NixOps: Towards The Final Frontier

NixOps (Nix Operations) is an application for deploying and provisioning multiple NixOS machines in a network or cloud environment. Let’s bid farewell to nix-rebuild (and many other cool tools) and say hello to nixops. This write up is by no means an authoritative Consult your friendly NixOps documentation. guide but rather a commentary on my current The current procedure of moving my entire external and personal infrastructure to NixOS. nixops approach.

Bootstrapping Remote Machines

NixOps requires NixOps version 2.0 supports non-root deployments. ssh access to the root user, therefore you should bootstrap a remote machine using its configuration.nix. Power users will use NixOS generators to speed Provision a specific NixOS configuration and convert between the many (too many) virtualization and cloud vendor formats. up this process.

{
  services.openssh = {
    enable = true;
    permitRootLogin = "prohibit-password";
    passwordAuthentication = false;
    challengeResponseAuthentication = false;
    extraConfig = "Compression no";
  };
  
  users.users.root = {
    openssh.authorizedKeys.keyFiles = [ ./ssh-key.pub ];
  };
}

The user’s authorized_keys is declaratively users.users.<name?> where <name?> is the actual user. set using the option users.users.<name?>.openssh.authorizedKeys.keyFiles.

Do not use users.users.<name?>.openssh.authorizedKeys.keys to configure public keys as it replaces the authorized_keys that nixops sets on the remote machine.

When nixops interfaces successfully with a remote machine it sets its public key, as well as a private key NixOps was first introduced as Charon [pdf]. called id_charon_vpn in the user’s .ssh folder. The remote machines are now ready to accept connections from nixops.

Directory Structure

Let’s take a look at my current directory layout before creating a deployment using nixops.

├── configuration.nix
├── deployments.nixops
├── configuration
│   ├── heron.nix
│   └── pigeon.nix
├── hardware
│   ├── heron.nix
│   └── pigeon.nix
├── helpers
│   ├── extra-builtins.nix
│   └── wrappers
│   	└── vault
├── keys
│   └── thedroneely.com.nix
├── mail
├── package
├── program
│   └── nix.nix
├── server
├── user
│   ├── pigeon.nix
│   ├── root.nix
│   └── thedro.nix
├── virtualisation
└── website
    └── thedroneely.com.nix

The deployment entry point is at configuration.nix. The machine specific configuration.nix and hardware.nix are in the configuration and hardware folders respectively. There are helpers, program specific configurations, as well as files responsible for secrets management. Deployment state The state file is rigid, however options for multiple state backends are in the works. is tracked in deployments.nixops.

NixOps Deployment and Commands

Populate the configuration entry point configuration.nix with a list of machines to provision in the deployment.

{
  network.description = "nixos";
  network.enableRollback = true;

  heron = {
    imports = [ ./hardware/heron.nix ./configuration/heron.nix ];
    deployment.targetHost = "heron.local";
  };

  talon = {
    imports = [ ./hardware/talon.nix ./configuration/talon.nix ];
    deployment.targetHost = "talon.local";
  };

  tiger = {
    imports = [ ./hardware/tiger.nix ./configuration/tiger.nix ];
    deployment.targetHost = "tiger.local";
  };

  hound = {
    imports = [ ./hardware/hound.nix ./configuration/hound.nix ];
    deployment.targetHost = "hound.local";
  };

  pigeon = {
    imports = [ ./hardware/pigeon.nix ./configuration/pigeon.nix ];
    deployment.targetHost = "pigeon.local";
  };
}

This entry point declares five machines to be operated on — each with their own hardware and configuration specific assets under the network nixos. Rollbacks are enabled for the nixops operator to switch generations in the case of a mishap.

Make a deployment using create by referencing the entry point file path and optionally The state file defaults to ~/.nixops/deployments.nixops setting the location of the deployment state file using the -s flag.

nixops create -d nixos configuration.nix -s deployments.nixops

List all available deployment The output of nixops list depends on the last nixops deploy run. configurations using the list argument.

$ nixops list -s deployments.nixops
+--------------------------------------+-------+-------------+------------+------+
| UUID                                 | Name  | Description | # Machines | Type |
+--------------------------------------+-------+-------------+------------+------+
| f56d276b-af54-11ea-b171-02422b1a33b6 | nixos | nixos       |          5 | none |
+--------------------------------------+-------+-------------+------------+------+

Run NixOps deployment the deployment nixos on all machines listed in the entrypoint configuration using the deploy argument.

nixops deploy -d nixos -s deployments.nixops

Run the deployment nixos but only on the machine with the cryptonym heron.

nixops deploy -d nixos -s deployments.nixops --include heron

Show the state for deployment nixos using the info argument. This command lists the status and current state of each machine.

$ nixops info -d nixos -s deployments.nixops

Network name: nixos
Network UUID: f56d276b-af54-11ea-b171-02422b1a33b6
Network description: nixos
Nix expressions: /nixos/configuration.nix
Nix profile: /nix/var/nix/profiles/per-user/thedro/nixops/f56d276b-af54-11ea-b171-02422b1a33b6

+--------+-----------------+------+----------------------------------------------------+------------+
| Name   |      Status     | Type | Resource Id                                        | IP address |
+--------+-----------------+------+----------------------------------------------------+------------+
| heron  |  Up / Outdated  | none | nixops-f56d276b-af54-11ea-b171-02422b1a33b6-heron  |            |
| hound  |  Up / Outdated  | none | nixops-f56d276b-af54-11ea-b171-02422b1a33b6-hound  |            |
| pigeon | Up / Up-to-date | none | nixops-f56d276b-af54-11ea-b171-02422b1a33b6-pigeon |            |
| talon  |  Up / Outdated  | none | nixops-f56d276b-af54-11ea-b171-02422b1a33b6-talon  |            |
| tiger  | Up / Up-to-date | none | nixops-f56d276b-af54-11ea-b171-02422b1a33b6-tiger  |            |
+--------+-----------------+------+----------------------------------------------------+------------+

Examine the machines more closely with the check argument. This command provides extra information such as load averages and failed units.

$ nixops check -d nixos -s deployments.nixops --include tiger pigeon
Machines state:
+--------+--------+-----+-----------+----------+----------------+----------------------------------------+-------+
| Name   | Exists | Up  | Reachable | Disks OK | Load avg.      | Units                                  | Notes |
+--------+--------+-----+-----------+----------+----------------+----------------------------------------+-------+
| pigeon | Yes    | Yes | Yes       | N/A      | 0.16 0.07 0.04 | proc-sys-fs-binfmt_misc.mount [failed] |       |
| tiger  | Yes    | Yes | Yes       | N/A      | 0.11 0.09 0.03 | proc-sys-fs-binfmt_misc.mount [failed] |       |
+--------+--------+-----+-----------+----------+----------------+----------------------------------------+-------+

List the generations for deployment nixos. The deployment fleet is currently on generation 203.

$ nixops list-generations -d nixos -s deployments.nixops
 200   2020-06-26 00:28:10   
 201   2020-06-26 04:14:40   
 202   2020-06-26 04:28:12   
 203   2020-06-26 21:58:55   (current)

Delete a machine resource with the delete argument but you must first remove the resource with destroy.

nixops destroy -d heron -s deployments.nixops
nixops delete -d heron -s deployments.nixops

NixOps Secrets Management

The nix store at /nix/store is world readable. Secrets entered directly into any nix configuration will be available to all users on the system. Avoid placing secrets in a nix configuration directly — as it will leak to unprivileged users. NixOps provides a powerful secrets management system in the form of password files.

Using a password file provides indirection and guards against writing secrets directly into your nix files. NixOps writes deployment keys to /run/keys. Users must be a part of the key group to access deployment keys on the system.

NixOps can set the permissions, owners, and groups of each key. The actual Use any preferred program as a secrets storage. secrets are sourced from a Vault server wrapper defined in builtins.extraBuiltins. This blog post gives an excellent rundown on this technique. A sample of the nix secrets configuration for this website is shown below.

{ config, ... }:

let user = "thedroneely"; in

{
  deployment.keys = {

    thedroneely_cockpit_api_token = {
      user = user;
      text = "${builtins.extraBuiltins.vault "thedroneely/thedroneely.com" "cockpit_api_token"}";
    };

    thedroneely_pgsql_database_password = {
      user = user; group = "postgres"; permissions = "0640";
      text = "${builtins.extraBuiltins.vault "thedroneely/thedroneely.com" "pgsql_database_password"}";
    };

    thedroneely_ssh_known_hosts = {
      inherit user;
      text = "${builtins.extraBuiltins.vault "thedroneely/thedroneely.com" "ssh_known_hosts"}";
    };
  };
}

Enable extraBuiltins by linking directly from the nix-plugins package on the nix operator host. Allow users in @wheel to become trusted users and wield this ultimate power.

{ pkgs, config, ... }:

{
  nix.extraOptions = ''
    trusted-users = root @wheel
    plugin-files = ${pkgs.nix-plugins}/lib/nix/plugins/libnix-extra-builtins.so
  '';
}

Create helpers/extra-builtins.nix and extend the builtins with custom functions. The vault function accepts a field and a path as arguments to an external wrapper.

{ exec, ... }:

{
  vault = path: field: exec [ ./wrappers/vault field path ];
}

The wrapper returns the secret result from vault as a string. NixOps is now aware of keys available from a vault server. This is done at evaluation time. Environment variables are available to wrappers called from extraBuiltins.

#!/bin/sh -eu
printf '"' && vault kv get -field "$1" "$2" && printf '"'

Load the extraBuiltins with any nixops command by appending the --option flag with the file path. Vault keys are called using builtins.extraBuiltins.vault.

nixops reboot -d nixos -s deployments.nixops --include pigeon --option extra-builtins-file $PWD/nixos/helpers/extra-builtins.nix

The keys in /run/keys are ephemeral and never touch storage. Do not Executing a reboot with nixops on the same nixops host won't end well if secrets are set. reboot the system through any other means except nixops. Your keys will be lost and any dependent service will fail. Use nixops to remotely reboot a machine and its keys will be seeded back into position. Wonderful.

Upload deployment keys immediately without waiting on a deploy by using the send-keys argument.

nixops send-keys -d nixos -s deployments.nixops --include pigeon

NixOps Pinning

NixOps depends on the nix channel set on the host. This is a very interesting moving Executing nixops on a machine with an uncertain channel is an assured gotcha. target. Shoot it and lock the host’s channel by prefixing or exporting a declaratively Currently using a nix shell until more of the code is read. defined NIX_PATH to your nixops commands.

NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs/archive/2b417708c282d84316366f4125b00b29c49df10f.tar.gz

Check the system’s channel version with nix-instantiate if you prefer to use nix-channel.

nix-instantiate --eval -E '(import <nixpkgs> {}).lib.version'

NixOps and Nix Channel

Syncing the nix channel as needed on the host and remote with nix-channel works out best To avoid silent drift when working with throw away nix-env or mkShell environments. for my use case. The official channel list that nix-channel sources can be viewed at release.nixos.org. Create a “lock” file called versions.nix that contain all sources of truth.

let version = "f8248ab6d9e"; in

{
  inherit version;
  
  # Channel list at https://releases.nixos.org/?prefix=nixos/20.03

  channel = "https://releases.nixos.org/nixos/20.03/nixos-20.03.2351.${version}";

  stable = import (builtins.fetchTarball {
    url= "https://github.com/NixOS/nixpkgs/archive/${version}.tar.gz";
    sha256 = "009i9j6mbq6i481088jllblgdnci105b2q4mscprdawg3knlyahk"; }) {};

  unstable = import (builtins.fetchTarball {
    url= "https://github.com/NixOS/nixpkgs/archive/c308da1c4b14b0ffdfe9c2e2a948320124938220.tar.gz";
    sha256 = "0ab9jhnlscxl1fl9rc7l978n26al1n77w32sr8gdbb11pfnhsizp"; }) {};

  kernelLock = import (builtins.fetchTarball {
    url= "https://github.com/NixOS/nixpkgs/archive/b0e3df2f8437767667bd041bb336e9d62a97ee81.tar.gz";
    sha256 = "0d34k96l0gzsdpv14vnxdfslgk66gb0nsjz7qcqz1ykb0i7n3n07"; }) {};

  intelKernelLock = import (builtins.fetchTarball {
    url= "https://github.com/NixOS/nixpkgs/archive/f761c14fd2f198f64cc5483ebf9f83222f9214aa.tar.gz";
    sha256 = "09w0n8qvldanab1m6ik507nl48aszam2a5ii3z2fvk72s66zmry7"; }) {};
}

Create a systemd service that uses restartTriggers to lazily watch versions.nix for attribute changes. The system will sync up as needed. This allows easier operation of nixos-rebuild if nixops is not available.

{ pkgs, ... }:

{
  systemd.services.nix-channel-update = {
    description = "Update Nix Channels";
    wantedBy = [ "multi-user.target" ];
    restartTriggers = [ (import ./versions.nix).channel ];
    path = [ pkgs.shellcheck pkgs.nix ];
    script = ''
      set -euxo pipefail
      shellcheck "$0" || exit 1

      nix-channel --add "${(import ../versions.nix).channel}" nixos
      nix-channel --update
    '';
    serviceConfig = { RemainAfterExit = "yes"; };
  };
}

Updated 4 September 2020