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Type of service (TOS) For “ICMP port unreachable” messages, the TOS is
examined. Most stack implementations use 0, but this can vary.
Fragmentation handling As pointed out by Thomas Ptacek and Tim
Newsham in their landmark paper “Insertion, Evasion, and Denial of
Service: Eluding Network Intrusion Detection” (http://www.clark.net/
~roesch/idspaper.html), different stacks handle overlapping fragments
differently. Some stacks will overwrite the old data with the new data
and vice versa when the fragments are reassembled. By noting how probe
packets are reassembled, you can make some assumptions about the target
operating system.
TCP options TCP options are defined by RFC 793 and more recently by RFC
1323 (www.ietf.org/rfc/rfc1323.txt). The more advanced options provided by
RFC 1323 tend to be implemented in the most current stack implementations.
By sending a packet with multiple options set, such as no operation, maximum
segment size, window scale factor, and timestamps, it is possible to make some
assumptions about the target operating system.
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