# Configuring apt

One of the tools used to install packages on a &debian-gnu; system is the program `apt`, from the `apt` package

Note that the program which actually installs the packages is called `dpkg`. However, this program is more of a low-level tool. `apt` is a higher-level tool, which will invoke `dpkg` as appropriate. It knows how to retrieve packages from your installation media, the network, or wherever. It is also able to automatically install other packages which are required to make the package you're trying to install work correctly.

. Other front-ends for package management, like `aptitude` and `synaptic`, are also in use. These front-ends are recommended for new users, since they integrate some additional features (package searching and status checks) in a nice user interface.

`apt` must be configured so that it knows from where to retrieve packages. The results of this configuration are written to the file `/etc/apt/sources.list`. You can examine and edit this file to your liking after the installation is complete.

If you are installing at default priority, the installer will largely take care of the configuration automatically, based on the installation method you are using and possibly using choices made earlier in the installation. In most cases the installer will automatically add a security mirror and, if you are installing the stable distribution, a mirror for the stable-updates service.

If you are installing at a lower priority (e.g. in expert mode), you will be able to make more decisions yourself. You can choose whether or not to use the security and/or stable-updates services, and you can choose to add packages from the contrib, non-free, and non-free-firmware sections of the archive.

## Installing from more than one DVD image

If you are installing from a DVD image that is part of a larger set, the installer will ask if you want to scan additional installation media. If you have such additional media available, you probably want to do this so the installer can use the packages included on them.

If you do not have any additional media, that is no problem: using them is not required. If you also do not use a network mirror (as explained in the next section), it can mean that not all packages belonging to the tasks you select in the next step of the installation can be installed.

Packages are included on DVD images in the order of their popularity. This means that for most uses only the first image of a set is needed and that only very few people actually use any of the packages included on the last images of a set.

It also means that buying or downloading and burning a full DVD set is just a waste of money as you'll never use most of them. In most cases you are better off getting only the first 1 to 2 DVDs and installing any additional packages you may need from the Internet by using a mirror.

If you do scan multiple installation media, the installer will prompt you to exchange them when it needs packages from one that isn't currently in the drive. Note that only discs that belong to the same set should be scanned. The order in which they are scanned does not really matter, but scanning them in ascending order will reduce the chance of mistakes.

## Using a network mirror

One question that will be asked during most installs is whether or not to use a network mirror as a source for packages. In most cases the default answer should be fine, but there are some exceptions.

If you are not installing from a DVD image, you really should use a network mirror as otherwise you will end up with only a very minimal system. However, if you have a limited Internet connection it is best not to select the `desktop` task in the next step of the installation.

If you are installing from DVD, any packages needed during the installation should be present on the first DVD image. Use of a network mirror is optional.

One advantage of adding a network mirror is that updates, that have occurred since the DVD images were created and have been included in a point release, will become available for installation, thus extending the life of your DVD set without compromising the security or stability of the installed system.

In summary: selecting a network mirror is generally a good idea, except if you do not have a good Internet connection. If the current version of a package is available from installation media, the installer will always use that. The amount of data that will be downloaded if you do select a mirror thus depends on

1. the tasks you select in the next step of the installation,

2. which packages are needed for those tasks,

3. which of those packages are present on the installation media you have scanned, and

4. whether any updated versions of packages included on the installation media are available from a mirror (either a regular package mirror, or a mirror for security or stable-updates).

Note that the last point means that, even if you choose not to use a network mirror, some packages may still be downloaded from the Internet if there is a security or stable-updates update available for them and those services have been configured.

## Choosing a network mirror

Unless you chose not to use a network mirror, you will be presented with a list of network mirrors based upon your country selection earlier in the installation process. Choosing the offered default is usually fine.

The offered default is deb.debian.org, which is not a mirror itself but will redirect to a mirror that should be up-to-date and fast. These mirrors support TLS (https protocol) and IPv6. This service is maintained by the Debian System Administration (DSA) team.

A mirror can also be specified by hand by choosing enter information manually . You can then specify a mirror host name and an optional port number. This actually has to be a URL base, i.e. when specifying an IPv6 address, one has to add square brackets around it, for instance [2001:db8::1].

If your computer is on an IPv6-only network (which is probably not the case for the vast majority of users), using the default mirror for your country might not work. All the mirrors in the list are reachable via IPv4, but only some of them can be used via IPv6. As connectivity of individual mirrors can change over time, this information is not available in the installer. If there is no IPv6 connectivity for the default mirror for your country, you can either try some of the other mirrors offered to you or choose the enter information manually option. You can then specify ftp.ipv6.debian.org as the mirror name, which is an alias for a mirror available via IPv6, although it will probably not be the fastest possible one.