Important configuration files

[H.S.Rai]

How to find important configuration files? I can copy those files or
keep backup of these files. It seems that with GUI, even man/info or
help don't throw light of configuration files it changes.

I would like to know files changed by "Synaptic Package Manager" when
we change proxy setting, when we select packages of installation or
removal, where those changes are recorded?

Like wise, when I enter IP, DNS, Gateway (network setting), which
files hold changes.

Or let me know some link or tutorial, which can help me for such things.

-- 
H.S.Rai
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