One way to defend against OS fingerprinting from tools such as nmap, queso, p0f, xprobe etc is to change the metrics that they base their analysis on.

One way to do this with OpenBSD is to use Sealing Wafter.

Goals of Sealing Wafter:
1. To reduce OS detection based on well known fingerprints network stack behavior.
2. To have the ability to load custom rules into the stack.
3. To unload, modify, reload the kernel module with on the fly rules. (great feature at packet parties)
4. To learn how the magic of tcpip stacks work.

What Sealing Wafter currently provides:
1. Hide from Nmap Syn/Xmas/Null scans, as well as the specific fingerprinting packets.
2. Ability to see what your stack is receiving without the need to drop your network device into promisc mode.
3. Complete control over rules that you can load on the fly todeal with specific incoming packets.
4. Initial support for several OS passive detection has been added for SYNs.

Weaknesses in current Sealing Wafter:
1. Full connection scans. e.g. nmap -sT will still find open ports. this is because I have yet to find anything that seperates a real tcp connection vs an nmap full connection. (most likely isn’t one.)
2. Can be very verbose when under heavy load. I have run this on my heaviest web servers, and have not noticed any major overhead.

Download the c code for the LKM here: Sealing Wafter