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APT Files
Chapter 2 Files

2.1 Distribution Source list (sources.list)

The distribution source list is used to locate archives of the debian distribution. It is designed to support any number of active sources and to support a mix of source media. The file lists one source per line, with the fastest source listed first. The format of each line is:

type uri args

The first item, type, indicates the format for the remainder of the line. It is designed to indicate the structure of the distribution the line is talking about. Currently the only defined value is deb which indicates a standard debian archive with a dists dir.

2.1.1 The deb Type

The deb type is to be a typical two level debian distributions, dist/distribution/component. Typically distribution is one of stable, unstable or frozen while component is one of main, contrib, non-free or non-us. The format for the deb line is as follows:

deb uri distribution component [component ...]

uri for the deb type must specify the base of the debian distribution. APT will automatically generate the proper longer URIs to get the information it needs. distribution can specify an exact path, in this case the components must be omitted and distribution must end in a slash.

Since only one distribution can be specified per deb line it may be necessary to list a number of deb lines for the same URI. APT will sort the URI list after it has generated a complete set to allow connection reuse. It is important to order things in the sourcelist from most preferred to least preferred (fastest to slowest).

2.1.2 URI specification

URIs in the source list support a large number of access schemes.

cdrom

The cdrom scheme is special in that If Modified Since queries are never performed and that APT knows how to match a cdrom to the name it was given when first inserted. APT also knows all of the possible mount points the cdrom drives and that the user should be prompted to insert a CD if it cannot be found. The path is relative to an arbitrary mount point (of APT's choosing) and must not start with a slash. The first pathname component is the given name and is purely descriptive and of the users choice. However, if a file in the root of the cdrom is called '.disk/info' its contents will be used instead of prompting. The name serves as a tag for the cdrom and should be unique.

        cdrom:Debian 1.3/debian

http

This scheme specifies a HTTP server for the debian archive. HTTP is preferred over FTP because If Modified Since queries against the Package file are possible as well as deep pipelining and resume capabilities.

        http://www.debian.org/archive

ftp

This scheme specifies a FTP connection to the server. FTP is limited because there is no support for IMS and is hard to proxy over firewalls.

        ftp://ftp.debian.org/debian

file

The file scheme allows an arbitrary directory in the file system to be considered as a debian archive. This is useful for NFS mounts and local mirrors/archives.

        file:/var/debian

smb

A possible future expansion may be to have direct support for smb (Samba servers).

        smb://ftp.kernel.org/pub/mirrors/debian

2.1.3 Hashing the URI

All permanent information acquired from any of the sources is stored in the lists directory. Thus, there must be a way to relate the filename in the lists directory to a line in the sourcelist. To simplify things this is done by quoting the URI and treating _'s as quoteable characters and converting / to _. The URI spec says this is done by converting a sensitive character into %xx where xx is the hexadecimal representation from the ASCII character set. Examples:

     http://www.debian.org/archive/dists/stable/binary-i386/Packages 
     /var/lib/apt/lists/www.debian.org_archive_dists_stable_binary-i386_Packages
     
     cdrom:Debian 1.3/debian/Packages
     /var/lib/apt/info/Debian%201.3_debian_Packages

The other alternative that was considered was to use a deep directory structure but this poses two problems, it makes it very difficult to prune directories back when sources are no longer used and complicates the handling of the partial directory. This gives a very simple way to deal with all of the situations that can arise. Also note that the same rules described in the Archive Directory section regarding the partial sub dir apply here as well.

2.2 Extra Status File (xstatus)

The extra status file serves the same purpose as the normal dpkg status file (/var/lib/dpkg/status) except that it stores information unique to apt. This includes the autoflag, target distribution and version and any other unique features that come up over time. It duplicates nothing from the normal dpkg status file. Please see other APT documentation for a discussion of the exact internal behaviour of these fields. The Package field is placed directly before the new fields to indicate which package they apply to. The new fields are as follows:

X-Auto

The Auto flag can be Yes or No and controls whether the package is in auto mode.

X-TargetDist

The TargetDist item indicates which distribution versions are offered for installation from. It should be stable, unstable or frozen.

X-TargetVersion

The target version item is set if the user selects a specific version, it overrides the TargetDist selection if both are present.

2.3 Binary Package Cache (pkgcache.bin)

Please see cache.sgml for a complete description of what this file is. The cache file is updated whenever the contents of the lists directory changes. If the cache is erased, corrupted or of a non-matching version it will be automatically rebuilt by all of the tools that need it. srcpkgcache.bin contains a cache of all of the package files in the source list. This allows regeneration of the cache when the status files change to use a prebuilt version for greater speed.

2.4 Downloads Directory (archives)

The archives directory is where all downloaded .deb archives go. When the file transfer is initiated the deb is placed in partial. Once the file is fully downloaded and its MD5 hash and size are verified it is moved from partial into archives/. Any files found in archives/ can be assumed to be verified.

No directory structure is transfered from the receiving site and all .deb file names conform to debian conventions. No short (msdos) filename should be placed in archives. If the need arises .debs should be unpacked, scanned and renamed to their correct internal names. This is mostly to prevent file name conflicts but other programs may depend on this if convenient. A conforming .deb is one of the form, name_version_arch.deb. Our archive scripts do not handle epochs, but they are necessary and should be re-inserted. If necessary _'s and :'s in the fields should be quoted using the % convention. It must be possible to extract all 3 fields by examining the file name. Downloaded .debs must be found in one of the package lists with an exact name + version match..

2.5 The Methods Directory (/usr/lib/apt/methods)

The Methods directory is more fully described in the APT Methods interface document.

2.6 The Mirror List

The mirror list is stored on the primary debian web server (www.debian.org) and contains a machine readable list of all known debian mirrors. It's format and style mirror the Package file.

Site

This is the proper host name of the site. It should not be a host within debian.org and generally cnames should be avoided here.

Aliases

These list any commonly used aliases for the site. This field is used to make sure that a site is not added twice.

Type

This field can either be Push-Primary or leaf. Push-Primary are authorized top level mirrors of the archive, all other mirrors are leaf.

Archive-[access]

The Archive field gives the path(s) to the debian archive. [access] specifies the access method and may be one of ftp, http, rsync, nfs, or smb. For many of the types it is possible to prefix the path with :### indicating that an alternate port should be used. Generally paths start with a / and end with a /, rsync is an exception in that the first directory component is not a path but a label.

WWW-[access]

The WWW field gives the path(s) to the debian web site.

CDImage-[access]

The WWW field gives the path(s) to the debian CD-ROM images

Incoming-[access]

The Incoming field gives the path(s) to a mirror of the debian incoming directory.

nonUS-[access]

The nonUS field gives the path(s) to a mirror of the non-US distribution.

Maintainer

This is the email address of the maintainer of the mirror.

Location

Location gives the general geographical region the mirror is in.

Sponsor

The Sponsor field indicates who owns the mirror and a URL to a web page describing the organization.

Comment

General free-form text.

Some form of network measurement will have to be used to gauge performance of each of the mirrors. This will be discussed later, initial versions will use the first found URI.

2.7 The Release File

This file plays and important role in how APT presents the archive to the user. Its main purpose is to present a descriptive name for the source of each version of each package. It also is used to detect when new versions of debian are released. It augments the package file it is associated with by providing meta information about the entire archive which the Packages file describes.

The full name of the distribution for presentation to the user is formed as 'label version archive', with a possible extended name being 'label version archive component'.

The file is formed as the package file (RFC-822) with the following tags defined:

Archive

This is the common name we give our archives, such as stable or unstable.

Component

Refers to the sub-component of the archive, main, contrib etc. Component may be omitted if there are no components for this archive.

Version

This is a version string with the same properties as in the Packages file. It represents the release level of the archive.

Origin

This specifies who is providing this archive. In the case of Debian the string will read 'Debian'. Other providers may use their own string

Label

This carries the encompassing name of the distribution. For Debian proper this field reads 'Debian'. For derived distributions it should contain their proper name.

Architecture

When the archive has packages for a single architecture then the Architecture is listed here. If a mixed set of systems are represented then this should contain the keyword mixed.

NotAutomatic

A Yes/No flag indicating that the archive is extremely unstable and its version's should never be automatically selected. This is to be used by experimental.

Description

Description is used to describe the release. For instance experimental would contain a warning that the packages have problems.

The location of the Release file in the archive is very important, it must be located in the same location as the packages file so that it can be located in all situations. The following is an example for the current stable release, 1.3.1r6

     Archive: stable
     Component: main
     Version: 1.3.1r6
     Origin: Debian
     Label: Debian
     Architecture: i386

This is an example of experimental,

     Archive: experimental
     Version: 0
     Origin: Debian
     Label: Debian
     Architecture: mixed
     NotAutomatic: Yes

And unstable,

     Archive: unstable
     Component: main
     Version: 2.1
     Origin: Debian
     Label: Debian
     Architecture: i386