Simple IPv6 PTR helper

ip6rev.sh

#! /bin/sh

# ip6rev - Generate IPv6 PTR DNS RR
# Copyright (C) 2012 Matous J. Fialka, <http://mjf.cz/>
# Released under the terms of "The MIT License"

if [ $# -lt 1 -o -$# -gt 3 ]; then
	cat <<- EOT
	ip6rev ip6 rev [oct]
	
	Where:
	   ip6	IPv6 address
	   rev	Hostname
	   oct	Octets (optional; default: 20)
	EOT
else
	pfx=$(
		sipcalc -r "$1" |
		sed -n 5p |
		cut -d. -f1-"${3:-20}"
	)
	printf '%s\tIN\tPTR\t%s\n' "$pfx" "${2%.}."
fi