Proposed patch for Port Randomization modifications according to RFC6056

[Ivo Vachkov]

Hello,

I would like to propose a patch (against FreeBSD RELENG_8) to extend
the port randomization support in FreeBSD, according to RFC6056
(https://www.rfc-editor.org/rfc/rfc6056.txt)

Currently the patch implements:
- Algorithm 1 (default in FreeBSD 8)
- Algorithm 2
- Algorithm 5
from the aforementioned RFC6056.

Any of those algorithms can be chosen with the sysctl variable
net.inet.ip.portrange.rfc6056_algorithm.

I deliberately skipped Algorithm 3 and Algorithm 4, because I believe
usage of cryptographic hash functions will introduce unnecessary
latency in vital network operations. However, in case of expressed
interest, I will be glad to add those too.

I would like to ask what is the proper way to validate the sysctl
input in order to accept only a specific values? In my case only '1',
'2' and '5'.

Thank you very much.

Ivo Vachkov
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