I am using an almost hidden FTP server in my DMZ behind a Palo Alto Networks firewall. FTP is only allowed from a few static IP addresses, hence no brute-force attacks on my server. Furthermore, I have an “allow ping and traceroute from any to DMZ” policy since ping is no security flaw but really helpful while troubleshooting.

Now, here comes the point: My FTP server logfile showed dozens of connections from many different IP addresses from the Internet. WHAT? For the first moment I was really shocked. Have I accidentally exposed my FTP server to the Internet? Here is what happened:

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