iptables command from RHEL documentation for cobbler, fails in RHEL 7

Solution 1:

Congratulations, you found an error in the RHEL documentation.

The iptables rule shown here uses the tcp matcher but then attempts to specify the udp protocol. This doesn't work; the tcp matcher can only be used with protocol 6, which happens to be tcp. Thus the error you received.

To correct the rule you have to first figure out which is wrong, the matcher or the protocol. Unfortunately this error has been propagated all over the Internet so this isn't so easy to figure out from an Internet search, and even the Cobbler documentation doesn't clearly mention it.

You can sort it out easily by inspecting the output of ss -nl, and there you should find cobbler listening on TCP port 25150, not UDP. Thus you replace udp with tcp and then report the documentation error to Red Hat.