# APT Method Interface

Jason Gunthorpe
jgg@debian.org
Version &apt-product-version;

This document describes the interface that APT uses to the archive access methods.

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.

## Introduction

### 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

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.

### 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.

## Specification

### 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.

### 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

- 351 Aux Request - Method requests an auxiliary file

- 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 351, 402 and 403 require that the method continue reading all other 6xx codes until the proper 600/602/603 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.

### 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, Local-Only, Pipeline, Send-Config, Needs-Cleanup, Removable, AuxRequests, Send-URI-Encoded

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

351 Aux Request

  Indicates a request for an auxiliary file to be downloaded by the acquire system (via another method) and made available for the requesting method. The requestor will get a 600 URI Acquire with the URI it requested and the filename will either be an existing file if the request was success or if the acquire failed for some reason the file will not exist. Fields: URI (of the file causing the need for the auxiliary file), MaximumSize, Aux-ShortDesc, Aux-Description, Aux-URI

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

### 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.