As of the date of writing this page (6 March 2022) nixos-unstable has the necessary changes. The changes needed to proxmox for supporting NixOS LXCs have been merged upstream, but a new release of pve-container with these changes is not available yet. To patch an older version of proxmox to support nixos, see the “patching pve-container” section below.

KVM

It is possible to generate generic qcow2 images and attach them to VMs with qm importdisk as shown here

A better option is to generate a VMA image that can be imported as a VM on proxmox. With this method, many VM configuration options such as CPU, memory, netowrk interfaces, and serial terminals can be specified in nix instead of manually setting them on the proxmox UI.

Generating VMA

The first run will take some time, as a patched version of qemu with support for the VMA format needs to be built

nix run github:nix-community/nixos-generators -- --format proxmox

Pass additional nix configuration to the template with --configuration filename.nix. In addition to NixOS module options, proxmox-specific options present in nixos/modules/virtualisation/proxmox-image.nix can be used to set core, memory, disk and other VM hardware options.

Deploying on proxmox

The generated vma.zst file can be copied to /var/lib/vz/dump/ (or any other configured VM dump storage path). A new VM can be spun up from it either using the GUI or the CLI:

qmrestore /var/lib/vz/dump/vzdump-qemu-nixos-21.11.git.d41882c7b98M.vma.zst <vmid> --unique true

note: the MAC accress of net0 defaults to 00:00:00:00:00:00. This must either be overriden thruogh proxmox.qemuConf.net0, or the unique attribute must be set to true when importing the image on proxmox.

By default, the generated image is set up to expose a serial terminal interface for ease of access.

root@proxmox-server:~# qm start <vmid>
root@proxmox-server:~# qm terminal <vmid>
starting serial terminal on interface serial0 (press Ctrl+O to exit)

<<< NixOS Stage 1 >>>

loading module dm_mod...
running udev...
Starting version 249.4
.
.
.
[  OK  ] Reached target Multi-User System.


<<< Welcome to NixOS 21.11.git.d41882c7b98M (x86_64) - ttyS0 >>>

Run 'nixos-help' for the NixOS manual.

nixos login: root (automatic login)


[root@nixos:~]#

Network configuration

Cloud-init can be enabled with

services.cloud-init.network.enable = true;

This will enable systemd-networkd, allowing cloud-init to set up network interfaces on boot.

LXC

Patching pve-container

This is not needed on pve-container versions newer than 4.1-4. As of 6 March 2022, there is no newer version availble. Until a new version is released, you will have to patch existing versions to add NixOS LXC support. If you have a newer version, skip ahead to the next section.

install some dependencies apt install git devscripts gdebi
clone https://github.com/proxmox/pve-container

root@pve:~# git clone https://github.com/proxmox/pve-container
...
root@pve:~# cd pve-container/

get the installed version of pve-container

root@pve:~/pve-container# pveversion -v | grep pve-container
pve-container: 4.1-2

check out the commit of the version you want

root@pve:~/pve-container# git log --grep "4.1-2"
commit 5d5f81f645bd1e8fd0ffff878fe249253e1be777
Author: Thomas Lamprecht <t.lamprecht@proxmox.com>
Date:   Fri Nov 12 19:21:25 2021 +0100

    bump version to 4.1-2

    Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

root@pve:~/pve-container# git checkout 5d5f81f645bd1e8fd0ffff878fe249253e1be777
Note: switching to '5d5f81f645bd1e8fd0ffff878fe249253e1be777'.
...
HEAD is now at 5d5f81f bump version to 4.1-2

cherry-pick the patch that adds NixOS LXC support

root@pve:~/pve-container# git cherry-pick 6226d0101652914744cb5c657414bf286ccd857d
Auto-merging src/PVE/LXC/Config.pm
[detached HEAD 6f3cd03] Setup: add NixOS support
 Author: Harikrishnan R via pve-devel <pve-devel@lists.proxmox.com>
 Date: Tue Feb 15 22:58:46 2022 +0530
 Committer: root <root@pve>
...
 4 files changed, 50 insertions(+), 1 deletion(-)
 create mode 100644 src/PVE/LXC/Setup/NixOS.pm

If the version of pve-container you’re applying the patch to is older than 4.1, it might encounter merge conflicts that would need to be manually resolved.

Install build deps

root@pve:~/pve-container# mk-build-deps
...
The package has been created.
Attention, the package has been created in the current directory,
not in ".." as indicated by the message above!

root@pve:~/pve-container# gdebi pve-container-build-deps_4.1-2_all.deb
Reading package lists... Done
...
Fetched 432 MB in 6s (17.8 MB/s)
...
Unpacking pve-container-build-deps (4.1-2) ...
Setting up pve-container-build-deps (4.1-2) ...

build the patched pve-container

root@pve:~/pve-container# make
...
dpkg-buildpackage: info: binary-only upload (no source included)
lintian pve-container_4.1-2_all.deb
warning: running with root privileges is not recommended!

install the deb

root@pve:~/pve-container# dpkg -i pve-container_4.1-2_all.deb

verify that the installed pve-container package added NixOS support

root@pve:~# ls /usr/share/perl5/PVE/LXC/Setup/NixOS.pm
/usr/share/perl5/PVE/LXC/Setup/NixOS.pm

Generating LXC template

nix run github:nix-community/nixos-generators -- --format proxmox-lxc

Privileged LXCs

While it’s not necessary, proxmoxLXC.privileged can be set to true to enable the DebugFS mount in privileged LXCs. If enabled on unprivileged LXCs, this will fail to mount.

Network configuration

The proxmox LXC template uses systemd-networkd by default to allow network configuration by proxmox. proxmoxLXC.manageNetwork can be set to true to disable this.

deploying on proxmox

Copy the tarball to proxmox, then create a new LXC with this template through the web UI or the CLI. The “nesting” feature needs to be enabled. Newer versions of proxmox will have it enabled by default.

As of now, not all of the configuration options on the web UI work for proxmox LXCs. Network configuration and adding SSH keys to root user work, while setting a password for the root user and setting hostname don’t.