Multiarch lets you install packages from multiple architectures on the same machine. This is useful in various ways, but the most common is installing both 64 and 32-bit software on the same machine and having dependencies correctly resolved automatically.

Concepts

There is a current machine architecture, as printed by dpkg --print-architecture. It is built-in to the currently installed dpkg package.

Packages are now specified as 'Package:Architecture' pretty-much anywhere that was previously just 'Package', so we have libc:i386 and lib:amd64. The bare name 'package' refers to the current machine architecture.

Other available architectures are shown by dpkg --print-foreign-architectures. dpkg will manage packages for these architectures as well as the machine architecture.

There is a 'Multi-Arch' header in the package metadata of any multiarch-aware package.

Existing packages work fine in a multiarch environment, just as before, but to gain the benefits of co-installation or cross-architecture dependencies, many packages need to be made 'multiarch-aware'.

Packages marked 'Multi-Arch : allowed also exist which can be treated as either :same or :foreign depending on how they are depended-on. See the multiarch spec and implementationhowto for details of this.

Availability

You need a multiarch-aware dpkg and apt.

check dpkg support is present with 

dpkg --assert-multiarch

In Debian this is present since 1.16.2. In Ubuntu this is present since natty (v1.16.0)

Apt is multiarch-aware if it supports -o APT::Architectures. This is available from version 0.8.13 onwards. However there are many multiarch-related improvements and bug-fixes in later apt versions, such as apt-get build-dep -a cross-dependency support, so the later the better in general up to at least 0.9.4.

Usage

Configuring architectures

To add an extra parchitecture: 

dpkg --add-architecture <arch>

e.g. 

dpkg-architecture --add-architecture armhf

To remove an achitecture 

dpkg --remove-architecture <arch>

dpkg architectures are stored in /var/lib/dpkg/arch.

setting up apt sources

Apt defaults to using the set of architectures reported by dpkg, and any unqualified architecture deb lines in /etc/apt/sources.list, which is usually what you wanted. This can be overridden using APT::Architecture=<arch> to set the defalt architecture or APT::Architectures="<arch> <arch>".

apt-sources can be architecture qualified with this syntax. This is very useful on Ubuntu's split archive. It is not normally necessary on Debian unless your normal archive does not mirror the extra architectures you are intereted in. 

deb [arch=amd64.i386] http://uk.archive.ubuntu.com/ubuntu/ quantal main universe
deb [arch=armel,armhf] http://ports.ubuntu.com/ubuntu-ports quantal main universe

Arch-qualifying deb-src lines doesn't make any sense.

Don't forget to apt-get update after adding new architectures.

Installing/removing packages

To install a package of the non-default architecture just specify that architecture on the command line: 

apt-get install package:architecture

That package's dependencies will be installed automatically for the correct architectures (same-arch library deps, machine-arch for other deps) e.g 

apt-get install links:i386

dpkg --i package:architecture

Sometimes (in May 2012) dpkg get stuck when it is not sure which arch package it should be configuring. (dpkg: error: --configure needs a valid package name but 'gcc-4.7-base' is not: ambiguous package name 'gcc-4.7-base' with more than one installed instance) dpkg --configure -a will unbung this.

Installing cross-dependencies

To install build-dependencies of a package before cross-building: 

apt-get build-dep -a <arch> <package>

Details of how this resolves are on the