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I'm securing a (company) webserver on AIX - I've been using nmap to scan from my linux box for open ports, but there's some that I miss that a collegue with a windows machine picks up (with languard). What's the combination of flags to test every port on a box, both TCP and UDP? I know there's certain ports open for websphere (> about 9000) that I'm not picking up on.
Well, ShieldsUp! at https://grc.com included port 0, so I included it too. Now I've checked my two versions of nmap, and 3.00 gives the error that you mention, but 3.48 doesn't. I can't remember what port 0 is about, but as far as I remember you can distinguish between different OS:s that way.
Commonly used to help determine the operating system. This works because on some systems, port 0 is "invalid" and will generate a different response when you connect to it vs. a normal closed port. One typical scan uses a destination IP address of 0.0.0.0 and sets the ACK bit, with broadcast at the Ethernet layer.