how can i verify what the running kernel is compiled to support without having the kernel config file?

In general, you can't. You could look for kernel module files under /lib/modules, but that will only tell you what is supported through modules, not what is or isn't compiled in.

However, recent 2.6 series kernels have started embedding the configuration file and making it available as /proc/config.gz, which you can view with: or extract to your home directory with:It should be noted that the option to supply this file can be turned off, but I've never known anyone actually do this.

it's the service consol of esx, it's running kernel 2.4.9-vmnix2, redhat 7.2. I want to see what they compiled the kernel to support. your tip worked on my workstation running NLD, but not on the server. Any other suggestions?

Isn't there a config file in your /boot directory?

Also maybe in

/usr/src/linux

Yes,it is.
Here rise the following question:
I use kernel 2.4.x .
If I use the /boot/config-2.4.x file for xconfig program , only to change some settings , in order to recompile a 2.6.10 kernel , is that gonna work ?
At least does this sound reasonable ?

if you:
cp /boot/config-2.4.whatever /usr/src/linux/
and then:
make xconfig
you'll get all the settings of your old kernel. problem is, any new bits added into the kernel will be set at their default value, which can sometimes be dangerous (or at the least, annoying, and could make your new kernel slow and/or big). so make sure you investigate anything that's marked as 'new'.

or instead, going for:
make oldconfig
will auto-skip any old bits, and only prompt for new stuff since your old kernel. but it takes ages (at least 200 changes from 2.6.4 to 2.6.6.11). lot slower, but you definately pick up all the changes.