postinst is in bootp 2.4.3-18build1.
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  | #!/bin/sh -e
# $Id: postinst,v 1.1 1999/03/14 04:26:30 herbert Exp $
update-inetd --group BOOT --add "#bootps		dgram	udp	wait	root	/usr/sbin/bootpd	bootpd -i -t 120"
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