### Why older kernel?

For whatever reason out there, you might be *forced* to run another kernel than the ones provided by Ubuntu. It may even take you back a few years for a kernel that is compatible with specific precompiled kernel modules, your Xen/container-based VPS provider may force you to use his kernel, etc.

I have this question for a long time, but [this](https://unix.stackexchange.com/questions/60731/overflow-tmp-ubuntu-12-04-1-lts#comment84116_60731) sparked it again today.

In such a case it would be very useful to be able to say whether you can blame the kernel for issues or whether you should even bother to try to set up a more recent version of Ubuntu in your situation.

### Policy, documentation?

I'm particularly interested in what the objectives of the developers/QA are in this regarding LTS releases and newer stable running the LTS-kernel. Some closely related questions:

* What is the **policy** on compatibility with earlier kernel versions? E.g. no bug reports accepted, must work with all kernel versions back to and including previous LTS, etc.
* Example case, practically: How likely will I be in trouble when running Lucid's kernel on Precise?
* To what extent is software relatively close to the kernel (udev, gvfs, mdadm etc.) being tested on other than the version provided with the release?
* How does Desktop/Server edition differ in this?

The most obvious place to look at this would be the [Release notes](https://wiki.ubuntu.com/PrecisePangolin/ReleaseNotes/UbuntuServer). However, besides updates/changes to the Ubuntu flavoured kernel, this doesn't mention anything about compatibility with other kernels, while kernel-related features are being mentioned in other parts of the notes, e.g.

> 
> Software RAID now supports bad block management (MD).
> 
> 
> 

Is Ubuntu simply not bothering about these cases or am I missing a resource on this? Besides the release notes, I have used Google to quite some extent using keywords: `Ubuntu 12.04 minimal kernel version required` and several variants to it. Yet, no statement about it appears to be made on those results. *I now find this question popping up as only relevant resource.* I did find [this answer](https://askubuntu.com/a/154300), tough, and it looks very promising, yet it's about a specific issue/environment and not really about server/desktop usage.

### Userspace vs kernel

I know most pieces of userland software should not bother about kernel versions, but it's getting more complicated for VPN software or applications interacting with hardware, like the one above, but also for example V4L2, Network Manager, Alsa, etc.

### Debian vs Ubuntu

Debian is really clear on this. Already for Wheezy, we know that if you rely on udev, it will require 2.6.26 to run properly from it's [release notes](http://www.debian.org/releases/testing/amd64/release-notes/ch-upgrading.en.html#upgrading-udev) (in the works):

> 
> The udev version in wheezy requires a kernel of version 2.6.26 or newer with [...]
> 
> 
> 

### What I'm not asking for

I am very well aware about the backports provided for newer kernels from newer releases to the current LTS version. This question is about the opposite.

Please avoid any discussion like "why would one want to run an older kernel?" - you just don't have a choice sometimes and it's not about what we want, but how one can deal with such a given situation.