I have the following requirements:
I have looked through the help and documentation but can't find any input on this. Will older OpenVPN versions that work on Linux kernel 2.4.21 (CentOS 3) be compatible with the iOS client?
If I were you I think I would approach this question in the following way. Given CentOS 3 is a hard requirement I would first find out what is the latest version of OpenVPN that will run on that version of Linux.
If you go to repoforge.org you'll find RPMs with an "el" in their names. The "el" stands for "Enterprise Linux". These are RPMs for Red Hat Enterprise Linux (aka. RHEL) - from which CentOS is derived. Looking through the list I see lots of choices for el3 - the equivalent RHEL3 for CentOS 3.
excerpt from package list on repoforge.org
I'd start with this version, 2.2.2-1: 64-bit or 32-bit.
Now I'd go to OpenVPN's main site and see what the latest version is, which at the time of this writing appears to be:
OpenVPN 2.3.2 -- released on 2013.06.03
So the 2.2.2-1 doesn't seem to be that old in comparison. Additionally if you scroll down on that same page the version 2.2 is mentioned as being released at this time:
OpenVPN 2.2.2 -- released on 2011.12.22
This is directly in the heart of when iOS devices have/had been released so I'd say there is a very good chance that version 2.2.2 will work just fine with iOS.
As a little bit more evidence I found this tutorial that describes setting up iOS 4.2 devices with OpenVPN, titled: On-Demand VPN using OpenVPN for iOS, so I think what you're proposing seems reasonable and doable to me.
