borgbackup: Moved documentation to NixOS manual and added examples for

* creating a local backup
* creating a borgbackup server
* backing up to a borgbackup server
* hints about the Vorta graphical desktop application
* Added documentation about Vorta desktop client

Tested the examples locally and with my borgbase.com account.

nixos/modules/services/backup/borgbackup.nix

@@ -189,6 +189,7 @@ let
 
 in {
   meta.maintainers = with maintainers; [ dotlambda ];
+  meta.doc = ./borgbackup.xml;
 
   ###### interface
 
@@ -197,10 +198,11 @@ in {
       Deduplicating backups using BorgBackup.
       Adding a job will cause a borg-job-NAME wrapper to be added
       to your system path, so that you can perform maintenance easily.
+      See also the chapter about BorgBackup in the NixOS manual.
     '';
     default = { };
     example = literalExample ''
-      {
+      { # for a local backup
         rootBackup = {
           paths = "/";
           exclude = [ "/nix" ];
@@ -213,6 +215,23 @@ in {
           startAt = "weekly";
         };
       }
+      { # Root backing each day up to a remote backup server. We assume that you have
+        #   * created a password less key: ssh-keygen -N '' -t ed25519 -f /path/to/ssh_key
+        #     best practices are: use -t ed25519, /path/to = /run/keys
+        #   * the passphrase is in the file /run/keys/borgbackup_passphrase
+        #   * you have initialized the repository manually
+        paths = [ "/etc" "/home" ];
+        exclude = [ "/nix" "'**/.cache'" ];
+        doInit = false;
+        repo =  "user3@arep.repo.borgbase.com:repo";
+        encryption = {
+          mode = "repokey-blake2";
+          passCommand = "cat /path/to/passphrase";
+        };
+        environment = { BORG_RSH = "ssh -i /path/to/ssh_key"; };
+        compression = "auto,lzma";
+        startAt = "daily";
+    };
     '';
     type = types.attrsOf (types.submodule (let globalConfig = config; in
       { name, config, ... }: {
@@ -268,6 +287,8 @@ in {
               <manvolnum>7</manvolnum></citerefentry>.
               If you do not want the backup to start
               automatically, use <literal>[ ]</literal>.
+              It will generate a systemd service borgbackup-job-NAME.
+              You may trigger it manually via systemctl restart borgbackup-job-NAME.
             '';
           };
 
@@ -303,6 +324,10 @@ in {
               you to specify a <option>passCommand</option>
               or a <option>passphrase</option>.
             '';
+            example = ''
+              encryption.mode = "repokey-blake2" ;
+              encryption.passphrase = "mySecretPassphrase" ;
+            '';
           };
 
           encryption.passCommand = mkOption {
@@ -538,6 +563,7 @@ in {
     description = ''
       Serve BorgBackup repositories to given public SSH keys,
       restricting their access to the repository only.
+      See also the chapter about BorgBackup in the NixOS manual.
       Also, clients do not need to specify the absolute path when accessing the repository,
       i.e. <literal>user@machine:.</literal> is enough. (Note colon and dot.)
     '';

nixos/modules/services/backup/borgbackup.xml

@@ -0,0 +1,227 @@
+<chapter xmlns="http://docbook.org/ns/docbook"
+         xmlns:xlink="http://www.w3.org/1999/xlink"
+         xmlns:xi="http://www.w3.org/2001/XInclude"
+         version="5.0"
+         xml:id="module-borgbase">
+ <title>BorgBackup</title>
+  <para>
+  <emphasis>Source:</emphasis>
+  <filename>modules/services/backup/borgbackup.nix</filename>
+ </para>
+ <para>
+  <emphasis>Upstream documentation:</emphasis>
+  <link xlink:href="https://borgbackup.readthedocs.io/"/>
+ </para>
+ <para>
+  <link xlink:href="https://www.borgbackup.org/">BorgBackup</link> (short: Borg)
+  is a deduplicating backup program. Optionally, it supports compression and
+  authenticated encryption.
+  </para>
+  <para>
+  The main goal of Borg is to provide an efficient and secure way to backup
+  data. The data deduplication technique used makes Borg suitable for daily
+  backups since only changes are stored. The authenticated encryption technique
+  makes it suitable for backups to not fully trusted targets.
+ </para>
+  <section xml:id="module-services-backup-borgbackup-configuring">
+  <title>Configuring</title>
+  <para>
+   A complete list of options for the Borgbase module may be found
+   <link linkend="opt-services.borgbackup.jobs">here</link>.
+  </para>
+</section>
+ <section xml:id="opt-services-backup-borgbackup-local-directory">
+  <title>Basic usage for a local backup</title>
+
+  <para>
+   A very basic configuration for backing up to a locally accessible directory
+   is:
+<programlisting>
+{
+    opt.services.borgbackup.jobs = {
+      { rootBackup = {
+          paths = "/";
+          exclude = [ "/nix" "/path/to/local/repo" ];
+          repo = "/path/to/local/repo";
+          doInit = true;
+          encryption = {
+            mode = "repokey";
+            passphrase = "secret";
+          };
+          compression = "auto,lzma";
+          startAt = "weekly";
+        };
+      }
+    };
+}</programlisting>
+  </para>
+  <warning>
+    <para>
+        If you do not want the passphrase to be stored in the world-readable
+        Nix store, use passCommand. You find an example below.
+    </para>
+  </warning>
+ </section>
+<section xml:id="opt-services-backup-create-server">
+  <title>Create a borg backup server</title>
+  <para>You should use a different SSH key for each repository you write to,
+    because the specified keys are restricted to running borg serve and can only
+    access this single repository. You need the output of the generate pub file.
+  </para>
+    <para>
+        <programlisting>
+# sudo ssh-keygen -N '' -t ed25519 -f /run/keys/id_ed25519_my_borg_repo
+# cat /run/keys/id_ed25519_my_borg_repo
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAID78zmOyA+5uPG4Ot0hfAy+sLDPU1L4AiIoRYEIVbbQ/ root@nixos</programlisting>
+    </para>
+    <para>
+      Add the following snippet to your NixOS configuration:
+      <programlisting>
+{
+  services.borgbackup.repos = {
+    my_borg_repo = {
+      authorizedKeys = [
+        "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAID78zmOyA+5uPG4Ot0hfAy+sLDPU1L4AiIoRYEIVbbQ/ root@nixos"
+      ] ;
+      path = "/var/lib/my_borg_repo" ;
+    };
+  };
+}</programlisting>
+    </para>
+</section>
+
+ <section xml:id="opt-services-backup-borgbackup-remote-server">
+  <title>Backup to the borg repository server</title>
+  <para>The following NixOS snippet creates an hourly backup to the service
+    (on the host nixos) as created in the section above. We assume
+    that you have stored a secret passphrasse in the file
+    <code>/run/keys/borgbackup_passphrase</code>, which should be only
+    accessible by root
+  </para>
+  <para>
+      <programlisting>
+{
+  services.borgbackup.jobs = {
+    backupToLocalServer = {
+      paths = [ "/etc/nixos" ];
+      doInit = true;
+      repo =  "borg@nixos:." ;
+      encryption = {
+        mode = "repokey-blake2";
+        passCommand = "cat /run/keys/borgbackup_passphrase";
+      };
+      environment = { BORG_RSH = "ssh -i /run/keys/id_ed25519_my_borg_repo"; };
+      compression = "auto,lzma";
+      startAt = "hourly";
+    };
+  };
+};</programlisting>
+  </para>
+  <para>The following few commands (run as root) let you test your backup.
+      <programlisting>
+> nixos-rebuild switch
+...restarting the following units: polkit.service
+> systemctl restart borgbackup-job-backupToLocalServer
+> sleep 10
+> systemctl restart borgbackup-job-backupToLocalServer
+> export BORG_PASSPHRASE=topSecrect
+> borg list --rsh='ssh -i /run/keys/id_ed25519_my_borg_repo' borg@nixos:.
+nixos-backupToLocalServer-2020-03-30T21:46:17 Mon, 2020-03-30 21:46:19 [84feb97710954931ca384182f5f3cb90665f35cef214760abd7350fb064786ac]
+nixos-backupToLocalServer-2020-03-30T21:46:30 Mon, 2020-03-30 21:46:32 [e77321694ecd160ca2228611747c6ad1be177d6e0d894538898de7a2621b6e68]</programlisting>
+    </para>
+</section>
+
+ <section xml:id="opt-services-backup-borgbackup-borgbase">
+  <title>Backup to a hosting service</title>
+
+  <para>
+    Several companies offer <link
+      xlink:href="https://www.borgbackup.org/support/commercial.html">(paid)
+      hosting services</link> for Borg repositories.
+  </para>
+  <para>
+    To backup your home directory to borgbase you have to:
+  </para>
+  <itemizedlist>
+  <listitem>
+    <para>
+      Generate a SSH key without a password, to access the remote server. E.g.
+    </para>
+    <para>
+        <programlisting>sudo ssh-keygen -N '' -t ed25519 -f /run/keys/id_ed25519_borgbase</programlisting>
+    </para>
+  </listitem>
+  <listitem>
+    <para>
+      Create the repository on the server by following the instructions for your
+      hosting server.
+    </para>
+  </listitem>
+  <listitem>
+    <para>
+      Initialize the repository on the server. Eg.
+      <programlisting>
+sudo borg init --encryption=repokey-blake2  \
+    -rsh "ssh -i /run/keys/id_ed25519_borgbase" \
+    zzz2aaaaa@zzz2aaaaa.repo.borgbase.com:repo</programlisting>
+  </para>
+  </listitem>
+  <listitem>
+<para>Add it to your NixOS configuration, e.g.
+<programlisting>
+{
+    services.borgbackup.jobs = {
+    my_Remote_Backup = {
+        paths = [ "/" ];
+        exclude = [ "/nix" "'**/.cache'" ];
+        repo =  "zzz2aaaaa@zzz2aaaaa.repo.borgbase.com:repo";
+          encryption = {
+          mode = "repokey-blake2";
+          passCommand = "cat /run/keys/borgbackup_passphrase";
+        };
+        BORG_RSH = "ssh -i /run/keys/id_ed25519_borgbase";
+        compression = "auto,lzma";
+        startAt = "daily";
+    };
+  };
+}}</programlisting>
+  </para>
+  </listitem>
+</itemizedlist>
+ </section>
+  <section xml:id="opt-services-backup-borgbackup-vorta">
+  <title>Vorta backup client for the desktop</title>
+  <para>
+    Vorta is a backup client for macOS and Linux desktops. It integrates the
+    mighty BorgBackup with your desktop environment to protect your data from
+    disk failure, ransomware and theft.
+  </para>
+  <para>
+    It is available as a flatpak package. To enable it you must set the
+    following two configuration items.
+  </para>
+  <para>
+    <programlisting>
+services.flatpak.enable = true ;
+# next line is needed to avoid the Error
+# Error deploying: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown:
+services.accounts-daemon.enable = true;
+    </programlisting>
+  </para>
+  <para>As a normal user you must first install, then run vorta using the
+    following commands:
+    <programlisting>
+flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
+flatpak install flathub com.borgbase.Vorta
+flatpak run --branch=stable --arch=x86_64 --command=vorta com.borgbase.Vorta
+</programlisting>
+    After running <code>flatpak install</code> you can start Vorta also via
+        the KDE application menu.
+  </para>
+  <para>
+    Details about using Vorta can be found under <link
+      xlink:href="https://vorta.borgbase.com/usage">https://vorta.borgbase.com
+      </link>.
+  </para>
+ </section>
+</chapter>