I have never compiled or installed a new kernel on a e-smith but I have done it for some times at a standard RedHat, for some time ago, so I guess I don't remember everything.
In RedHat it is not really difficult, to compile or include a new kernel. There is a kernel compilation kit running under Gnome. This graphical kernel development environment contains a some kind of step by step guide, so it is not so difficult if you have a kernel source code.
I think the Linux kernel had a rather major revision from the 2.2.x to the 2.4.x version.
The 2.2.x vere running the "ipchains" old type firewall while the 2.4.x kernel got a completely new mechanism for datatransportation and firewalling and the iptables configuration tool. On the other hand the 2.4.x kernel had an optional kernel module that gave it backwards compability with the ipchains and the 2.2.x kernel. At least RedHat 7.0 and 7.1 were delivered with ipchains as the default configuration.
To say it more easy, if there is not a correct version and konfiguration of the kernel, or if you change to another version, you will have to redisign the firewall and nat mechanism completely.
The 2.4.x kernel is a rather modularized kernel, made up of a lot of small parts that kan be loaded dynamically, or it can be compiled in another way so it will work in a more static way as a monolitic kernel. I think the 2.2.x kernel had a more static disign without theese dynamic options.
If your e-smith are using the ipchains or iptables generation of firewall is easy to find out by issuing the command "iptables -l" (Small L) and "ipchains -l" (Same small L) You will then see which command that work and lists the firewall status and not. (The Netfilter/iptables firewall actually came with the latest verion of the 2.2.x kernel.
In my e-smith 5.6 home server, I think the 2.4.x kernel prinsipple of dynamic loading of kernel modules is used quite heavily. I think you can check this with the command "modprobe -l" (Small L) Dont know if this command will work in any way with the 2.2.x kernel.
I have been thinking a little about trying to transfer a newer kernel to the e-smith 5.6/6-0 but I havent tried so far. The way I think it possibly could work (but I don't really know) would be to make a RedHat installation with Gnome and kernel compilation kit. Then I would have tried to compile a new monolitic / non dynamic kernel, and then I would try to transfer this to the e-smith.
The reason for monolitic/non dynamic is that this, I belive will not make it neccessary to transfer all thos small kernel modules that has to be there to be dynamic loadable.
The bad thin about using a monolitic kernel is that it will be less effective and use more memmory as all "unnessesary", drivers, etc are there all the time also when they are not needed. The good thing is that it might be simpler.
I believe there is no kernel rpm. I belive that the kernel is not a part of the ordinary rpm pacage system, so I believe you can not only pick it from a newer CD. (But still I'm not shore if a kernel upgrade can be made as an rpm, I will guess this is possible.)
If some of you other knows something more or if I'm wrong about things, please leave a msg !!