Remote disk unlocking

If you want to unlock your computer remotely via SSH or even through Tor, and you are facing the problem, that you can’t reach your computer before your computer is unlocked. Tor will help you to reach your computer, even during the boot process.

Setup

Generate host key for the SSH daemon which will run in initrd during boot

# ssh-keygen -t rsa -N "" -f /etc/secrets/initrd/ssh_host_rsa_key

Enable SSH daemon in initrd

/etc/nixos/configuration.nix

boot.kernelParams = [ "ip=dhcp" ];
boot.initrd = {
  availableKernelModules = [ "r8169" ];
  systemd.users.root.shell = "/bin/cryptsetup-askpass";
  network = {
    enable = true;
    ssh = {
      enable = true;
      port = 22;
      authorizedKeys = [ "ssh-rsa AAAAyourpublic-key-here..." ];
      hostKeys = [ "/etc/secrets/initrd/ssh_host_rsa_key" ];
    };
  };
};

Adapt following parts according to your setup

• authorizedKeys: Add the SSH public keys for the users which should be able to authenticate to the SSH daemon to the authorizedKeys option.
• availableKernelModules: Most likely your network card is not working without its kernel module being part of the initrd, so you have to find out which module is used for your network. Use lspci -v | grep -iA8 'network\|ethernet' for that.
• kernelParams: Instead of using DHCP you could also configure a static IP, for example with kernel parameter boot.kernelParams = [ "ip=10.25.0.2::10.25.0.1:255.255.255.0:myhost::none" ];, where 10.25.0.2 is the client IP, 10.25.0.1 is the gateway IP. See the kernel documentation for more information on the ip= parameter. When using DHCP, make sure your computer is always attached to the network and is able to get an IP adress, or the boot process will hang.

The shell option is necessary to get a password prompt instead of a shell. If you omit it, you will get dropped into /bin/ash, and you will have to manually run cryptsetup-askpass to enter the password. Alternatively, the shell option can be set to /bin/conspy for passwords which expect stdin. This binary included by default, and provided by busybox.

Usage

After reboot, connect to the initrd SSH daemon using

# ssh root@10.25.0.2

Where 10.25.0.2 is the IP which is acquired via DHCP or configured via the kernel parameter.

Tips and tricks

Bcachefs unlocking

Unlocking encrypted Bcachefs root filesystems is not yet supported. As a workaround, following script can be used as SSH shell, to unlock the disk /dev/vda2.

/etc/nixos/configuration.nix

boot.initrd.systemd = let
  askPass = pkgs.writeShellScriptBin "bcachefs-askpass" ''
    keyctl link @u @s
    mkdir /sysroot
    until bcachefs mount /dev/vda2 /sysroot
    do
      sleep  1
    done
  '';
  in {
    enable = true;
    initrdBin = with pkgs; [ keyutils ];
    storePaths = ["${askPass}/bin/bcachefs-askpass"];
    users.root.shell = "${askPass}/bin/bcachefs-askpass";
  };
};

Using systemd in initrd automatically continues the boot process after the target /sysroot is mounted.

Tor in initrd

An example with an ssh server listening at a tor hidden service address can be found at krebs/2configs/tor/initrd.nix in stockholm

Prepare the Onion ID

You need 3 files to create an onion id (a.k.a. tor hidden service).

• hostname
• hs_ed25519_public_key
• hs_ed25519_secret_key

To create these files, you have to run tor once, with a dummy configuration.

DataDirectory /tmp/my-dummy.tor/
SOCKSPort 127.0.0.1:10050 IsolateDestAddr
SOCKSPort 127.0.0.1:10063
HiddenServiceDir /home/tony/tor/onion
HiddenServicePort 1234 127.0.0.1:1234

Let’s asume you created this file in /home/tony/tor/tor.rc.

Verify that everything is tor.rc awesome, by running tor -f /home/tony/tor/tor.rc --verify-config. If you don’t see any errors, just run tor -f /home/tony/tor/tor.rc.

You will get some output like this.

May 21 18:38:39.000 [notice] Bootstrapped 80% (ap_conn): Connecting to a relay to build circuits
May 21 18:38:39.000 [notice] Bootstrapped 85% (ap_conn_done): Connected to a relay to build circuits
May 21 18:38:39.000 [notice] Bootstrapped 89% (ap_handshake): Finishing handshake with a relay to build circuits
May 21 18:38:39.000 [notice] Bootstrapped 90% (ap_handshake_done): Handshake finished with a relay to build circuits
May 21 18:38:39.000 [notice] Bootstrapped 95% (circuit_create): Establishing a Tor circuit
May 21 18:38:40.000 [notice] Bootstrapped 100% (done): Done

Hit Ctrl-C and the files you need, should be in /home/tony/tor/onion.

Setup Tor

Now that you have your 3 files, you have to script a bit, but it’s not too complicated.

# copy your onion folder
boot.initrd.secrets = {
  "/etc/tor/onion/bootup"; = /home/tony/tor/onion; # maybe find a better spot to store this.
};

# copy tor to you initrd
boot.initrd.extraUtilsCommands = ''
  copy_bin_and_libs ${pkgs.tor}/bin/tor
'';

# start tor during boot process
boot.initrd.network.postCommands = let
  torRc = (pkgs.writeText "tor.rc" ''
    DataDirectory /etc/tor
    SOCKSPort 127.0.0.1:9050 IsolateDestAddr
    SOCKSPort 127.0.0.1:9063
    HiddenServiceDir /etc/tor/onion/bootup
    HiddenServicePort 22 127.0.0.1:22
  '');
in ''
  echo "tor: preparing onion folder"
  # have to do this otherwise tor does not want to start
  chmod -R 700 /etc/tor

  echo "make sure localhost is up"
  ip a a 127.0.0.1/8 dev lo
  ip link set lo up

  echo "tor: starting tor"
  tor -f ${torRc} --verify-config
  tor -f ${torRc} &
'';

That was it. Tor should be running during your boot process.

Setup haveged

If your system doesn't gather enough entropy the startup time of tor is rather long (2:42 vs 0:06 on a RPi 4b). Counter it by starting haveged.

Append in your boot.initrd.extraUtilsCommands.

  copy_bin_and_libs ${pkgs.haveged}/bin/haveged

Then use this snippet before echo "tor: starting tor" in your boot.initrd.network.postCommands.

      echo "haveged: starting haveged"
      haveged -F &

Setup ntpdate

If your system doesn't utilize a RTC you've to ensure time is correctly set before startup of tor.

Append in your boot.initrd.extraUtilsCommands.

  copy_bin_and_libs ${pkgs.ntp}/bin/ntpdate

Then use this snippet before echo "tor: starting tor" in your boot.initrd.network.postCommands.

      echo "ntp: starting ntpdate"
      echo "ntp   123/tcp" >> /etc/services
      echo "ntp   123/udp" >> /etc/services
      ntpdate w.x.y.z # pick one IP from https://www.ntppool.org/

Usage

When your computer boots, and asks for the LUKS password. Now you can unlock your encrypted Hard drive using:

torify ssh root@<onion.id>.onion -p 22 'my-secret-password'