Browse Source

move special READMEs into doc/ and format as md

The formatting is very basic and more should probably be done, but we at
least get the files out of the root directory which in case of the
various READMEs was confusing salsa which one display as (central) README.
tags/devuan/2.0.1+devuan1
David Kalnischkies 2 years ago
parent
commit
e85987ab22
10 changed files with 203 additions and 174 deletions
  1. +14
    -0
      README.md
  2. +0
    -69
      README.progress-reporting
  3. +6
    -4
      debian/apt-doc.docs
  4. +101
    -95
      doc/acquire-additional-files.md
  5. +1
    -1
      doc/apt-verbatim.ent
  6. +0
    -0
      doc/external-dependency-solver-protocol.md
  7. +0
    -0
      doc/external-installation-planner-protocol.md
  8. +0
    -0
      doc/json-hooks-protocol.md
  9. +76
    -0
      doc/progress-reporting.md
  10. +5
    -5
      doc/srv-records-support.md

+ 14
- 0
README.md View File

@@ -198,3 +198,17 @@ block the correct installation of very important packages. Which option to use d
if you are interested in the topology sorting (`Debug::pkgOrderList`), the dependency-aware
cycle and unconfigured prevention (`Debug::pkgPackageManager`) or the actual calls
to dpkg (`Debug::pkgDpkgPm`).


Additional documentation
------------------------

Many more things could and should be said about APT and its usage but are more
targeted at developers of related programs or only of special interest.

* [Protocol specification of APTs communication with external dependency solvers (EDSP)](./doc/external-dependency-solver-protocol.md)
* [Protocol specification of APTs communication with external installation planners (EIPP)](./doc/external-installation-planner-protocol.md)
* [Howto use and configure APT to acquire additional files in 'update' operations](./doc/acquire-additional-files.md)
* [Download and package installation progress reporting details](./doc/progress-reporting.md)
* [Remarks on DNS SRV record support in APT](./doc/srv-records-support.md)
* [Protocol specification of APT interfacing with external hooks via JSON](./doc/json-hooks-protocol.md)

+ 0
- 69
README.progress-reporting View File

@@ -1,69 +0,0 @@
Install-progress reporting
--------------------------

If the apt options: "APT::Status-Fd" is set, apt will send status
reports to that fd. The status information is separated with a ':',
there are the following status conditions:

status = {"pmstatus", "dlstatus", "conffile-prompt", "error", "media-change" }

The reason for using a fd instead of a OpProgress class is that many
apt front-end fork a (vte) terminal for the actual installation.

The reason to do the mapping and l10n of the dpkg states to human
readable (and translatable) strings is that this way the translation
needs to be done only once for all front-ends.


pmstatus
--------
Status of the package manager (dpkg). This is send when packages
are installed/removed.
pmstatus:pkgname:TotalPercentage:action-description

PkgName = the name of the package
TotalPercentage = the total progress between [0..100]
description = a i18ned human readable description of the current action


Example:
# ./apt-get install -o APT::Status-Fd=2 3dchess >/dev/null
pmstatus:3dchess:20:Preparing 3dchess
pmstatus:3dchess:40:Unpacking 3dchess
pmstatus:3dchess:60:Preparing to configure 3dchess
pmstatus:3dchess:80:Configuring 3dchess
pmstatus:3dchess:100:Installed 3dchess

pmerror
-------
pmerror:deb:TotalPercentage:error string

Example:
pmerror: /var/cache/apt/archives/krecipes_0.8.1-0ubuntu1_i386.deb : 75% : trying to overwrite `/usr/share/doc/kde/HTML/en/krecipes/krectip.png', which is also in package krecipes-data


pmconffile
----------
pmconffile:conffile:percent:'current-conffile' 'new-conffile' useredited distedited


media-change
------------
media-change:medium:drive:human-readable string

example:
media-change: Ubuntu 5.10 _Breezy Badger_ - Alpha i386 (20050830):/cdrom/:Please insert the disc labeled: 'Ubuntu 5.10 _Breezy Badger_ - Alpha i386 (20050830)' in the drive '/cdrom/' and press enter.


dlstatus
--------
dlstatus:AlreadDownloaded:TotalPercentage:action-description

AlreadyDownloaded = the number of already downloaded packages
TotalPercentage = the total progress between [0..100]
description = a i18ned human readable description of the current action

Example:
dlstatus:1:9.05654:Downloading file 1 of 3 (4m40s remaining)
dlstatus:1:9.46357:Downloading file 1 of 3 (4m39s remaining)
dlstatus:1:9.61022:Downloading file 1 of 3 (4m38s remaining)

+ 6
- 4
debian/apt-doc.docs View File

@@ -1,4 +1,6 @@
README.progress-reporting
doc/acquire-additional-files.txt
doc/external-dependency-solver-protocol.txt
doc/external-installation-planner-protocol.txt
doc/acquire-additional-files.md
doc/external-dependency-solver-protocol.md
doc/external-installation-planner-protocol.md
doc/json-hooks-protocol.md
doc/progress-reporting.md
doc/srv-records-support.md

doc/acquire-additional-files.txt → doc/acquire-additional-files.md View File

@@ -7,12 +7,12 @@ process which is hard to implement correctly and securely.
APT front-ends share the code and binaries to make this happen in libapt
with the Acquire system, supported by helpers shipped in the apt package
itself and additional transports in individual packages like
apt-transport-https.
`apt-transport-https`.

For its own operation libapt needs or can make use of Packages, Sources
and Translation-* files, which it will acquire by default, but
a repository might contain more data files (e.g. Contents) a front-end
(e.g. apt-file) might want to use and would therefore need to be
For its own operation libapt needs or can make use of *Packages*, *Sources*
and *Translation-* files, which it will acquire by default, but
a repository might contain more data files (e.g. `Contents`) a front-end
(e.g. `apt-file`) might want to use and would therefore need to be
downloaded as well.

This file describes the configuration scheme such a front-end can use to
@@ -23,8 +23,8 @@ instruct the Acquire system to download those additional files.
The Acquire system uses the same configuration settings to implement the
files it downloads by default. These settings are the default, but if
they would be written in a configuration file the configuration
instructing the Acquire system to download the Packages files would look
like this (see also apt.conf(5) manpage for configuration file syntax):
instructing the Acquire system to download the *Packages* files would look
like this (see also `apt.conf(5)` manpage for configuration file syntax):

Acquire::IndexTargets::deb::Packages {
MetaKey "$(COMPONENT)/binary-$(ARCHITECTURE)/Packages";
@@ -37,72 +37,74 @@ like this (see also apt.conf(5) manpage for configuration file syntax):
Optional "no";
};

All files which should be downloaded (nicknamed 'Targets') are mentioned
below the Acquire::IndexTargets scope. 'deb' is here the type of the
sources.list entry the file should be acquired for. The only other
supported value is hence 'deb-src'. Beware: You can't specify multiple
types here and you can't download the same (evaluated) MetaKey from
All files which should be downloaded (nicknamed *Targets*) are mentioned
below the `Acquire::IndexTargets` scope. `deb` is here the type of the
`sources.list` entry the file should be acquired for. The only other
supported value is hence `deb-src`. Beware: You can't specify multiple
types here and you can't download the same (evaluated) `MetaKey` from
multiple types!

After the type you can pick any valid and unique string which preferable
refers to the file it downloads (In the example we picked 'Packages').
refers to the file it downloads (In the example we picked *Packages*).
This string is used as identifier (if not explicitly set otherwise) for
the target class and accessible as 'Identifier' and 'Created-By' e.g.
in the "apt-get indextargets" output as detailed below. The identifier
the target class and accessible as `Identifier` and `Created-By` e.g.
in the `apt-get indextargets` output as detailed below. The identifier
is also used to allow user to enable/disable targets per sources.list
entry.

All targets have three main properties you can define:
* MetaKey: The identifier of the file to be downloaded as used in the

* `MetaKey`: The identifier of the file to be downloaded as used in the
Release file. It is also the relative location of the file from the
Release file. You can neither download from a different server
entirely (absolute URI) nor access directories above the Release file
(e.g. "../../").
* ShortDescription: Very short string intended to be displayed to the
* `ShortDescription`: Very short string intended to be displayed to the
user e.g. while reporting progress. apt will e.g. use this string in
the last line to indicate progress of e.g. the download of a specific
item.
* Description: A preferable human understandable and readable identifier
* `Description`: A preferable human understandable and readable identifier
of which file is acquired exactly. Mainly used for progress reporting
and error messages. apt will e.g. use this string in the Get/Hit/Err
progress lines.
An identifier of the site accessed as seen in the sources.list (e.g.
"http://example.org/debian" or "file:/path/to/a/repository") is
`http://example.org/debian` or `file:/path/to/a/repository`) is
automatically prefixed for this property.


Additional optional properties:
* Identifier: The default value is the unique string identifying this
file (in the example above it was 'Packages') also accessible as
Created-By. The difference is that using this property multiple files

* `Identifier`: The default value is the unique string identifying this
file (in the example above it was *Packages*) also accessible as
`Created-By`. The difference is that using this property multiple files
can be subsumed under one identifier e.g. if you configure multiple
possible locations for the files (with Fallback-Of), but the front-end
possible locations for the files (with `Fallback-Of`), but the front-end
doesn't need to handle files from the different locations differently.
* DefaultEnabled: The default value is 'yes' which means that apt will
try to acquire this target from all sources. If set to 'no' the user
* `DefaultEnabled`: The default value is `yes` which means that apt will
try to acquire this target from all sources. If set to `no` the user
has to explicitly enable this target in the sources.list file with the
Targets option(s) – or override this value in a config file.
* Optional: The default value is 'yes' and should be kept at this value.
`Targets` option(s) – or override this value in a config file.
* `Optional`: The default value is `yes` and should be kept at this value.
If enabled the acquire system will skip the download if the file isn't
mentioned in the Release file. Otherwise this is treated as a hard
mentioned in the `Release` file. Otherwise this is treated as a hard
error and the update process fails. Note that failures while
downloading (e.g. 404 or hash verification errors) are failures,
regardless of this setting.
* KeepCompressed: The default is the value of Acquire::GzipIndexes,
which defaults to false. If true, the acquire system will keep the
* `KeepCompressed`: The default is the value of `Acquire::GzipIndexes`,
which defaults to `false`. If `true`, the acquire system will keep the
file compressed on disk rather than extract it. If your front-end can't
deal with compressed files transparently you have to explicitly set
this option to false to avoid problems with users setting the option
globally. On the other hand, if you set it to true or don't set it you
this option to `false` to avoid problems with users setting the option
globally. On the other hand, if you set it to `true` or don't set it you
have to ensure your front-end can deal with all compressed fileformats
supported by apt (libapt users can e.g. use FileFd, others can use
the cat-file command of /usr/lib/apt/apt-helper).
* Fallback-Of: Is by default not set. If it is set and specifies another
target name (see Created-By) which was found in the Release file the
supported by apt (libapt users can e.g. use `FileFd`, others can use
the `cat-file` command of `/usr/lib/apt/apt-helper`).
* `Fallback-Of`: Is by default not set. If it is set and specifies another
target name (see `Created-By`) which was found in the *Release* file the
download of this target will be skipped. This can be used to implement
fallback(chain)s to allow transitions like the rename of target files.
The behavior if cycles are formed with Fallback-Of is undefined!
* flat{MetaKey,Description}: APT supports two types of repositories:
* `flatMetaKey`, `flatDescription`: APT supports two types of repositories:
dists-style repositories which are the default and by far the most
common which are named after the fact that the files are in an
elaborated directory structure. In contrast a flat-style repository
@@ -116,27 +118,28 @@ file if it is available and uncompress it for you, just as it will also
use PDiff patching if provided by the repository and enabled by the
user. You only have to ensure that the Release file contains the
information about the compressed files/PDiffs to make this happen.
*NO* properties have to be set to enable this!
**NO** properties have to be set to enable this!


More properties exist, but these should *NOT* be set by front-ends
More properties exist, but these should **NOT** be set by front-ends
requesting files. They exist for internal and end-user usage only.
Some of these are – which are documented here only to ensure that they
aren't accidentally used by front-ends:
* PDiffs: controls if apt will try to use PDiffs for this target.
Defaults to the value of Acquire::PDiffs which is true by default.

* `PDiffs`: controls if apt will try to use PDiffs for this target.
Defaults to the value of `Acquire::PDiffs` which is *true* by default.
Can be overridden per-source by the sources.list option of the same
name. See the documentation for both of these for details.
* By-Hash: controls if apt will try to use an URI constructed from
* `By-Hash`: controls if apt will try to use an URI constructed from
a hashsum of the file to download. See the documentation for config
option Acquire::By-Hash and sources.list option By-Hash for details.
* CompressionTypes: The default value is a space separated list of
compression types supported by apt (see Acquire::CompressionTypes).
option `Acquire::By-Hash` and sources.list option `By-Hash` for details.
* `CompressionTypes`: The default value is a space separated list of
compression types supported by apt (see `Acquire::CompressionTypes`).
You can set this option to prevent apt from downloading a compression
type a front-end can't open transparently. This should always be
a temporary workaround through and a bug should be reported against
the front-end in question.
* KeepCompressedAs: The default value is a space separated list of
* `KeepCompressedAs`: The default value is a space separated list of
compression types supported by apt (see previous option) which is
sorted by the cost-value of the compression in ascending order,
except that cost=0 "compressions" (like uncompressed) are listed last.
@@ -144,7 +147,7 @@ aren't accidentally used by front-ends:

# More examples

The stanzas for Translation-* files as well as for Sources files would
The stanzas for `Translation-*` files as well as for `Sources` files would
look like this:

Acquire::IndexTargets {
@@ -176,20 +179,20 @@ by the acquire system. The following variables are known; note that
unknown variables have no default value nor are they touched: They are
printed as-is.

* $(RELEASE): This is usually an archive- or codename, e.g. "stable" or
"stretch". Note that flat-style repositories do not have an archive-
* `$(RELEASE)`: This is usually an archive- or codename, e.g. *stable* or
*stretch*. Note that flat-style repositories do not have an archive-
or codename per-se, so the value might very well be just "/" or so.
* $(COMPONENT): as given in the sources.list, e.g. "main", "non-free" or
"universe". Note that flat-style repositories again do not really
* `$(COMPONENT)`: as given in the sources.list, e.g. *main*, *non-free* or
*universe*. Note that flat-style repositories again do not really
have a meaningful value here.
* $(LANGUAGE): Values are all entries (expect "none") of configuration
option Acquire::Languages, e.g. "en", "de" or "de_AT".
* $(ARCHITECTURE): Values are all entries of configuration option
APT::Architectures (potentially modified by sources.list options),
e.g. "amd64", "i386" or "armel" for the 'deb' type. In type 'deb-src'
this variable has the value "source".
* $(NATIVE_ARCHITECTURE): The architecture apt treats as the native
architecture for this system configured as APT::Architecture
* `$(LANGUAGE)`: Values are all entries (expect *none*) of configuration
option `Acquire::Languages`, e.g. *en*, *de* or *de_AT*.
* `$(ARCHITECTURE)`: Values are all entries of configuration option
`APT::Architectures` (potentially modified by sources.list options),
e.g. *amd64*, *i386* or *armel* for the *deb* type. In type *deb-src*
this variable has the value *source*.
* `$(NATIVE_ARCHITECTURE)`: The architecture apt treats as the native
architecture for this system configured as `APT::Architecture`
defaulting to the architecture apt itself was built for.

Note that while more variables might exist in the implementation, these
@@ -198,73 +201,76 @@ you have a need for other variables contact us.

# Accessing files

Do NOT hardcode specific file locations, names or compression types in
Do **NOT** hardcode specific file locations, names or compression types in
your application! You will notice that the configuration options give
you no choice over where the downloaded files will be stored. This is by
design so multiple applications can download and use the same file
rather than each and every one of them potentially downloads and uses
its own copy somewhere on disk.

"apt-get indextargets" can be used to get the location as well as other
information about all files downloaded (aka: you will see Packages,
Sources and Translation-* files here as well). Provide a line of the
`apt-get indextargets` can be used to get the location as well as other
information about all files downloaded (aka: you will see *Packages*,
*Sources* and *Translation-* files here as well). Provide a line of the
default output format as parameter to filter out all entries which do
not have such a line. With --format, you can further more define your
not have such a line. With `--format`, you can further more define your
own output style. The variables are what you see in the output, just all
uppercase and wrapped in $(), as in the configuration file.
uppercase and wrapped in `$()`, as in the configuration file.

To get all the filenames of all *Translation-en* files you can e.g. call:

To get all the filenames of all Translation-en files you can e.g. call:
apt-get indextargets --format '$(FILENAME)' "Identifier: Translations" "Language: en"

The line-based filtering and the formatting is rather crude and feature-
less by design: The default format is Debian's standard format deb822
less by design: The default format is Debian's standard format `deb822`
(in particular: Field names are case-insensitive and the order of fields
in the stanza is undefined), so instead of apt reimplementing powerful
filters and formatting for this command, it is recommend to use piping
and dedicated tools like 'grep-dctrl' if you need more than the basics
and dedicated tools like `grep-dctrl` if you need more than the basics
provided.

Accessing this information via libapt is done by reading the
sources.lists (pkgSourceList), iterating over the metaIndex objects this
creates and calling GetIndexTargets() on them. See the source code of
"apt-get indextargets" for a complete example.
sources.lists (`pkgSourceList`), iterating over the metaIndex objects this
creates and calling `GetIndexTargets()` on them. See the source code of
`apt-get indextargets` for a complete example.

Note that by default targets are not listed if they weren't downloaded.
If you want to see all targets, you can use the --no-release-info, which
also removes the Codename, Suite, Version, Origin, Label and Trusted
If you want to see all targets, you can use the `--no-release-info`, which
also removes the *Codename*, *Suite*, *Version*, *Origin*, *Label* and *Trusted*
fields from the output as these also display data which needs to be
downloaded first and could hence be inaccurate [on the pro-side: This
mode is faster as it doesn't require a valid binary cache to operate].
The most notable difference perhaps is in the Filename field through: By
The most notable difference perhaps is in the *Filename* field through: By
default it indicates an existing file, potentially compressed (Hint:
libapt users can use FileFd to open compressed files transparently). In
the --no-release-info mode the indicated file doesn't need to exist and
libapt users can use `FileFd` to open compressed files transparently). In
the `--no-release-info` mode the indicated file doesn't need to exist and
it will always refer to an uncompressed file, even if the index would be
(or is) stored compressed.

Remarks on fields only available in (default) --release-info mode:
* Trusted: Denotes with a 'yes' or 'no' if the data in this file is
Remarks on fields only available in (default) `--release-info mode`:

* `Trusted`: Denotes with a *yes* or *no* if the data in this file is
authenticated by a trust chain rooted in a trusted gpg key. You should
be careful with untrusted data and warn the user if you use it.
* Codename, Suite, Version, Origin and Label are fields from the Release
file, are only present if they are present in the Release file and
contain the same data.
* `Codename`, `Suite`, `Version`, `Origin` and `Label` are fields from the
*Release* file, are only present if they are present in the *Release* file
and contain the same data.

Remarks on other available fields:
* MetaKey, ShortDesc, Description, Site, Release: as defined

* `MetaKey`, `ShortDesc`, `Description`, `Site`, `Release`: as defined
by the configuration and described further above.
* Identifier: Defaults to the value of Created-By, but can be set
* `Identifier`: Defaults to the value of `Created-By`, but can be set
explicitly in the configuration (see above). Prefer this field over
Created-By to subsume multiple file(location)s (see Fallback-Of).
* Created-By: configuration entity responsible for this target
* Target-Of: type of the sources.list entry
* URI, Repo-URI: avoid using. Contains potentially username/password.
Prefer 'Site', especially for display.
* Optional, DefaultEnabled, KeepCompressed: Decode the options of the
`Created-By` to subsume multiple file(location)s (see `Fallback-Of`).
* `Created-By`: configuration entity responsible for this target
* `Target-Of`: type of the sources.list entry
* `URI`, `Repo-URI`: avoid using. Contains potentially username/password.
Prefer `Site`, especially for display.
* `Optional`, `DefaultEnabled`, `KeepCompressed`: Decode the options of the
same name from the configuration.
* Language, Architecture, Component: as defined further above, but with
* `Language`, `Architecture`, `Component`: as defined further above, but with
the catch that they might be missing if they don't effect the target
(aka: They weren't used while evaluating the MetaKey template).
(aka: They weren't used while evaluating the `MetaKey` template).

Again, additional fields might be visible in certain implementations,
but you should avoid using them and instead talk to us about a portable
@@ -275,7 +281,7 @@ implementation.
It is highly encouraged that applications talk to each other and to us
about which files they require. It is usually best to have a common
package ship the configuration needed to get the files, but specific
needs might require specific solutions. Again: talk to us.
needs might require specific solutions. Again: **talk to us**.

Bad things will happen if multiple front-ends request the same file(s)
via different targets, which is another reason why coordination is very
@@ -309,10 +315,10 @@ You can't modify the files apt has downloaded as apt keeps state with
e.g. the modification times of the files and advanced features like
PDiffs break.

You can however install an APT::Update::Post-Invoke{-Success,} hook
You can however install an `APT::Update::Post-Invoke{-Success,}` hook
script and use them to copy (modified) files to a different location.
Use 'apt-get indextargets' (or similar) to get the filenames – do not
look into /var/lib/apt/lists directly!
Use `apt-get indextargets` (or similar) to get the filenames – do not
look into `/var/lib/apt/lists` directly!

Please avoid time consuming calculations in the scripts and instead just
trigger a background task as there is little to no feedback for the user

+ 1
- 1
doc/apt-verbatim.ent View File

@@ -225,7 +225,7 @@
</citerefentry>"
>

<!ENTITY apt-acquire-additional-files "<literal><filename>/usr/share/doc/apt-doc/acquire-additional-files.txt</filename></literal>">
<!ENTITY apt-acquire-additional-files "<literal><filename>/usr/share/doc/apt-doc/acquire-additional-files.md.gz</filename></literal>">

<!-- Boiler plate docinfo section -->
<!ENTITY apt-email "


doc/external-dependency-solver-protocol.txt → doc/external-dependency-solver-protocol.md View File


doc/external-installation-planner-protocol.txt → doc/external-installation-planner-protocol.md View File


README.json-hooks.md → doc/json-hooks-protocol.md View File


+ 76
- 0
doc/progress-reporting.md View File

@@ -0,0 +1,76 @@
Install-progress reporting
--------------------------

If the apt options `APT::Status-Fd` is set, apt will send status
reports to that fd. The status information is separated with a '`:`',
there are the following status conditions:

* pmstatus
* dlstatus
* conffile-prompt
* error
* media-change

The reason for using a fd instead of an OpProgress class is that many
apt front-end fork a (vte) terminal for the actual installation.

The reason to do the mapping and l10n of the dpkg states to human
readable (and translatable) strings is that this way the translation
needs to be done only once for all front-ends.


pmstatus
--------
Status of the package manager (dpkg). This is send when packages
are installed/removed.
pmstatus:pkgname:TotalPercentage:action-description

* PkgName = the name of the package
* TotalPercentage = the total progress between [0..100]
* description = a i18ned human readable description of the current action

Example:

# ./apt-get install -o APT::Status-Fd=2 3dchess >/dev/null
pmstatus:3dchess:20:Preparing 3dchess
pmstatus:3dchess:40:Unpacking 3dchess
pmstatus:3dchess:60:Preparing to configure 3dchess
pmstatus:3dchess:80:Configuring 3dchess
pmstatus:3dchess:100:Installed 3dchess

pmerror
-------
pmerror:deb:TotalPercentage:error string

Example:

pmerror: /var/cache/apt/archives/krecipes_0.8.1-0ubuntu1_i386.deb : 75% : trying to overwrite `/usr/share/doc/kde/HTML/en/krecipes/krectip.png', which is also in package krecipes-data


pmconffile
----------
pmconffile:conffile:percent:'current-conffile' 'new-conffile' useredited distedited


media-change
------------
media-change:medium:drive:human-readable string

Example:

media-change: Ubuntu 5.10 _Breezy Badger_ - Alpha i386 (20050830):/cdrom/:Please insert the disc labeled: 'Ubuntu 5.10 _Breezy Badger_ - Alpha i386 (20050830)' in the drive '/cdrom/' and press enter.


dlstatus
--------
dlstatus:AlreadDownloaded:TotalPercentage:action-description

* AlreadyDownloaded = the number of already downloaded packages
* TotalPercentage = the total progress between [0..100]
* description = a i18ned human readable description of the current action

Example:

dlstatus:1:9.05654:Downloading file 1 of 3 (4m40s remaining)
dlstatus:1:9.46357:Downloading file 1 of 3 (4m39s remaining)
dlstatus:1:9.61022:Downloading file 1 of 3 (4m38s remaining)

README.SrvRecords → doc/srv-records-support.md View File

@@ -2,17 +2,17 @@ DNS SRV record support in apt
=============================

Apt supports a subset of the DNS SRV server records protocol as
described in RFC 2782 for service discovery.
described in [RFC 2782](https://tools.ietf.org/html/rfc2782) for service discovery.

Before connecting to the requested server APT will send a SRV
record request of the form "_$protocol._tcp._$host", e.g.
"_http._tcp.ftp.debian.org" or "_http._tcp.security.debian.org".
record request of the form `_$protocol._tcp._$host`, e.g.
`_http._tcp.ftp.debian.org` or `_http._tcp.security.debian.org`.

If the server sends SRV records
as a reply APT will use those to connect to the server(s) in
this reply. It will honor the "priority" field in the reply.
this reply. It will honor the `priority` field in the reply.

However it does not implement the "weight" algorithm as described
However it does not implement the `weight` algorithm as described
in RFC 2782. It will use an equal weight for each server of the
same priority.


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