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Quickstart Guide: nixos-anywhere

Install NixOS everywhere via ssh

Documentation Index

Introduction

This guide documents a simple installation of NixOS using nixos-anywhere on a target machine running x86_64 Linux with kexec support. The example used in this guide installs NixOS on a Hetzner cloud machine. The configuration may be different for some other instances. We will be including further examples in the How To Guide as and when they are available.

You will need:

  • A flake that controls the actions to be performed
  • A disk configuration containing details of the file system that will be created on the new server.
  • A target machine that is reachable via SSH, either using keys or a password, and the privilege to either log in directly as root or a user with password-less sudo.

nixos-anywhere doesn’t need to be installed. You can run it directly from the Github repository.

Details of the flake, the disk configuration and the CLI command are discussed below.

Steps required to run nixos-anywhere

1. Enable Flakes

Check if your nix has flakes enabled by running nix flake. It will tell you if it's not. To enable flakes, refer to the NixOS Wiki.

2. Initialize a Flake

The easiest way to start is to copy our example flake.nix into a new directory. This example is tailored for a virtual machine setup similar to one on Hetzner Cloud, so you might need to adapt it for your setup.

Hardware-Specific Configuration: If you're not using a virtual machine, you'll need to generate a custom hardware configuration with nixos-generate-config.

If you already have a flake, you can use it by adding disko configuration to it.

Get nixos-generate-config onto the Target Machine

  1. Option 1: If NixOS is not installed, boot into an installer without first installing NixOS.
  2. Option 2: Use the kexec tarball method, as described here.

Generate Configuration

Run the following command on the target machine:

nixos-generate-config --no-filesystems --dir /mnt/etc/nixos

This creates the necessary configuration files under /mnt/etc/nixos/, which you can then customize as needed and copy them to your local machine in order to include them in your flake.

3. Configure your SSH key

If you cloned our nixos-anywhere-example you will also replace the SSH key like this: In your configuration, locate the line that reads:

# change this to your ssh key
            "CHANGE"

Replace the text CHANGE with your own SSH key. This is crucial, as you will not be able to log into the target machine post-installation without it.

4. Configure Storage

In the same directory, create a file named disk-config.nix. This will be used to specify the disk layout to the disko tool, which nixos-anywhere uses to partition, format and mount the disks.

For a simple installation you can paste the contents from the example here. This configures a standard GPT (GUID Partition Table) partition compatible with both EFI and BIOS systems, and mounts the disk as /dev/sda. If this doesn’t meet your requirements, choose an example that suits your disk layout from the disko examples. For more information about this configuration, refer to the disko documentation.

5. Lock your Flake

nix flake lock

This will download your flake dependencies and make a flake.lock file that describes how to reproducably build your system.

Optionally, you can commit these files to a repo such as Github, or you can simply reference your local directory when you run nixos-anywhere. This example uses a local directory on the source machine.

6. Connectivity to the Target Machine

nixos-anywhere will create a temporary SSH key to use for the installation. If your SSH key is not found, you will be asked for your password. If you are using a non-root user, you must have access to sudo without a password. To avoid SSH password prompts, set the SSHPASS environment variable to your password and add --env-password to the nixos-anywhere command.

7. (Optional) Test your NixOS and Disko configuration

The following command will automatically test your nixos configuration and run disko inside a virtual machine, where

  • <path to configuration> is the path to the directory or repository containing flake.nix and disk-config.nix

  • <configuration name> must match the name that immediately follows the text nixosConfigurations. in the flake, as indicated by the comment in the example.

nix run github:nix-community/nixos-anywhere -- --flake <path to configuration>#<configuration name> --vm-test

8. Run it

You can now run nixos-anywhere from the command line as shown below, where:

  • <path to configuration> is the path to the directory or repository containing flake.nix and disk-config.nix

  • <configuration name> must match the name that immediately follows the text nixosConfigurations. in the flake, as indicated by the comment in the example.

  • <ip address> is the IP address of the target machine.

nix run github:nix-community/nixos-anywhere -- --flake <path to configuration>#<configuration name> root@<ip address>

The command would look  like this if you had created your files in a directory named /home/mydir/test and the IP address of your target machine is 37.27.18.135:

nix run github:nix-community/nixos-anywhere -- --flake /home/mydir/test#hetzner-cloud root@37.27.18.135

nixos-anywhere will then run, showing various output messages at each stage. It may take some time to complete, depending on Internet speeds. It should finish by showing the messages below before returning to the command prompt.

Installation finished. No error reported.
Warning: Permanently added '<ip-address>' (ED25519) to the list of known hosts

When this happens, the target server will have been overwritten with a new installation of NixOS. Note that the server's public SSH key will have changed.

If you have previously accessed this server using SSH, you may see the following message the next time you try to log in to the target.

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ED25519 key sent by the remote host is
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.
Please contact your system administrator.
Add correct host key in ~/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in ~/.ssh/known_hosts:6
  remove with:
  ssh-keygen -f ~/.ssh/known_hosts" -R "<ip address>"
Host key for <ip_address> has changed and you have requested strict checking.
Host key verification failed.

This is because the known_hosts file in the .ssh directory now contains a mismatch, since the server has been overwritten. To solve this, use a text editor to remove the old entry from the known_hosts file (or use the command ssh-keygen -R <ip_address>). The next connection attempt will then treat this as a new server.

The error message line Offending ECDSA key in ~/.ssh/known_hosts:6 gives the line number that needs to be removed from the known_hosts file (line 6 in this example).

Finished!

nixos-anywhere's job is now done, as it is a tool to install NixOS onto the target machine.

Any future changes to the configuration should be made to your flake. You would reference this flake when using the NixOS nixos-rebuild command or a separate 3rd party deployment tool of your choice i.e. deploy-rs, colmena, nixinate.

To update on the machine locally (replace <URL to your flake> with your flake i.e. .# if your flake is in the current directory):

nixos-rebuild switch --flake <URL to your flake>

To update remotely you will need to have configured an ssh server and your ssh key for the root user:

nixos-rebuild switch --flake <URL to your flake> --target-host "root@<ip address>"

See the Nix documentation for use of the flake URL-like syntax.

For more information on different use cases of nixos-anywhere please refer to the How to Guide, and for more technical information and explanation of known error messages, refer to the Reference Manual.