how to hide os type from scaning of nmap or nessus

Applications Cybersecurity
1

my os is freebsd 7.1
just open sshd and hide the sshd banner
nessus still report correctly the os type
how to hide os type from scaning of nmap or nessus?

Thanx so much.

my ipfw rule:
ipfwcmd="ipfw -q add allow tcp"
localip="192.168.1.254"
$ipfwcmd from any to $localip 22 in setup keep-state

2

Some tools use network fingerprinting to determine the OS, if you interested the Center (sic) for Internet Security produce documents on hardening and benchmarking scripts to measure the hardness of your build, I imagine they cover amending the network fingerprint for BSD.

3

Thanx so much!
Master:)
i posted the question in four forums only U reply me.