $IFACE tso on gso on gro on

auto vmbr0
iface vmbr0 inet dhcp
    bridge-ports eno1
    bridge-stp off
    bridge-fd 0

iface wlp0s20f3 inet manual

source /etc/network/interfaces.d/*