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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, reachable via SSH, with your SSH public key deployed and and the privilege to either login directly as root or to use password-less sudo.

nixos-anywhere doesn’t need to be installed. You can run it directly from Numtide's repository on Github.

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

Steps required to run nixos-anywhere

  1. Enable Flakes:

    Ensure that flakes are enabled on your system. 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.

  • 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 --root /mnt

    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.

  1. Find SSH Key Line:
    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.

  2. 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. Again, 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.

  3. Run the following command to create the flake.lock file:

    nix flake lock

    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.

  4. On the target machine, make sure you have access as root via ssh by adding your SSH key to the file authorized_keys in the directory /root/.ssh

  5. (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

  6. 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 [email protected]

      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 addrress>"
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. The next connection attempt will then treat this as a new server.

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

      The new server's configurations are defined in the flake. nixos-anywhere does not create etc/nixos/configuration.nix, since it expects the server to be administered remotely. Any future changes to the configuration should be made to the flake, and you would reference this flake when doing the nixos-rebuild command or a deployment tool of your choice i.e. colmena, nixinate.

      This example can be run from the machine itself for updating (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>

      You can also run nixos-rebuild to update a machine remotly, if you have set up an openssh server and your ssh key for the root user:

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

      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.