feat(hosts): add the Raspberry Pi Zero 2 W Psion sidecar
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A headless aarch64 companion for a Psion 5MX: PPP over RS232 with NAT out to wifi and a telnet login, plus a cleartext POP3/SMTP proxy for the Psion's mail client. - hosts/PiZero2W/: host config, serial-ppp.nix, email-proxy.nix, an SD-image variant, and a hardware-configuration.nix placeholder. - Host table entry on nixos-hardware's raspberry-pi-3 profile; the Zero 2 W is the Pi 3's BCM2837 SoC. nixpkgs' linuxPackages_rpi02w is deprecated and warns that the linux-rpi series is being removed in favour of nixos-hardware. - The host owns its firmware partition (hardware.raspberry-pi.firmware), which is what puts the disable-bt and uart0/ctsrts overlays in config.txt so /dev/ttyAMA0 is the RS232 header rather than Bluetooth. uboot.enable keeps the U-Boot -> extlinux boot path the rewritten config.txt would otherwise lose. - packages.aarch64-linux.zero2w-sd-image: the host's own configuration as an installable card. The board has no Ethernet and no free serial port, so a generic image would leave no way in. - The mail proxy comes from the legacy-email-proxy flake, which provides the package and the NixOS module; nothing about it is vendored here. - docs/hosts/pizero2w.md, plus README host table and shared-layer notes. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Emma Thorpe
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Claude Opus 5
Claude Opus 5
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1ff333a896
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# Raspberry Pi Zero 2 W (`lyrathorpe-zero2w`)
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Headless `aarch64-linux` "Psion sidecar": an RS232 companion for a Psion 5MX,
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after [Kian Ryan's PPP modem and terminal
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write-up](https://www.kianryan.co.uk/2022-11-28-psion-sidecar-ppp-modem-and-terminal/).
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Two roles, split into submodules:
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- **PPP link + telnet** (`serial-ppp.nix`) — `pppd` on `/dev/ttyAMA0`, the Psion
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on the far end of a null-modem cable, NAT out to Wi-Fi, and a telnet login for
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the Psion's terminal client.
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- **Legacy mail proxy** (`email-proxy.nix`) — cleartext POP3/SMTP for the
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Psion's built-in mail client, forwarded to authenticated IMAPS/SMTPS by
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[legacy-email-proxy](https://code.emmathe.dev/lyrathorpe/legacy-email-proxy).
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That project ships its own package and NixOS module, so `email-proxy.nix`
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here is only `services.legacy-email-proxy.enable` plus a path to the
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credentials — nothing about the proxy is vendored into this flake.
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`sd-image.nix` in the same directory is not part of the running system: it is
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the one-shot install card, built as `packages.aarch64-linux.zero2w-sd-image`.
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See "Install".
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## Hardware and boot
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The Zero 2 W is a BCM2837 — the Pi 3's SoC — so the host table uses
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`nixos-hardware`'s `raspberry-pi-3` profile for the kernel, firmware and device
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tree. Boot is the same U-Boot + extlinux path as the other Pi.
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Unlike the Pi 5, this host owns the firmware partition declaratively
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(`hardware.raspberry-pi.firmware.enable`): every `switch` rewrites
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`/boot/firmware`, including `config.txt`. Two settings there matter:
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| `config.txt` | Why |
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| ------------------------ | --------------------------------------------------------------------------------------------------------------------------------------------------- |
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| `dtoverlay=disable-bt` | Moves the PL011 UART off Bluetooth onto GPIO 14/15, so `/dev/ttyAMA0` is the RS232 header. The mini UART (`ttyS0`) drifts at 115200. |
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| `dtoverlay=uart0,ctsrts` | RTS/CTS on GPIO 16/17. Both `pppd` and the Psion's modem profile use hardware flow control. |
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| `kernel=u-boot.bin` | `hardware.raspberry-pi.firmware.uboot.enable`. Without it the rewritten `config.txt` would have no `kernel=` line and the board would stop booting. |
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`gpu_mem=16`, `start_x=0`, `camera_auto_detect=0` and `display_auto_detect=0`
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hand the VideoCore the minimum: the board has 512 MB total and no display.
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## Never build on the Pi
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512 MB of RAM and an SD card. It cannot compile its own system, and there is
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deliberately no swap partition (SD cards wear out under swap writes) — zram
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takes its place. Build somewhere else and push the result:
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```sh
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# from a workstation, using another aarch64 machine as the builder
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nixos-rebuild switch --flake .#lyrathorpe-zero2w \
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--build-host lyrathorpe@lyrathorpe-rpi5 \
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--target-host lyrathorpe@<pi-address> --use-remote-sudo
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```
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The `raspberry-pi-3` profile builds the vendor kernel from source and it is not
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in the binary cache, so the first build is long (hours on the Pi 5, less on the
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MacBook). Later builds reuse it. The same applies to the SD image below: it
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contains that kernel, so it needs an `aarch64-linux` builder too. From an
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`x86_64` box or a Mac, that means a remote builder (`nix.buildMachines`) or, on
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Darwin, `nix.linux-builder.enable`.
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## Install
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The card is built from this flake, not downloaded. A generic NixOS image would
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boot, but there would be no way into the machine afterwards: it has no Ethernet,
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no wifi credentials, and this configuration hands the serial port to `pppd`, so
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there is no console either. Building the host's own image sidesteps all three —
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the first boot is already the real system, with the SSH key from the registry
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in place.
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1. **Set the SSID.** `networking.wireless.networks` in `configuration.nix` still
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says `CHANGE-ME-SSID`. It is baked into the image at build time; only the PSK
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is read at runtime.
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2. **Build and write the card.** On an `aarch64-linux` machine (or with one
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configured as a builder):
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```sh
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nix build .#packages.aarch64-linux.zero2w-sd-image
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sudo dd if=result/sd-image/nixos-zero2w.img of=/dev/sdX bs=4M conv=fsync status=progress
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```
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Check `/dev/sdX` twice. `dd` does not ask.
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3. **Seed the secrets before first boot.** They are not in the image. Mount the
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card's second partition (the ext4 root) and write both files described under
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"Secrets" below:
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```sh
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sudo mount /dev/sdX2 /mnt
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sudo mkdir -p /mnt/var/lib/wpa_supplicant /mnt/var/lib/legacy-email-proxy
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printf 'psk_home=%s\n' 'the-pre-shared-key' \
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| sudo tee /mnt/var/lib/wpa_supplicant/secrets.conf > /dev/null
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sudo chmod 600 /mnt/var/lib/wpa_supplicant/secrets.conf
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# ... and /mnt/var/lib/legacy-email-proxy/backend.env, same permissions
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sudo umount /mnt
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```
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Skip the PSK and the board boots with no network at all.
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4. **Boot it.** Give it a few minutes on first boot — it resizes the root
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partition and generates host keys on a slow card. Then:
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```sh
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ssh lyrathorpe@lyrathorpe-zero2w.local # mDNS; services.avahi publishes it
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```
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5. **Give the login user a password** (`passwd lyrathorpe`) if you want console
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or telnet login; the SSH key from
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[`users/registry.nix`](https://code.emmathe.dev/lyrathorpe/nixfiles/src/branch/main/users/registry.nix)
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already works without one.
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6. Thereafter, rebuild from another machine as in the previous section.
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`hosts/PiZero2W/hardware-configuration.nix` is a **placeholder** — but its
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layout (`/` on label `NIXOS_SD`, `/boot/firmware` on label `FIRMWARE`) is
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exactly what the SD image produces, so there is nothing to regenerate for a card
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install. Run `nixos-generate-config` and replace it only if you deviate from
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that layout.
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If the board never appears on the network, it is almost always the PSK file.
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Re-mount the card and check it. Failing that, a mini-HDMI monitor and a
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micro-USB keyboard get you a console on `tty1` — the serial port will not,
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because `pppd` holds it.
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## Secrets (not in the Nix store)
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Both files are created on the device, owned by root, mode `0600`. Neither is
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managed by this flake; the units that read them fail loudly if they are absent.
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**Wi-Fi PSK** — `/var/lib/wpa_supplicant/secrets.conf`:
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```
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psk_home=<the pre-shared key>
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```
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The SSID itself _is_ in `configuration.nix` and is currently the placeholder
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`CHANGE-ME-SSID`; set it to the real network. `wpa_supplicant` resolves
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`pskRaw = "ext:psk_home"` against this file at runtime.
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**Mail backend** — `/var/lib/legacy-email-proxy/backend.env`, a systemd
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`EnvironmentFile`:
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```
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BACKEND_IMAP_HOST=imap.example.com
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BACKEND_IMAP_USER=someone@example.com
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BACKEND_IMAP_PASS=<app password>
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BACKEND_SMTP_HOST=smtp.example.com
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BACKEND_SMTP_USER=someone@example.com
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BACKEND_SMTP_PASS=<app password>
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```
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Ports and TLS default sensibly (IMAPS 993, SMTPS 465); the full variable list is
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in the proxy's README.
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### Why POP3 and not IMAP
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The Psion's built-in mail client speaks POP only, so POP3 is what the proxy
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exposes. If a third-party IMAP client is ever installed on the device, the
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answer is **not** to add an IMAP frontend to the proxy: the backend is already
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IMAP, so there is no protocol to translate, only TLS to remove. An `stunnel`
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client (plaintext 143 on the PPP link, IMAPS 993 outbound) does that in a few
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lines with no code, and credentials pass straight through — IMAP clients always
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authenticate.
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SMTP stays on the proxy either way. A client of this vintage cannot do SMTP
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AUTH, which is exactly why the proxy injects the backend credentials.
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## Psion configuration
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Matches the addressing in `serial-ppp.nix` (`10.0.0.1` the Pi, `10.0.0.2` the
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Psion):
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- **Modem** control panel, a "Direct Cable Connection" profile: 115200 baud,
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Hardware (RTS/CTS) flow control; on the Advanced tab, Terminal Detect and
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Carrier Detect both **off**.
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- **Internet** control panel, a new profile: Connection Type **Direct**, Manual
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Login **True**. Addresses: get IP from server **False**, static **10.0.0.2**.
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Get DNS from server **True** — `pppd` sends resolvers over the link
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(`ms-dns`), so nothing is hard-coded on the Psion.
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- Advanced: PPP extensions **False**, plain-text authentication **True**.
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- Terminal client: telnet to **10.0.0.1 port 23**. It renders non-ANSI output
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far better than the raw serial console does.
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- Mail client: POP3 and SMTP server **10.0.0.1**, no encryption, no
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authentication.
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## Security
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Everything on this host that the Psion talks to is unauthenticated and
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unencrypted, because a 1999 palmtop speaks no TLS:
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- **telnet on 23** — cleartext login, including the password.
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- **POP3 on 110 / SMTP on 25** — full mailbox access and an open relay to anyone
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who reaches them.
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The confinement is the firewall, and it is the only thing standing there:
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`ppp0` is a trusted interface, `wlan0` is not, and those ports are never opened
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on it. The proxy binds `0.0.0.0` rather than `10.0.0.1` on purpose — the PPP
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address only exists while the Psion is plugged in, and a bind-time dependency on
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a serial cable is a restart loop waiting to happen. Do not add these ports to
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`networking.firewall.allowedTCPPorts`, and do not put this board on an untrusted
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network.
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Only sshd (port 22, key-only, via
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[`modules/ssh.nix`](https://code.emmathe.dev/lyrathorpe/nixfiles/src/branch/main/modules/ssh.nix))
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is reachable over Wi-Fi.
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## Troubleshooting
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| Symptom | Check |
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| ----------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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| No PPP at all | `systemctl status pppd-psion`, then `journalctl -u pppd-psion -f` while the Psion dials. `passive`/`persist` mean it waits, not fails. |
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| PPP negotiates, then hangs | Flow control. Confirm `dtoverlay=uart0,ctsrts` is in `/boot/firmware/config.txt` and that the Psion's modem profile is set to Hardware. |
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| `/dev/ttyAMA0` missing or is a Bluetooth device | `disable-bt` did not apply — the firmware partition was not rewritten. Confirm `/boot/firmware` is a mounted partition; the activation script skips with a warning if it is not. |
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| Something else holds the port | `systemctl status serial-getty@ttyAMA0` — it is disabled in `serial-ppp.nix`, and must stay that way. |
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| Mail proxy dead | `systemctl status legacy-email-proxy`. A missing `backend.env` fails the unit before it starts. |
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