postinst is in iputils-tracepath 3:20161105-1.
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 | #!/bin/sh
set -e
if [ "$1" = configure ]; then
	update-alternatives \
		--install /usr/bin/traceroute6 traceroute6 /usr/bin/traceroute6.iputils 100 \
		--slave /usr/share/man/man8/traceroute6.8.gz traceroute6.8.gz /usr/share/man/man8/traceroute6.iputils.8.gz
	if command -v setcap > /dev/null; then
		if setcap cap_net_raw+ep /usr/bin/traceroute6.iputils; then
            chmod u-s /usr/bin/traceroute6.iputils
		else
			echo "Setcap failed on /usr/bin/traceroute6.iputils, falling back to setuid" >&2
			chmod u+s /usr/bin/traceroute6.iputils
		fi
	else
		echo "Setcap is not installed, falling back to setuid" >&2
		chmod u+s /usr/bin/traceroute6.iputils
	fi
fi
exit 0
# Local variables:
# mode: shell-script
# tab-width: 4
# indent-tabs-mode: nil
# end:
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