Linux – chroot build script somewhere

centoschrootlinuxscripting

I am about to develop a little script to gather information for a chroot-jail.

In my case this looks (at the first glance) pretty simple:
The application has a clean rpm-install and did install almost all files into a sub-directory of /opt.

My idea is:

Do a find of all binaries
Check their library-dependencies
Record the results into a list
Do a rsync of that list into the chroot-target-directory before startup of the application

Now I wonder – ist there any script around that already does such a job (perl/bash/python)?

So far I found only specialized solutions for single applications (like sftp-chroot).

Although is does not matter (imho) – OS is CentOS 5 x86_64 current minor release and patch-level.

rpm -ql is IMHO not generic enough, since it will only cover rpm-based distributions. The mention of the "clean install" above was just to mention that the files of the software are not distributed across the whole file-system. So my starting point is – at the moment – a find /opt/directory/… that should work on almost any system (even not Linux).

Best Answer

I would suggest creating a template chroot and installing all the packages you want just like it was a normal OS. After that you can manage the chroot using your typical tools (update scripts, package manager, etc.) and rsync the updates into each chroot built using that template.

There are a few advantages to this approach. The two big ones are you can manage the template using familiar tools (no strange hoops to jump through to upgrade your chroot), and if you have one chroot which can't be updated for some reason (say it needs a particular version of some package) you can exclude it from the rsync upgrade process and manage it independently as though it were a standalone machine, marking the package as "held" or equivalent so it doesn't get stomped on.

Your mileage (and implementation requirements) may vary...

Do I need support?

Support has two complementary meanings in our world. The first is that a product has a given feature (e.g. "Postfix supports SMTP"). The second is that the vendor will talk to you about it. Which definition is meant is not always clear from context.

To accomplish a task, you obviously need support in the first sense. Where vendor support comes in is to assist you in resolving issues and giving the vendor feedback as to what features need to exist or be improved. Many sites pay a fortune for vendor support when they have the in-house expertise to resolve any issues that may arise, faster and even cheaper than the vendor could. Whether to buy vendor support is ultimately a business decision you will have to make (or advise management on).

Why not do an in-place upgrade?

This is what Red Hat says about it:

Red Hat does not support in-place upgrades between any major versions of Red Hat Enterprise Linux. A major version is denoted by a whole number version change. For example, Red Hat Enterprise Linux 5 and Red Hat Enterprise Linux 6 are both major versions of Red Hat Enterprise Linux.

In-place upgrades across major releases do not preserve all system settings, services or custom configurations. Consequently, Red Hat strongly recommends fresh installations when upgrading from one major version to another.

They further warn:

However, note the following limitations before you choose to upgrade your system:

Individual package configuration files may or may not work after performing an upgrade due to changes in various configuration file formats or layouts.

If you have one of Red Hat's layered products (such as the Cluster Suite) installed, it may need to be manually upgraded after the Red Hat Enterprise Linux upgrade has been completed.

Third party or ISV applications may not work correctly following the upgrade.

Of course, they then describe how to do an in-place upgrade via method 1, just in case you really want to do it. The feature exists and Red Hat puts development time into it, so it is supported in that the feature exists. But if something goes wrong, Red Hat will tell you to install fresh; they will not provide vendor support for things that break as a result of the upgrade.

For the record, I've never actually had a problem with an in-place upgrade of a RHEL/CentOS or Fedora system that I couldn't resolve myself. The typical problems come from renamed packages, third party repositories and the occasional version mismatch between the i386 and x86_64 architectures of a package. The installer is a bit better at handling these than yum, I think.

How should I upgrade?

I generally warn people that they should plan on a maintenance window every 3-4 years to update RHEL systems from one major version to the next. While upgrades generally go smoothly, the unexpected can always happen.

For both of your environments, I expect an in-place upgrade would work, though I strongly recommend testing it thoroughly first. P2V a representative sample of the servers and run through the in-place upgrade on the virtual systems to see what problems you're going to run into. You can then plan the actual production upgrade based on better knowledge of what will happen.

For a large deployment such as you have here, consider using Limoncelli's "one-some-many" approach. Upgrade one machine, see what problems occur, solve them, then use lessons learned when upgrading a small batch of machines, repeat the lessons learned thing, then when you believe you have all the kinks worked out, upgrade large batches of them.

At a time like this, I also recommend taking a long hard look at your application deployment process. If it isn't sufficiently automated that you can kick it off with a single command and be reasonably sure that the app will be deployed correctly, then perhaps the developers need to get to work on that. Having such a deployment process would make it much easier to do a fresh installation of the newer version of EL and then deploy onto it.

Will switching distributions help?

Debian-based distributions do have a supported in-place upgrade method, and it mostly works, but it is not immune from problems. Lots of things broke for people upgrading from Ubuntu 10.04 LTS to 12.04 LTS via the supported method, for instance. It's not clear that Debian or Canonical are putting a sufficient amount of development time into "supporting" this feature, i.e., making sure it works. And you still actually have to buy vendor support for this distribution if you want someone to hold your hand. So I doubt you will gain much from switching to such a distribution.

You may gain by switching to a rolling-release distribution such as Gentoo or Arch. However, this also doesn't make you immune to problems; it just means you have to deal with the upgrade problems continuously over the life of the server (e.g. whenever you or the developers decide to update something on the system), rather than all at once at a well-planned distribution upgrade time. You also have no vendor to provide support.

What does the future hold?

The Fedora Project is working on a tool to improve in-place upgrades. They had a tool called preupgrade which was abandoned and replaced with a new tool called fedup beginning with Fedora 18. This was added to RHEL7 and now in-place upgrades have full support, at least from RHEL 6 to RHEL 7. From my own experience I can say that while fedup still has some kinks, it is shaping up to be a very useful tool.

CentOS is also experimenting with a rolling-release type of repository, but it only applies between minor versions (e.g. 6.3-6.4).

Linux – the best method of installing programs into a chroot / jail

There are multiple ways to setup chroot jails. Personally I've seen over 50 different scripts for that. All jails are similar, but have smaller or bigger differences, so there is no generic way to do anything inside them.
The most common usage of chroot jails regarding installing software, is installing everything from source. Sometimes chroot jails are used especially to install software from source, avoiding dependiences installed on local machine from packages (eg. zlib, libpng or other common libraries).
What is unfortunately common between many different chroot jail scripts, is that they don't have any "upgrade mode", so each time you want to upgrade your base system software packages, or software inside jail, you have to setup this jail again. This of course can be scripted, but this differs from normal package upgrading.
Search for OpenVZ or LXC paravirtualization technologies. Both of them are used to create "containers" (separate systems with shared kernel). Both are based on chroot concept, but with added separation on many other layers inside kernel.

LXC is newer and more modern, however OpenVZ provides much better security isolation. And in both of them you can install almost complete system, with package manager, libraries from packages etc., so you can normally install and upgrade software, just like in your base system.