Yes, there are. I use one for my work servers that is iptables based.
I don't have any links for you unfortunately but I have seen them. If
you are really interested I can probably track down one I saw that used
iptables and was a combination style. I also know of an open source
"magic packet" style that I could probably find a link for if you were
interested.
boger said the following:
Hello Kirk,
Is there IPtables based port knocker?
I dislike idea opening ports for this purpose because they can be distinguished by some way.
Promiscuous mode port knockers consume a lot of processor and
I don't think it's good for production server.
KH> A port knocker of some sort is a much more secure solution that will
KH> allow you to block all unwanted IP's but still allow for dynamic
KH> addresses. There are port knockers that listen on various ports and
KH> work like a combination lock to open the port, and there are others that
KH> use a more secure one time pad "magic packet" kind of authentication to
KH> open the port for your IP. It is more work to setup, but it is more
KH> secure than just changing the port. Remember a few years ago when ssh
KH> had a remote exploit? You probably shouldn't leave that port open.
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