postinst is in wireshark-common 1.10.6-1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 | #!/bin/sh
set -e
. /usr/share/debconf/confmodule
PROGRAM=/usr/bin/dumpcap
GROUP=wireshark
if ! dpkg-statoverride --list $PROGRAM > /dev/null; then
    db_get wireshark-common/install-setuid
    if [ -e "$PROGRAM" ]; then
	if [ "$RET" = "false" ] ; then
	    chown root:root $PROGRAM
	    chmod u=rwx,go=rx $PROGRAM
	else
	    if ! addgroup --quiet --system $GROUP; then
		echo "Executing \"addgroup --quiet --system $GROUP\" failed."
		echo "Most probably the $GROUP group exists, but is not a system group."
		echo "Please delete the existing group or re-create it as a system group and try configuring wireshark-common again."
		exit 1
	    fi
	    chown root:$GROUP $PROGRAM
	    if which setcap > /dev/null ; then
                chmod u=rwx,g=rx,o=r $PROGRAM
                if ! setcap cap_net_raw,cap_net_admin=eip $PROGRAM; then
		    echo "Setting capabilities for dumpcap using Linux Capabilities failed."
		    echo "Falling back to setting set-user-id bit."
		    chmod u=rwxs,g=rx,o=r $PROGRAM
                fi
	    else
                chmod u=rwxs,g=rx,o=r $PROGRAM
	    fi
        fi
    fi
else
    echo "Preserving owner and mode for $PROGRAM set by dpkg-statoverride:"
    dpkg-statoverride --list $PROGRAM
fi
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