Glitch,
Good questions really. The best way to go about this is to use your basic knowledge. So... if you want to look at MTU for PPPOE from experience you may know that you want the file /var/service/pppoe/run.pppoe.conf
However, when you look at that files you'll see a message warning that you should modify this file. That's because it's dynamically created after certain setting changes in the server-manager... so writing directly to the file will be a waste of time because your work will be overwritten.
So... take a look at the template that created this file:
cd /etc/e-smith/tempates/var/service/pppoe/run.pppoe.conf
You'll see three files there. You can edit these, but an update to the rpm that created them. So we should instead create a special place to old our modifications
mkdir -pv /etc/e-smith/tempates-custom/var/service/pppoe/run.pppoe.conf
cp device /etc/e-smith/tempates-custom/var/service/pppoe/run.pppoe.conf
and modify the file 'device' in the templates-custom directory.
Okay... so lets assume you don't know where to find your config file, but you have a general idea of a word that may be in the file. In your example let's look for any occurance of 'mtu' in the /etc directory
grep -R -i mtu /etc/*
-R means look in all the subdirs, -i means match on lowercase.
You are going to get a lot of junk in most cases. You can either filter it out with further commands or redirect it to a file to search for it.... or search google for where it's controlled.
Hope this helps.