libxt_mark.man

This module matches the netfilter mark field associated with a packet
(which can be set using the
.B MARK
target below).
.TP
[\fB!\fP] \fB\-\-mark\fP \fIvalue\fP[\fB/\fP\fImask\fP]
Matches packets with the given unsigned mark value (if a \fImask\fP is
specified, this is logically ANDed with the \fImask\fP before the
comparison).