/usr/src/xtables-addons-2.6/extensions/libxt_length2.man is in xtables-addons-dkms 2.6-1.
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This module matches the length of a packet against a specific value or range of
values.
.TP
[\fB!\fR] \fB\-\-length\fR \fIlength\fR[\fB:\fR\fIlength\fR]
Match exact length or length range.
.TP
\fB\-\-layer3\fR
Match the layer3 frame size (e.g. IPv4/v6 header plus payload).
.TP
\fB\-\-layer4\fR
Match the layer4 frame size (e.g. TCP/UDP header plus payload).
.TP
\fB\-\-layer5\fR
Match the layer5 frame size (e.g. TCP/UDP payload, often called layer7).
.PP
If no \-\-layer* option is given, \-\-layer3 is assumed by default. Note that
using \-\-layer5 may not match a packet if it is not one of the recognized
types (currently TCP, UDP, UDPLite, ICMP, AH and ESP) or which has no 5th
layer.
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