APT Method Interface
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Jason Gunthorpe
<jgg@debian.org>
Version 1.4.6
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Copyright © 1998 Jason Gunthorpe
License Notice
"APT" and this document are free software; you can redistribute them and/
   or modify them under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version.
   For more details, on Debian systems, see the file /usr/share/
common-licenses/GPL for the full license.
Abstract
This document describes the interface that APT uses to the archive access
methods.
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Table of Contents
1. Introduction
1.1. General
1.2. Terms
2. Specification
2.1. Overview
2.2. Message Overview
2.3. Header Fields
2.4. Notes
Chapter 1. Introduction
Table of Contents
1.1. General
1.2. Terms
1.1. General
The APT method interface allows APT to acquire archive files (.deb),
   index files (Packages, Release, Mirrors) and source files (.tar.gz,
.diff). It is a general, extensible system designed to satisfy all of
these requirements:
1. Remote methods that download files from a distant site
2. Resume of aborted downloads
3. Progress reporting
4. If-Modified-Since (IMS) checking for index files
5. In-Line MD5 generation
  Â
6. No-copy in-filesystem methods
7. Multi-media methods (like CD's)
8. Dynamic source selection for failure recovery
9. User interaction for user/password requests and media swaps
10. Global configuration
Initial releases of APT (0.1.x) used a completely different method
   interface that only supported the first 6 items. This new interface deals
with the remainder.
1.2. Terms
Several terms are used through out the document, they have specific
   meanings which may not be immediately evident. To clarify they are
summarized here.
source
Refers to an item in source list. More specifically it is the broken
down item, that is each source maps to exactly one index file.
Archive sources map to Package files and Source Code sources map to
Source files.
archive file
Refers to a binary package archive (.deb, .rpm, etc).
source file
Refers to one of the files making up the source code of a package. In
debian it is one of .diff.gz, .dsc. or .tar.gz.
URI
Universal Resource Identifier (URI) is a super-set of the familiar
URL syntax used by web browsers. It consists of an access
specification followed by a specific location in that access space.
The form is <access>:<location>. Network addresses are given with the
form <access>://[<user>[:<pas>]@]hostname[:port]/<location>. Some
examples:
file:/var/mirrors/debian/
ftp://ftp.debian.org/debian
   ftp://jgg:MooCow@localhost:21/debian
nfs://bigred/var/mirrors/debian
rsync://debian.midco.net/debian
cdrom:Debian 2.0r1 Disk 1/
method
There is a one to one mapping of URI access specifiers to methods. A
method is a program that knows how to handle a URI access type and
operates according to the specifications in this file.
method instance
A specific running method. There can be more than one instance of
each method as APT is capable of concurrent method handling.
message
A series of lines terminated by a blank line sent down one of the
communication lines. The first line should have the form xxx TAG
where xxx are digits forming the status code and TAG is an
informational string
acquire
The act of bring a URI into the local pathname space. This may simply
be verifying the existence of the URI or actually downloading it from
a remote site.
Chapter 2. Specification
Table of Contents
2.1. Overview
2.2. Message Overview
2.3. Header Fields
2.4. Notes
2.1. Overview
All methods operate as a sub process of a main controlling parent. 3 FD's
   are opened for use by the method allowing two way communication and
emergency error reporting. The FD's correspond to the well known unix
FD's, stdin, stdout and stderr.
Through operation of the method communication is done via http style
plain text. Specifically RFC-822 (like the Package file) fields are used
   to describe items and a numeric-like header is used to indicate what is
happening. Each of these distinct communication messages should be sent
quickly and without pause.
   In some instances APT may pre-invoke a method to allow things like file
URI's to determine how many files are available locally.
2.2. Message Overview
The first line of each message is called the message header. The first 3
digits (called the Status Code) have the usual meaning found in the http
   protocol. 1xx is informational, 2xx is successful and 4xx is failure. The
6xx series is used to specify things sent to the method. After the status
code is an informational string provided for visual debugging.
* 100 Capabilities - Method capabilities
* 101 Log - General Logging
* 102 Status - Inter-URI status reporting (login progress)
* 200 URI Start - URI is starting acquire
* 201 URI Done - URI is finished acquire
* 400 URI Failure - URI has failed to acquire
   * 401 General Failure - Method did not like something sent to it
* 402 Authorization Required - Method requires authorization to access
the URI. Authorization is User/Pass
* 403 Media Failure - Method requires a media change
* 600 URI Acquire - Request a URI be acquired
* 601 Configuration - Sends the configuration space
* 602 Authorization Credentials - Response to the 402 message
* 603 Media Changed - Response to the 403 message
Only the 6xx series of status codes is sent TO the method. Furthermore
the method may not emit status codes in the 6xx range. The Codes 402 and
   403 require that the method continue reading all other 6xx codes until
the proper 602/603 code is received. This means the method must be
capable of handling an unlimited number of 600 messages.
The flow of messages starts with the method sending out a 100
Capabilities and APT sending out a 601 Configuration. After that APT
begins sending 600 URI Acquire and the method sends out 200 URI Start,
201 URI Done or 400 URI Failure. No synchronization is performed, it is
   expected that APT will send 600 URI Acquire messages at -any- time and
that the method should queue the messages. This allows methods like http
to pipeline requests to the remote server. It should be noted however
that APT will buffer messages so it is not necessary for the method to be
constantly ready to receive them.
2.3. Header Fields
   The following is a short index of the header fields that are supported
URI
URI being described by the message
Filename
Location in the filesystem
Last-Modified
A time stamp in RFC1123 notation for use by IMS checks
IMS-Hit
The already existing item is valid
Size
Size of the file in bytes
Resume-Point
Location that transfer was started
MD5-Hash
Computed MD5 hash for the file
Message
String indicating some displayable message
Media
String indicating the media name required
Site
String indicating the site authorization is required for
User
Username for authorization
   Password
Password for authorization
Fail
Operation failed
Drive
Drive the media should be placed in
Config-Item
A string of the form item=value derived from the APT configuration
space. These may include method specific values and general values
not related to the method. It is up to the method to filter out the
ones it wants.
Single-Instance
Requires that only one instance of the method be run This is a yes/no
value.
Pipeline
The method is capable of pipelining.
Local
The method only returns Filename: fields.
Send-Config
Send configuration to the method.
Needs-Cleanup
The process is kept around while the files it returned are being
used. This is primarily intended for CD-ROM and File URIs that need
to unmount filesystems.
Version
Version string for the method
   This is a list of which headers each status code can use
100 Capabilities
Displays the capabilities of the method. Methods should set the
pipeline bit if their underlying protocol supports pipelining. The
only known method that does support pipelining is http. Fields:
Version, Single-Instance, Pre-Scan, Pipeline, Send-Config,
Needs-Cleanup
101 Log
A log message may be printed to the screen if debugging is enabled.
This is only for debugging the method. Fields: Message
102 Status
Message gives a progress indication for the method. It can be used to
show pre-transfer status for Internet type methods. Fields: Message
200 URI Start
Indicates the URI is starting to be transferred. The URI is specified
along with stats about the file itself. Fields: URI, Size,
Last-Modified, Resume-Point
201 URI Done
Indicates that a URI has completed being transferred. It is possible
to specify a 201 URI Done without a URI Start which would mean no
data was transferred but the file is now available. A Filename field
is specified when the URI is directly available in the local pathname
space. APT will either directly use that file or copy it into another
location. It is possible to return Alt-* fields to indicate that
another possibility for the URI has been found in the local pathname
space. This is done if a decompressed version of a .gz file is found.
Fields: URI, Size, Last-Modified, Filename, MD5-Hash
400 URI Failure
Indicates a fatal URI failure. The URI is not retrievable from this
source. As with 201 URI Done 200 URI Start is not required to precede
this message Fields: URI, Message
   401 General Failure
Indicates that some unspecific failure has occurred and the method is
unable to continue. The method should terminate after sending this
message. It is intended to check for invalid configuration options or
other severe conditions. Fields: Message
402 Authorization Required
The method requires a Username and Password pair to continue. After
sending this message the method will expect APT to send a 602
Authorization Credentials message with the required information. It
is possible for a method to send this multiple times. Fields: Site
403 Media Failure
A method that deals with multiple media requires that a new media be
inserted. The Media field contains the name of the media to be
inserted. Fields: Media, Drive
600 URI Acquire
APT is requesting that a new URI be added to the acquire list.
Last-Modified has the time stamp of the currently cache file if
applicable. Filename is the name of the file that the acquired URI
should be written to. Fields: URI, Filename Last-Modified
601 Configuration
APT is sending the configuration space to the method. A series of
Config-Item fields will be part of this message, each containing an
entry from the configuration space. Fields: Config-Item.
602 Authorization Credentials
This is sent in response to a 402 Authorization Required message. It
contains the entered username and password. Fields: Site, User,
Password
603 Media Changed
This is sent in response to a 403 Media Failure message. It indicates
that the user has changed media and it is safe to proceed. Fields:
Media, Fail
2.4. Notes
   The methods supplied by the stock apt are:
1. cdrom - For Multi-Disc CD-ROMs
2. copy - (internal) For copying files around the filesystem
   3. file - For local files
4. gzip - (internal) For decompression
5. http - For HTTP servers
The two internal methods, copy and gzip, are used by the acquire code to
parallelize and simplify the automatic decompression of package files as
well as copying package files around the file system. Both methods can be
   seen to act the same except that one decompresses on the fly. APT uses
them by generating a copy URI that is formed identically to a file URI.
The destination file is send as normal. The method then takes the file
specified by the URI and writes it to the destination file. A typical set
of operations may be:
http://foo.com/Packages.gz -> /bar/Packages.gz
   gzip:/bar/Packages.gz -> /bar/Packages.decomp
rename Packages.decomp to /final/Packages
The http method implements a fully featured HTTP/1.1 client that supports
deep pipelining and reget. It works best when coupled with an apache 1.3
server. The file method simply generates failures or success responses
   with the filename field set to the proper location. The cdrom method acts
the same except that it checks that the mount point has a valid cdrom in
it. It does this by (effectively) computing a md5 hash of 'ls -l' on the
mountpoint.
