nixfiles/hosts/PiZero2W/serial-ppp.nix

# The serial half of the Psion sidecar: a PPP link to a Psion 5MX over
# /dev/ttyAMA0 (RS232 level shifter on the GPIO header, 115200 8N1 with
# RTS/CTS), masqueraded out of wifi, plus a telnet login for the Psion's
# terminal client.
#
# Cleartext telnet and unauthenticated PPP are safe *only* because the link is
# a two-node cable: the peer is a machine from 1999 that speaks no TLS. Nothing
# here is exposed to wlan0.
{ pkgs, ... }:
let
  # Point-to-point addresses for the serial link; nothing else routes here.
  piAddress = "10.0.0.1";
  psionAddress = "10.0.0.2";
in
{
  # pppd needs exclusive use of the port. NixOS starts a getty on any serial
  # console named in boot.kernelParams; ttyAMA0 is not one today, but disable it
  # explicitly so a later kernel-param change cannot silently steal the line.
  systemd.services."serial-getty@ttyAMA0".enable = false;

  services.pppd = {
    enable = true;
    peers.psion.config = ''
      /dev/ttyAMA0
      115200
      ${piAddress}:${psionAddress}

      # Hardware flow control, matching the Psion's modem profile.
      crtscts

      # A null-modem cable has no carrier detect and no peer to authenticate.
      local
      noauth

      # The systemd unit is Type=notify, so pppd must stay in the foreground.
      nodetach
      lock

      # Wait for the Psion rather than failing when it is unplugged, and keep
      # waiting for the next time it is plugged back in.
      passive
      persist
      maxfail 0
      holdoff 1

      # Hand the Psion resolvers over the link, so its Internet profile can set
      # "get DNS from server = True" instead of hard-coding them.
      ms-dns 1.1.1.1
      ms-dns 8.8.8.8
    '';
  };

  # The Psion's route to the internet. The original write-up used pppd's
  # proxyarp instead; NAT keeps the Psion out of the LAN broadcast domain and
  # does not depend on what the wifi router tolerates.
  networking.nat = {
    enable = true;
    externalInterface = "wlan0";
    internalIPs = [ "${psionAddress}/32" ];
  };

  # Everything the Psion connects to (telnet here, POP3/SMTP in
  # ./email-proxy.nix) is reachable over the PPP link and nowhere else.
  networking.firewall.trustedInterfaces = [ "ppp0" ];

  # The Psion's terminal client speaks telnet over TCP, which it renders far
  # better than the raw serial console. Socket-activated, one process per
  # connection; busybox's telnetd in inetd mode hands straight over to login.
  systemd.sockets.telnetd = {
    description = "Telnet login socket for the Psion";
    wantedBy = [ "sockets.target" ];
    listenStreams = [ "${piAddress}:23" ];
    socketConfig = {
      Accept = true;
      # ppp0 (and with it 10.0.0.1) only exists while the Psion is connected;
      # FreeBind lets the socket be listening before that.
      FreeBind = true;
    };
  };

  systemd.services."telnetd@" = {
    description = "Telnet login for the Psion";
    serviceConfig = {
      ExecStart = "-${pkgs.busybox}/bin/busybox telnetd -i -l ${pkgs.shadow}/bin/login";
      StandardInput = "socket";
      StandardError = "journal";
    };
  };
}