gen-ipv6-address

#!/usr/bin/perl

# Generates a random IPv6 address in the same subnet as the machine is currently in
# Written by Dustin Lundquist <dustin@null-ptr.net>

use strict;
use warnings;


sub getglobaladdress {
	open (IP, '-|', '/sbin/ip address show')
		or return undef;
	while (my $line = <IP>) {
		# inet6 2607:f700:1:a0:230:48ff:fe57:b20e/64 scope global
		if ($line =~ m/^\s+inet6 ((?:[0-9a-f]{0,4}\:)+[0-9a-f]{0,4})\/(\d+) scope global/) {
			close (IP);
			return ($1, $2);
		}
	}
	close (IP);
	return undef;
}

sub getprefix {
	my ($addr, $bits)= getglobaladdress
		or return undef;

	my $remaining_bits = $bits;

	my $prefix = '';
	while ($remaining_bits > 0) {
		if ($remaining_bits >= 16 and $addr =~ /^([0-9a-f]{1,3}\:)[0-9a-f]/) {
			$remaining_bits -= 16;
			$prefix .= $1;
			$addr =~ s/^$1//;
			next;
		} elsif ($remaining_bits >= 16 and $addr =~ /^([0-9a-f]{1,3}\:)((?:\:[0-9a-f]{1,3})+)/) {
			# :: in the prefix protion of the address
			$remaining_bits -= 16;
			$prefix .= $1;
			my $rest = $2;



			die ":: in network portion of address not supported at this point\n";
		}
		die "prefixes which are not multiples of 16 not supported";
	}
	return($prefix, $bits);
}

my ($address, $bits) = getprefix
	or die ("Unable to fetch a public prefix\n");

while ($bits < 128) {
	if ($bits + 16 < 128) {
		$address .= sprintf("%x", rand(65533) + 1) . ':'; # Avoid all 0's and all 1's addresses
		$bits += 16;
		next;
	}
	if ($bits + 16 == 128) {
		$address .= sprintf("%x", rand(65533) + 1);
		$bits += 16;
		next;
	}
	die "prefixes which are not multiples of 16 not supported ($address $bits)";
}
print "$address\n";