Markus Wernig
UNIX/Network Security Engineer
 CCSA, CCSE
CISSP


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How to check for valid IPv6 addresses

How to calculate IP6.ARPA format from IPv6 addresses

Abstract

This article shows how to check if an IPv6 address is valid with a regular expression and how to calculate the "reverse" (ie. IP6.ARPA) address query string.

This is the (Perl) regex to check if an IPv6 address is a valid interface address:

my $isvalid = ((($ip =~ s/:/:/g) <= 7) && $ip =~ /^(::([a-f0-9]{1,4}:){0,6}|([a-f0-9]{1,4}:){7}|([a-f0-9]{1,4}:){0,7}([a-f0-9]{1,4}::){1}([a-f0-9]{1,4}:){0,7})[a-f0-9]{1,4}(%[a-z]+[0-9]+){0,1}$/i);

This checks only for valid interface addresses, not network addresses in the zero-compressed form (eg. 2a00::).

Let's have a look at the parts of the regex:

my $isvalid =
(
  (
    ($ip =~ s/:/:/g) <= 7
  )
&&
  $ip =~ /^
  (
    ::([a-f0-9]{1,4}:){0,6}

  |
    ([a-f0-9]{1,4}:){7}
  |
    ([a-f0-9]{1,4}:){0,7}
    ([a-f0-9]{1,4}::){1}
    ([a-f0-9]{1,4}:){0,7})
  )
  [a-f0-9]{1,4}
  (%[a-z]+[0-9]+){0,1}$
  /i
);


# address contains at most 7 colons

# AND (ie. both conditions must match)
# address starts with:

# a double colon (::), followed by 0 to 6 (1-2-byte) hex
# numbers, each followed by a single colon
# OR address starts with:
# 7 hex numbers, with trailing colon (uncompressed form)
# OR address starts with:
# 0 to 7 hex numbers, with trailing colon, followed by
# 1 hex number, with trailing double colon, followed by
# 0 to 7 hex numbers, with trailing colon

# address ends with a hex number, optionally
# followed by up to 1 interface identifier (for link-local)
# all matches are case-insensitive

And here's how to calculate the "reverse" IP form from an IPv6 address (still in Perl):

#!/usr/bin/perl -w

# small script that calculates the ip6.arpa format of an IPv6 address
# public at wernig.net 2014

use strict;

my ($ip) = @ARGV or exit 1;
chomp($ip);

unless ((($ip =~ s/:/:/g) <= 7) && $ip =~ /^(::([a-f0-9]{1,4}:){0,6}|([a-f0-9]{1,4}:){7}|([a-f0-9]{1,4}:){0,7}([a-f0-9]{1,4}::){1}([a-f0-9]{1,4}:){0,7})[a-f0-9]{1,4}(%[a-z]+[0-9]+){0,1}$/i) {
  print "$ip is not a valid IPv6 address\n" and exit 1;
}

($ip, undef) = split(/\%/, $ip); # remove trailing interface identifier for link-local
my @num = split(/:/, $ip);
my $revip;
my $missing = 8 - $#num; # how many double-octets have to be expanded?
my $exp = 0; # wether or not we have already expanded
for (my $i=$#num; $i>=0; $i--) {
  if($num[$i] =~ /^$/ and not $exp) { # empty, "::"
    for (my $j=0; $j<$missing; $j++) { # expand "::" with missing zeroes
      $revip .= "0.0.0.0."; # until we have right length
    }
    $exp = 1; # record this, so we expand only once
  }
  else { # got a number string
    my @lit = split(//, $num[$i]); # split into single chars
    my $blk ='';
    for (my $j=$#lit; $j>=0; $j--) { # append single chars backwards
      $blk .= $lit[$j] . '.';
    }
    for (my $j = 0; $j < (3-$#lit); $j++) { # append "0." for every char less than 4
      $blk .= "0.";
    }
    $revip .= $blk;
  }
}
$revip .= "ip6.arpa";

print "Reverse IP is $revip\n";

Feel free to send any suggestions, questions and corrections to the webmaster link below (requires javascript).

Markus Wernig

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