By default, Linux OS systems have a firewall called IPTABLES. It can even be configured right after installation to block almost everything (as happens, for example, right after installing CentOS).
That would all be fine, but then the urge comes up to tweak a couple of rules in this firewall. Right away the question arises - where is the default file located that IPTABLES reads its rules from?
So,
For Linux Debian:
By default the file isn't created, but if rules are already present - look at this path:
/etc/network/if-pre-up.d/iptables
at this path there should be a bash file with a directive telling it where to pull instructions from (or the rules themselves may be written directly in it).
For Linux CentOS / RedHat:
/etc/sysconfig/iptables
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