[pyritschard]

It refers to a technique also known as "port knocking" which consists of leaving a port closed by default and opening it upon receiving a message by another channel (in this case, a UDP packet).

It was mostly in use when TLS hadn't made it's way into most common protocols

[justusthane]

No, that’s not what this is referring to, since that doesn’t involve blocking IPs. This page[1] provides some detail:

> A recent change goes a step further and sends a UDP packet to my OpenBSD firewall containing the IP to be banned, and a small Ruby server running there adds the IP to a pf table, immediately blocking all further IP access from the bot.

[1]: https://jcs.org/2022/07/15/kludge

[hexmiles]

Thank you,