/usr/src/xtables-addons-2.3/libxt_lscan.man is in xtables-addons-dkms 2.3-1.
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Detects simple low-level scan attempts based upon the packet's contents.
(This is
different from other implementations, which also try to match the rate of new
connections.) Note that an attempt is only discovered after it has been carried
out, but this information can be used in conjunction with other rules to block
the remote host's future connections. So this match module will match on the
(probably) last packet the remote side will send to your machine.
.TP
\fB\-\-stealth\fR
Match if the packet did not belong to any known TCP connection
(Stealth/FIN/XMAS/NULL scan).
.TP
\fB\-\-synscan\fR
Match if the connection was a TCP half-open discovery (SYN scan), i.e. the
connection was torn down after the 2nd packet in the 3-way handshake.
.TP
\fB\-\-cnscan\fR
Match if the connection was a TCP full open discovery (connect scan), i.e. the
connection was torn down after completion of the 3-way handshake.
.TP
\fB\-\-grscan\fR
Match if data in the connection only flew in the direction of the remote side,
e.g. if the connection was terminated after a locally running daemon sent its
identification. (E.g. openssh, smtp, ftpd.) This may falsely trigger on
warranted single-direction data flows, usually bulk data transfers such as
FTP DATA connections or IRC DCC. Grab Scan Detection should only be used on
ports where a protocol runs that is guaranteed to do a bidirectional exchange
of bytes.
.PP
NOTE: Some clients (Windows XP for example) may do what looks like a SYN scan,
so be advised to carefully use xt_lscan in conjunction with blocking rules,
as it may lock out your very own internal network.
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