printserver/README.md

# Printserver

A containerized CUPS print server that shares a **Brother HL-4150CDN** network
printer over IPP and advertises it via AirPrint (mDNS/DNS-SD). It prints by
sending **PostScript (BR-Script3)** straight to the printer, so no vendor driver
and no 32-bit libraries run in the container.

The container bundles:

- CUPS (IPP + web admin on port 631) with cups-filters for format conversion
- Avahi / DNS-SD for AirPrint discovery (UDP 5353)
- Brother's official HL-4150CDN **BR-Script3 PPD** for device options

## How it works

The HL-4150CDN emulates PCL6 and BR-Script3 (PostScript 3), and accepts
PostScript over raw AppSocket (port 9100). CUPS converts whatever a client
sends into PostScript and the BR-Script3 PPD turns the selected queue options
into PostScript `setpagedevice` instructions:

```
 macOS / iOS / Windows / Linux              container                    printer
 ┌─────────────────┐   IPP/AirPrint   ┌──────────────────────────┐   raw   ┌────────────┐
 │ AirPrint (URF)  │ ───────────────▶ │ CUPS                     │ ──────▶ │ HL-4150CDN │
 │ PDF / PostScript│                  │  URF/PWG/PDF → PostScript│ :9100   │ BR-Script3 │
 └─────────────────┘                  │  pstops → setpagedevice  │         └────────────┘
                                      └──────────────────────────┘
```

For example, `Duplex=DuplexNoTumble BRPrintQuality=Color InputSlot=Tray2`
becomes:

```postscript
<</Duplex true /Tumble false>>setpagedevice
<</BRProcessColor 4 /BRHrc 0>>setpagedevice
<</ManualFeed false /BRTraysw false /BRFeeder 1>> setpagedevice
```

## Layout

| Path | Purpose |
| --- | --- |
| `Containerfile` | Image definition (Debian base, CUPS + cups-filters + Avahi, BR-Script3 PPD) |
| `cupsd.conf` | Container CUPS configuration (listen on all interfaces, share printers) |
| `entrypoint.sh` | Starts D-Bus, Avahi and CUPS, then registers the printer queue |
| `assets/Brother HL-4150CDN/ppd/` | Official BR-Script3 PPD used for the queue |
| `assets/Brother HL-4150CDN/Windows/` | Vendor Windows driver the PPD was extracted from |

## Prerequisites

- Podman (or a compatible builder such as Docker/Buildah)
- The printer reachable on the network, with a known IP address or hostname

## Build

Run from the repository root:

```sh
podman build -t printserver:latest .
```

The image is architecture-independent (no i386 or other 32-bit binaries), so it
builds natively on x86_64 and arm64 hosts alike.

## Run

mDNS does not work through a normal container network, so use **host
networking**:

```sh
podman run -d --name printserver \
  --network host \
  -e PRINTER_IP=192.168.100.7 \
  -e PRINTER_NAME=HL-4150CDN \
  printserver:latest
```

### Environment variables

| Variable | Default | Description |
| --- | --- | --- |
| `PRINTER_IP` | *(empty)* | IP address / host name of the network printer. A device URI is derived from it (see `PRINTER_PROTOCOL`). |
| `PRINTER_URI` | *(empty)* | Full device URI. Overrides `PRINTER_IP` when set. |
| `PRINTER_PROTOCOL` | `socket` | Protocol used with `PRINTER_IP`: `socket`, `lpd` or `ipp`. |
| `PRINTER_NAME` | `HL4150CDN` | CUPS queue name. Also used as the name advertised via AirPrint/DNS-SD. |
| `PRINTER_DESCRIPTION` | *(empty)* | CUPS printer description (`lpadmin -D`), shown in the CUPS web UI. |
| `PPD_FILE` | `/usr/share/cups/model/BR4150_2.PPD` | PPD used for the queue. |

No queue is created when neither `PRINTER_IP` nor `PRINTER_URI` is set.

### Choosing the connection

The HL-4150CDN accepts PostScript directly, so the default is raw AppSocket.
Set `PRINTER_IP` and let the entrypoint build the URI:

| `PRINTER_PROTOCOL` | Resulting URI |
| --- | --- |
| `socket` (default) | `socket://<ip>:9100` — raw AppSocket/JetDirect |
| `lpd` | `lpd://<ip>/binary_p1` — LPR/LPD |
| `ipp` | `ipp://<ip>/ipp/print` |

For example, a printer at `192.168.100.7`:

```sh
-e PRINTER_IP=192.168.100.7
```

Or specify the URI explicitly for full control:

```sh
-e PRINTER_URI=socket://192.168.100.7:9100
```

### Advertised name

`PRINTER_NAME` is used as the CUPS queue name and as the name clients see in
AirPrint discovery. To show a descriptive label in the CUPS web interface,
set `PRINTER_DESCRIPTION` (this does not change the advertised name):

```sh
-e PRINTER_NAME=HL4150CDN -e PRINTER_DESCRIPTION="Brother HL-4150CDN (Office)"
```

## Printing options

The BR-Script3 PPD exposes the HL-4150CDN's hardware options. Common ones:

| Option | PPD keyword | Values |
| --- | --- | --- |
| Paper source | `InputSlot` | `AutoSelect`, `Tray1`, `Tray2`, `MPTray` |
| Two-sided printing | `Duplex` | `None`, `DuplexNoTumble` (long edge), `DuplexTumble` (short edge) |
| Colour / mono | `BRPrintQuality` | `Auto`, `Color`, `Black` |
| Colour mode | `BRColorMode` | `Normal`, `Vivid`, `None` |
| Media type | `BRMediaType` | `Plain`, `Thin`, `Thick`, `Recycled`, `Glossy`, `Label`, envelopes, … |
| Print quality | `CAPT` | `Fine` (600×600), `SuperFine` (2400 dpi class) |
| Toner save | `TonerSaveMode` | `True`, `False` |
| Secure print | `BRJobHold` | `None`, `Private` (+ `BRJobPIN`) |

They can be selected per job, e.g.:

```sh
podman exec printserver lp -d HL4150CDN \
  -o Duplex=DuplexNoTumble -o BRPrintQuality=Color -o InputSlot=Tray1 report.pdf
```

## Client support

| Platform | Path | Notes |
| --- | --- | --- |
| macOS | AirPrint / IPP Everywhere | Discovered via mDNS; no driver needed |
| iOS / iPadOS | AirPrint | Discovered via mDNS; no driver needed |
| Linux | IPP / IPP Everywhere | Native or with the shared PPD |
| Windows | IPP or raw 9100 | Install Brother's **BR-Script3** Windows driver, pointed at the printer or the shared queue |

Windows uses a client-side Brother driver; the server does not share via SMB.

## Verify

Browse the CUPS web interface:

```sh
open http://localhost:631/            # printers
open http://localhost:631/admin       # administration
```

From the host, or inside the container:

```sh
podman exec printserver lpstat -t                        # queues and status
podman exec printserver lpstat -v HL4150CDN              # device URI
podman exec printserver lpoptions -p HL4150CDN -l        # available options
podman exec printserver avahi-browse -rt _ipp._tcp       # mDNS advertisement
```

A print job can be submitted with:

```sh
podman exec printserver lp -d HL4150CDN /etc/hostname
```

On macOS/iOS the shared queue should appear in the printer list via AirPrint.

## Notes and limitations

- **Host networking is required** for Avahi to send and receive mDNS traffic.
  `-p 631:631` alone is enough for direct IPP access but not for AirPrint
  discovery.
- **Printer state is not persisted by default.** The queue is recreated on every
  start from `PRINTER_IP`/`PRINTER_URI`, so nothing has to survive a restart. To
  keep CUPS state (extra queues, subscriptions) across container replacement,
  mount a volume over `/etc/cups` — **use a named volume**:

  ```sh
  -v printserver-cups:/etc/cups
  ```

  Podman seeds a *new named volume* with the image's `/etc/cups`, so `cupsd.conf`
  is present. A **bind mount to an empty host directory does not**: it hides the
  image's configuration, `cupsd` then exits immediately (status 1, silently —
  its default `ErrorLog` is syslog), the container stops, and the entrypoint's
  `lpadmin` fails with a misleading
  `lpadmin: Unable to connect to server: Bad file descriptor`. This is easiest
  to hit from a quadlet that bind-mounts a share, e.g.
  `Volume=/nfs/share/print-server/:/etc/cups/:Z`. If you must bind-mount, make
  sure the directory already contains a `cupsd.conf`; also note that a read-only
  share, NFS `root_squash` or SELinux relabeling can still prevent `cupsd` from
  writing its state there. For a named volume in a quadlet, use
  `Volume=printserver-cups:/etc/cups` (a bare name, no leading `/`).
- Colour handling is done by the printer's own PostScript RIP. Output may differ
  slightly from Brother's host-side (rasterising) driver.
- The queue advertises AirPrint capabilities (`image/urf`, `image/pwg-raster`,
  `sides-supported`, `color-supported`). macOS and iOS printing is converted by
  cups-filters; verify end to end on your network.

## Troubleshooting

### The container runs but nothing answers on 631

The queue can be advertised over mDNS while nothing is actually listening, so
clients discover a printer they cannot reach. Check inside the container:

```sh
podman exec printserver curl -s -o /dev/null -w '%{http_code}\n' http://127.0.0.1:631/
podman exec printserver tail -20 /var/log/cups/error_log
```

**`Unable to open listen socket for address 0.0.0.0:631 - Permission denied`** —
`cupsd` never bound port 631. This is the normal outcome of **rootless Podman
with `Network=host`**: port 631 is privileged, and a rootless container does not
hold `CAP_NET_BIND_SERVICE` in the host network namespace (adding the capability
does not help, because the capability must belong to the *host* user namespace).
`cupsd` keeps running on its domain socket, so `lpadmin`/`lpstat` inside the
container still work — which makes the failure easy to miss. Fix one of:

- allow unprivileged binding of port 631 on the host, persistently (needs root):

  ```sh
  echo 'net.ipv4.ip_unprivileged_port_start=631' | sudo tee /etc/sysctl.d/90-unprivileged-ports.conf
  sudo sysctl --system
  ```

- run the container as a rootful (system) service rather than a user quadlet;
- or serve on an unprivileged port (CUPS advertises the actual port in its SRV
  record, but be aware some AirPrint clients assume 631).

On SELinux systems, verify there is no port-labelling denial as well
(`sudo ausearch -m avc -ts recent`). Also ensure 631/tcp and 5353/udp are open in
the firewall.

### macOS/iOS sees the printer but reports it cannot connect

If plain IPP works but the *secure* connection fails, this is the `_ipps`
service: macOS prefers `_ipps._tcp`, which CUPS advertises unconditionally
(also when no certificate exists). CUPS creates its TLS certificate lazily on
the first TLS connection, in `/etc/cups/ssl` — so that directory must be
writable. If it is not, CUPS logs
`Unable to encrypt connection: Unable to create server credentials` and the
secure connection fails while plain IPP keeps working. Use a **writable (named)
volume** for `/etc/cups`; see the bind-mount caveat under *Notes and
limitations*. After changing anything, remove the printer in macOS and add it
again — macOS caches the failed attempt.

## CUPS 3 and the future of PPDs

Adding the queue with a PPD makes `lpadmin` print:

```
lpadmin: Printer drivers are deprecated and will stop working in a future version of CUPS.
```

This is an advance notice, not an error. OpenPrinting deprecated PPDs, printer
drivers and filter executables during the CUPS 2.x series, and **CUPS 3.x drops
them entirely** in favour of an all-IPP architecture (a *local* server for
temporary driverless queues plus a *sharing* server for persistent ones, with
filtering limited to PDF, PWG Raster, Apple Raster and PCLm). Any queue created
with a PPD (`-P`) or a driver model (`-m <driver>`) triggers the message — it is
not specific to Brother, and there is no way to keep the PPD and silence it. A
raw queue and `-m everywhere` (driverless) do *not* print it.

The driverless replacement is a **Printer Application**: a PAPPL-based service
that emulates a driverless IPP printer and does the format/option conversion on
the other side. For a PostScript device like the HL-4150CDN that role is filled
by the **PostScript Printer Application** (`ps-printer-app`), which takes over
exactly what our BR-Script3 PPD does today — the PPD does not disappear, it just
moves out of CUPS and into the printer application.

Note the HL-4150CDN is itself *not* IPP Everywhere / AirPrint capable (Brother
lists it as such), so the plain `-m everywhere` shortcut is not available here.
This container instead advertises AirPrint to its *clients* and performs the
PostScript conversion itself.

Nothing needs to change right now: the distro images used here ship CUPS 2.4.x
and PPD-based drivers keep working for years. When you eventually move to
CUPS 3.x, run `ps-printer-app` (or `legacy-printer-app`) alongside CUPS and
create the queue driverlessly against it (`ipp://…`), leaving the PPD inside the
printer application.