spec-browsing.html [plain text]
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- SECTION: Specifications -->
<head>
<title>CUPS Browse Protocol</title>
<meta name='keywords' content='Programming'>
<LINK REL="STYLESHEET" TYPE="text/css" HREF="../cups-printable.css">
</head>
<body>
<!--
"$Id$"
CUPS Browse Protocol specification for CUPS.
Copyright 2008-2011 by Apple Inc.
Copyright 1997-2005 by Easy Software Products.
These coded instructions, statements, and computer programs are the
property of Apple Inc. and are protected by Federal copyright
law. Distribution and use rights are outlined in the file "LICENSE.txt"
which should have been included with this file. If this file is
file is missing or damaged, see the license at "http://www.cups.org/".
-->
<BLOCKQUOTE><B>Note:</B>
<P>The CUPS Browse Protocol is deprecated and will no longer be used in a future release of CUPS.</P>
</BLOCKQUOTE>
<H1 CLASS="title">CUPS Browse Protocol</H1>
<h2><a name='INTRO'>Introduction</a></h2>
<p>This specification describes the CUPS browsing protocol which is used for
printer sharing. The protocol is a UDP/IP-based broadcast service that
operates on IP service port 631 by default. Each broadcast packet describes
a single printer or class being shared.</p>
<p>For simple networks with a single subnet, a CUPS system sharing a printer
(the <em>server</em>) will periodically broadcast that printer's availability
and status information to the subnet. Every other CUPS system on the subnet
(the <em>clients</em>) will receive the broadcast and make that printer
available to local users. If a client stops receiving broadcasts from the
server, or if the server sends a special "deleted" broadcast message, the
client will remove its copy of the printer.</p>
<p>For larger networks with multiple subnets, a relay configuration can be used
where one or more client systems poll the server and then broadcast the
availability and status information for the server's shared printers to the
clients' local subnets.</p>
<p>A key feature of CUPS printer sharing is support for <em>implicit
classes</em>, which are automatically-created classes for printers that are
shared by multiple servers. These implicit classes provide automatic load
balancing and fail-safe printing functionality transparently to the user.</p>
<h2><a name='SECURITY'>Security Considerations</a></h2>
<p>Like most discovery protocols, CUPS browse packets are not encrypted or
signed, so it is possible for malicious systems on a network to advertise
or remove printers on that network to cause denial of service or information
disclosure. In order to combat this, CUPS logs incoming browse packets and
provides access controls to limit browse packet reception to known hosts.</p>
<h2><a name='FORMAT'>Browse Packet Format</a></h2>
<p>Each broadcast packet is an ASCII text string of up to 1450 bytes ending
with a line feed (0x0a). The general format is:</p>
<p class='summary'>
printer-type printer-state printer-uri "printer-location" "printer-info"
"printer-make-and-model" name=value name2=value2 ...
</p>
<p>Each of the fields contains the value of the corresponding IPP attribute.
The trailing "name=value" information is used to convey default job template
attribute values (job-sheets-default, media-default, etc.), authentication
requirements (auth-info-required), and additional IPP URI options that are
requested by the server (ipp-options).</p>
<h3>ABNF Definition</h3>
<p>The following ABNF definition [RFC4234, RFC3986] defines the format of each
browse packet:</p>
<pre class='command'>
PACKET = TYPE WSP STATE WSP URI WSP LOCATION WSP INFO WSP
MAKE-AND-MODEL WSP *[ WSP ATTR-NAME "=" ATTR-VALUE ] LF
TYPE = 1*HEXDIG
STATE = "3" / "4" / "5"
URI = "ipp://" ( 1*NAMECHAR / IP-literal / IPv4address )
[ ":" 1*DIGIT ] ( "/printers/" / "/classes/" ) 1*NAMECHAR
NAMECHAR = %x21.22.24.26-2E.30-7E / %x25 HEXDIG HEXDIG
IP-literal = See RFC 3986
IPv4address = See RFC 3986
LOCATION = QUOTED-STRING
INFO = QUOTED-STRING
MAKE-AND-MODEL = QUOTED-STRING
ATTR-NAME = 1*( ALPHA / DIGIT / "-" / "." )
ATTR-VALUE = QUOTED-STRING / 1*UNQUOTE-CHAR
QUOTED-STRING = DQUOTE *QUOTED-CHAR DQUOTE
QUOTED-CHAR = %x20.21.23-5B.5D-7E / UTF8-CHAR / %x5C %x5C / %x5C %x22
UNQUOTE-CHAR = %x21.23-26.28-5B.5D-7E / UTF8-CHAR
UTF8-CHAR = %xC0.DF %x80.BF / %xE0.EF %x80.BF %x80.BF /
%xF0.F7 %x80.BF %x80.BF %x80.BF
</pre>
</body>
</html>