mesh-catalog/modules/brother/README.md

brother — the Brother multifunction device: scan, printer state, admin pages

One module for one device (novox/hq ADR 0467, issue 1453): the scanner and the printer are one machine on the network, so one module serves both. It claims two seats, each held once for the mesh: scanner (verbs scan, status, list, document) and printer (verbs status, settings, print, upload; restart follows in a later round). It replaces the scan module of issue 1440 (ADR 0464), which ran on no node and is gone from the catalogue.

The scanner seat

Built for issue 1440 as novox/hq ADR 0464 decides. It scans the glass of a network scanner (any scanner that speaks eSCL, also called AirScan or Mopria), cuts the document out of the lid's white, files it as a one-page PDF in a folder and says scan.added on the bus, so other modules can react. A scan happens when a tool is called: the module runs no process of its own, and it runs as the node's operator account.

What a scan does

1. Asks the scanner for its glass size and puts one job to it (POST <scanner>/ScanJobs): the platen, the whole glass, at the configured resolution and colour mode, as a JPEG; it fetches the page once.
2. Reads the resolution from the image's own JFIF density, never from the request: a scanner may answer a 300 dpi request with 200 dpi, and the page size and the event's millimetres follow what it gave.
3. Crops: the lid's level is measured in the corner farthest from where documents lie, a pixel is document when it is darker than the lid, and the rows and columns holding enough of them give the box. No document, or one filling nearly the whole glass, keeps the whole image (cropped false), and so does crop false.
4. Files a one-page PDF holding the JPEG: written whole beside its place as .scan-*.tmp and renamed to scan-YYYY-MM-DD-HHMMSS.pdf (local time); a taken name gets -2, -3, and no file is ever overwritten.
5. Says scan.added once, after the rename, under the file's sha256 as its message id. It never carries the image.

A scanner that refuses or gives no page files nothing and says nothing on the bus; the tool's error says why.

Config

key	meaning	default
scanner	the scanner's eSCL address, such as http://192.0.2.10/eSCL (required, trusted)	none
scan-folder	the absolute path the PDFs are filed in (required, trusted)	none
printer	the printer's host, such as 192.0.2.10, for its admin page and IPP (required, trusted)	none
printer-certificate	the SHA-256 fingerprint of the certificate the admin page may serve, AA:BB:...; several, separated by commas, when it serves more than one (required, trusted)	none
resolution	dpi asked: 100, 200, 300 or 600	300
color-mode	color or gray	color
crop	cut the document out of the lid's white	true

The values reach the code in one file, settings.json in the module's state directory, which the mesh writes from the config; the code reads it at every call. With scanner or scan-folder unset, every scanner verb says which and writes and asks nothing; printer and printer-certificate are the printer verbs' to say.

The scan folder is not a manifest resource: a directory's path cannot take a config value (issue 1441), so the code makes it (mode 0755) when it files the first scan.

Tools

verb (tool)	does
scanner.scan	asks the device process to scan, crop, keep the PDF in the bucket under its sha256 and say scan.added; then fetches it in parts, checks the sha256 and files it in scan-folder. Waits up to 180 s. Answers the event's body and path
scanner.document	one part of a kept scan (object, part from 1): part, parts, size, sha256 (of the whole), data (base64). Join the parts and check the sha256
scanner.status	whether the scanner answers, its state (Idle or Processing) and its document feeder's state; a scanner that does not answer is answers: false with why. This is how its health is read: the module has no long-running resource to declare a health check on
scanner.list	the newest filed scans, newest first (limit, default 20). Read-only

The printer seat

verb (tool)	does
printer.status	without a login: over IPP (Get-Printer-Attributes, encoded by hand, on port 631) the state (idle, processing, stopped), the state reasons (such as toner-low-warning) and the markers' names, levels (percent; below 0 is unknown) and colours; over eSCL the scanner's state and its feeder's. A part that does not answer is answers: false with why. Needs printer and scanner
printer.upload	one part of a document to print later (upload id, part, parts, sha256 of the whole, data base64): stored as uploads/<upload>.pdf or .jpg once all parts came and the sha256 matches; a wrong sha256, a part out of range or a document that is neither PDF nor JPEG stores nothing
printer.print	prints a kept scan (object) or a completed upload (upload) as an IPP Print-Job (operation 0x0002, port 631) in its own format, copies 1 to 99 (default 1), sides one-sided, two-sided-long-edge or two-sided-short-edge (default one-sided), media (default iso_a4_210x297mm); answers the job's id and state
printer.settings	asks the device process (below) to log in, read one admin page and log out, and waits up to 60 s for its answer. Argument page: a path on the admin site, starting with / and ending in .html (default /general/status.html). Answers the page's title, links (href and text) and forms with their fields (name, type, value, label); a password field's value is never given. Read-only: it sends the login and the logout and nothing else

Trust and the password

The admin page is HTTPS with a self-signed certificate. The module verifies no chain: it accepts the one certificate whose SHA-256 fingerprint is one of those in printer-certificate and refuses any other, in the handshake, so no password is sent to it; the error names the served and the pinned fingerprints. Give each as 32 upper-case hex pairs joined by colons, several separated by commas. The printer serves two self-signed certificates for one name, an ECDSA one and an RSA one, and which it serves depends on the signature algorithms the client offers, so pin both (issue 1503): read them with openssl s_client -connect <printer>:443 -sigalgs ECDSA+SHA256 and -sigalgs RSA-PSS+SHA256, each piped to openssl x509 -noout -fingerprint -sha256.

The admin password is the module's own secret admin-password (ADR 0467 clause 4; ADR 0283): the operator gives it at their desk, and the mesh never generates or rotates it. It is owned by the account brother-device, which the module makes, because the tools run as the operator account and must never read it (ADR 0436 §1, §12; the shape of mesh-mail's sender). The file is ${dir:state}/admin-password.secret, mode 0600, and BROTHER_ADMIN_PASSWORD names it for the device process alone; the tools' bundle is given no such name, and admin_boundary_test.go checks that no tool file imports the package that reads it. While it is not given, printer.settings answers that the secret is not given, and the printer is not contacted.

Documents in parts

The bus refuses a message over 1 MiB, so a document crosses it in parts of at most 700 KiB (both the size this module cuts and the most it accepts, as ADR 0468 clause 3 says), counted from 1, each carried base64-encoded in JSON. A caller joins the parts and checks the SHA-256. The bucket (s3-bucket, from minio) is spoken to by the device process alone, from store.json (owner brother-device, mode 0600), with the S3 client mesh-mail uses; a scan is kept once as <sha256>.pdf and an upload as uploads/<id>.<ext>.

The device process

brother device runs as brother-device with a bus account of its own (the own secret broker-device), never a seat, and does all the device and bucket work: the scan (and scan.added, emitted by it with the object's key and no path, since filing in the scan folder is the tool's), the documents, the printing, and the admin pages. It consumes the work-queue stream device-asks (event device.asked, body id, page, asked-at) and writes each answer, once, into the state device-answers under the ask's id (ten minutes' life): the page's title, links and form fields, or error. It alone reads the password, pins the certificate, logs in, reads the page and logs out. An ask older than two minutes is dropped without contacting the printer; at most three deliveries. It reads settings.json (mode 0644, no secret in it) and the state directory is 0711 so that it can reach its own files; both are the choices that let its account read what it needs. Its health is its unit.

The login reads the form each time: the device generates the password field's name, so the code takes the name from the form, posts it with the form's hidden fields, and keeps the session cookie. A login is failed when the answer is the login page again. The logout follows a logout link or form on the page; a page with none is said in the answer (logout), and the session is left to expire on the device.

The event

scan.added, said by this module, on the wire mesh.mod.brother.event.scan.added; a consumer declares brother.scan.added. Its body: path, node, bytes, sha256, width-mm, height-mm, resolution (dpi, as the scanner gave it), color-mode, cropped, scanned-at (RFC 3339, UTC).

Files it does not know

Each directory or archive this module declares says how it treats a file it did not write (ADR 0422, ADR 0416).

• ${dir:state}: everything in it is the program's; the mesh moves nothing out (ADR 0422 §2).
• The scan folder (scan-folder) is declared by no manifest. The module writes only the scan-*.pdf files it files and their .scan-*.tmp while they are written; it never changes, moves or removes any other file there, and scanner.list only reads.

Tests

The unit tests use the captured answers of a real scanner (cmd/brother/testdata/escl, captured 2026-10-20) served by a test HTTP server, the captured IPP exchange (testdata/ipp) replayed, and the captured login page (testdata/admin/login-page.html) served by a test TLS server with its own certificate. The pages behind the login are synthetic (testdata/admin/synthetic-*.html) until round 2 captures the real ones. No test reaches the real device.