Details and examples are in the kernel module source.
+config NETFILTER_XT_MATCH_OSF
+ tristate '"osf" Passive OS fingerprint match'
+ depends on NETFILTER_ADVANCED && NETFILTER_NETLINK
+ help
+ This option selects the Passive OS Fingerprinting match module
+ that allows to passively match the remote operating system by
+ analyzing incoming TCP SYN packets.
+
+ Rules and loading software can be downloaded from
+ http://www.ioremap.net/projects/osf
+
+ To compile it as a module, choose M here. If unsure, say N.
+
endif # NETFILTER_XTABLES
endmenu