Nmap does not notice ACK packets

[Richard van den Berg <richard.vandenberg () ins com>]

I am scanning a fairly large network using -sS and I have some hosts
respond to nmap's SYN packet with only an ACK. I know this is a strange
way to behave for a host. Has anyone ever seens this before? It seems
intermittent because when I scan the host a second time, all is good.
Even when I craft the exact same packets using hping2, the host will
responds with SYN ACK (as it should).

The thing is, nmap 4.20 never reacts to these ACK packet. The port shows
up as filtered, and is not used to send TCP probes to either. I am not
sure what "state" nmap should give to such a port. Maybe open|filtered ?

-- 
Richard van den Berg
Senior Consultant, INS

E-mail: richard.vandenberg () ins com
Mobile: +31 (0)6-52071109

PGP Key ID: 0x6614D2AC
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