libxt_connmark.man

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