/usr/src/xtables-addons-3.0/extensions/libxt_psd.man is in xtables-addons-dkms 3.0-0.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Attempt to detect TCP and UDP port scans. This match was derived from
Solar Designer's scanlogd.
.TP
\fB\-\-psd\-weight\-threshold\fP \fIthreshold\fP
Total weight of the latest TCP/UDP packets with different
destination ports coming from the same host to be treated as port
scan sequence.
.TP
\fB\-\-psd\-delay\-threshold\fP \fIdelay\fP
Delay (in hundredths of second) for the packets with different
destination ports coming from the same host to be treated as
possible port scan subsequence.
.TP
\fB\-\-psd\-lo\-ports\-weight\fP \fIweight\fP
Weight of the packet with privileged (<=1024) destination port.
.TP
\fB\-\-psd\-hi\-ports\-weight\fP \fIweight\fP
Weight of the packet with non-priviliged destination port.
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