#@deckyard/escl

A client for eSCL, the driverless scan protocol behind AirScan (AirPrint scanning) and Mopria Scan. Most network scanners and multifunction printers made in the last decade speak it: XML documents and image data over HTTP or HTTPS, no driver. The deck reads what a scanner can do, checks your settings against it, runs the job and hands back the pages, or writes them to files.

#Conventions

Snippets assume this import:

import "@deckyard/escl/" as escl;

The deck needs the net capability, so it runs on the default jennifer binary, not jennifer-tiny.

Lengths are in the protocol's unit, 1/300 inch: A4 is 2480 x 3508, US Letter 2550 x 3300. escl.fromMm(210.0) converts.

Errors: every failure the deck raises is an Error of kind "escl" whose message starts with escl: - a setting the scanner does not offer (named, with what it does offer), an empty feeder, a refused or aborted job (with the scanner's reasons, e.g. a paper jam), an unreachable scanner, a malformed or oversized reply, a job past its page or byte limit. The one exception is an error your own scanEach handler throws: it reaches you with its own kind, and if cancelling the job then fails too, that is added to its message.

Text the deck returns from a scanner (makeAndModel, state, job reasons, a page's contentType, discovered names, ...) has every control character replaced by ?, so printing it cannot drive a terminal, and every bidirectional-text control and invisible format character too (zero-width spaces, line separators, the byte order mark), so a name cannot display reversed or hide text; text quoted in an error is cut to 80 characters. A text element over 1024 characters, a document over 1 MiB or 10,000 elements (refused before it is parsed), or a page whose Content-Type is not a plain type/subtype, is an error.

try {
    escl.scan($s, $st);
} catch (e) {
    io.printf("%s\n", $e.message);
}

#Finding the scanner

escl.discover(2000) asks the network once over mDNS and returns every scanner that answers within two seconds, with a ready url:

for (def f in escl.discover(2000)) {
    io.printf("%s  %s\n", $f.name, $f.url);
}

Answers are not authenticated (mDNS has no way to): any host on the network can announce a scanner, so check name and url before scanning anything sensitive. Names come back with control characters as ?. Only answers from port 5353 with no error code count. The query leaves by the default route's interface, and only IPv4 addresses are read; a scanner on another subnet or VLAN, or behind Wi-Fi client isolation, does not answer. Then give the address yourself. A scanner announces itself as _uscan._tcp (_uscans._tcp for HTTPS), with its port and, in the TXT record rs, its eSCL root (almost always eSCL):

avahi-browse -rt _uscan._tcp
curl -s http://192.168.1.40:8080/eSCL/ScannerCapabilities | head

Many devices use port 80; the HP LaserJet M28 series uses 8080. When the second command prints a ScannerCapabilities document, the deck can drive the scanner.