Chapter 3. Building Custom Packages

There are many things that might go wrong when building software packages. This is especially true when these packages must be delivered and installed through Red Hat Network. This chapter provides an overview of how to build packages for successful delivery via Red Hat Network. Topics covered include why to use RPM, how to build packages for RHN, and how to properly sign packages.

3.1. Building packages for Red Hat Network

Red Hat Network uses the RPM Package Manager (RPM) technology to determine what software additions and updates are applicable to each client system. Packages retrieved from Red Hat Network are usually in RPM format. Entire ISO images, however, are available through the Software tab of the Red Hat Network website, but are not available in RHN Satellite Server installations. If your Satellite has Solaris support enabled, you can use RHN Push to upload Solaris packages to custom channels used by Solaris clients.

RPM is a tool that provides users with a simple method for installing, uninstalling, upgrading, and verifying software packages. It also allows software developers to package the source code and compiled versions of a program for end users and developers.

3.1.1. RPM Benefits

RPM provides the following advantages:

Easy Upgrades: Using RPM, you upgrade individual components of a system without completely reinstalling. When Red Hat releases a new version of Red Hat Enterprise Linux, users do not have to reinstall in order to upgrade. RPM allows intelligent, fully-automated, in-place upgrades of your system. Configuration files in packages are preserved across upgrades so users do not lose customizations. There are no special upgrade files needed to update a package because the same RPM file is used to install and upgrade the package.
Package Querying: RPM provides querying options that allows you to search through your entire RPM database for all packages or just for certain files. You can also easily find out what package a file belongs to and from where the package came. The files contained in the package are in a compressed archive, with a custom binary header containing useful information about the package and its contents. RPM queries the headers quickly and easily.
System Verification: Another feature is the ability to verify packages. If you are worried a file related to a package was deleted, you can verify the package to check the status of the files it provides. The verification notifies you of any anomalies. If errors do exist, you can reinstall the files easily. Modified configuration files are preserved during reinstallation.
Pristine Sources: A crucial design goal of RPM is to allow the use of pristine software sources, as distributed by the original authors of the software. With RPM, the pristine sources can be packaged, along with any patches that were used, plus complete build instructions. This is an important advantage for several reasons. For instance, if a new version of a program is released, you do not necessarily have to start from scratch to make it compile. You can look at the patch to see what you might need to do. All the compiled-in defaults and changes made to get the software to build properly are easily visible using this technique.
Keeping sources pristine may seem important only to developers, but it results in higher quality software for end users, as well.

3.1.2. RHN RPM Guidelines

The strength of RPM lies in its ability to define dependencies and identify conflicts accurately. Red Hat Network relies on this aspect of RPM. Red Hat Network offers an automated environment, which means that no manual intervention can take place during the installation of a package. Therefore, when building RPMs for distribution through Red Hat Network, it is imperative to follow these rules:

Learn RPM. It is crucial to have a fundamental understanding of the important features of RPM to build packages properly. For more information about RPM, start with the following resources:
When building an RPM for a child channel, build the package on a fresh install of Red Hat Enterprise Linux of the same version as the child's base channel. Be sure to apply all updates from Red Hat Network first.
The RPM package must install without using the --force or --nodeps options. If you cannot install an RPM cleanly on your build system, Red Hat Network cannot install it automatically on a system.
The RPM package filename must be in the NVR (name, version, release) format and must contain the architecture for the package. The proper format is name-version-release.arch.rpm. For example, a valid RPM package filename is pkgname-0.84-1.i386.rpm, where name is pkgname, version is 0.84, release is 1, and arch is i386.
The RPM package should be signed by the maintainer of the package. Unsigned packages may be distributed through Red Hat Network, but the Red Hat Update Agent (up2date) must be forced to accept them. Signing packages is highly recommended and is covered in Section 3.2, “Digital Signatures for RHN Packages”.
If the package is changed in any way, including changing the signature or recompiling, the version or release must be increased incrementally. In other words, the NVRA (including architecture) for each RPM distributed through RHN must correspond to a unique build to avoid ambiguities.
No RPM package may obsolete itself.
If a package is split into separate packages, be extremely careful with the dependencies. Do not split an existing package unless there is a compelling reason to do so.
No package may rely upon interactive pre-install, post-install, pre-uninstall, or post-uninstall scripts. If the package requires direct user intervention during installation, it cannot work with Red Hat Network.
Any pre-install, post-install, pre-uninstall, and post-uninstall scripts should never write anything to stderr or stdout. Redirect the messages to /dev/null if they are not necessary. Otherwise, write them to a file.
When creating the spec file, use the group definitions from /usr/share/doc/rpm-<version>/GROUPS. If there is not an exact match, select the next best match.
Use the RPM dependency feature to make sure the program runs after it is installed.

Important

Do not create an RPM by archiving files and then unarchiving them in the post-install script. This defeats the purpose of RPM.

If the files in the archive are not included in the file list, they cannot be verified or examined for conflicts. In the vast majority of cases, RPM itself can pack and unpack archives most effectively anyway. For instance, do n't create files in a %post that you do not clean up in a %postun section.