Netfilter, Linux's in-kernel "packet mangling" infrastructure, and iptables, the administrative tool that manages it, represent a substantial improvement over ipchains, the previous option ...

iptables has a lot more to offer than these simple packet-filtering criteria. Some of the extras are fairly well known and even may make their way into some off-the-shelf Linux distributions, but some ...

Mastering iptables could take a while, but if you have a few rules to cover the basic security needs, you'll be well on your way to protecting your Linux system. Jack Wallen explains some key ...

Linux comes with a powerful firewall built-in, although the interface can be a little intimidating. This is the first in a multi-part tutorial on how to master basic and not-so-basic IPTables ...

The iptables -L command, by the way, doesn't really show enough info to be useful for troubleshooting. We'd need to see the actual script you're running, at least the parts relevant to SMTP.

Firewalld and iptables both talk to netfilter (part of kernel that does firewall). You can have one or the other. RHEL/Centos default to firewalld and for regular server where you just need to ...