natxo has asked for the wisdom of the Perl Monks concerning the following question:

I would like to convert a blob holding ip address(es) (it's the dnsrecord attribute of the MS AD ldap dnsnode objectclass.

It looks like this (ldif format):

dnsRecord:: BAABAAXwAACkAAAAAAABLAAAAAAAAAAArBQEKA==

This is the format of the dnsRecord string: https://msdn.microsoft.com/en-us/library/ee898781.aspx

I am really not a Perl guru, so I would really appreciate some handholding ;-) with unpacking this. Any help greatly appreciated. I will look into perlpacktut but I am a bit over my head with it right now.

update: I have tried this: 

my $blob = "BAABAAXwAACGAAAAAAABLAAAAAAAAAAArBQEKA==";

my (
    $dataLength,    # 2 bytes
    $type,          # 2 bytes
    $version,       # 1 byte
    $rank,          # 1 byte
    $flags,         # 2 bytes 
    $serial,        # 4 bytes 
    $ttl,           # 4 bytes 
    $reserved,      # 4 bytes 
    $timestamp,     # 4 bytes
    $data ) = unpack( 'S S C C S L N L L a*', $blob );

print $dataLength, "\n";
print "$type\n";
print "$version\n";
print "$rank\n";
print "$flags\n";
print "$serial\n";
print "$ttl\n";
print "$reserved\n";
print "$timestamp\n";
print "$data\n";

$ perl /tmp/kkk.pl

16706
16961
65
65
30552
1195589953
1094795585
1111572801
1094795596
AAAAAAAArBQEKA==

[download]
Obviously not what I expected.

Thanks in advance.

Replies are listed 'Best First'.
Re: unpack blob
by VinsWorldcom (Prior) on Jan 12, 2016 at 22:07 UTC

    I think you data is base64 encoded. Try decoding it first:

    use Socket;
my $out = `echo BAABAAXwAACkAAAAAAABLAAAAAAAAAAArBQEKA==|base64 -d`;

print (unpack "H*", $out);
print "\n";
print substr ((unpack "H*", $out), -8);
print "\n";
print inet_ntoa(substr $out, -4);

    [download]

    and output

    VinsWorldcom@C:\Users\VinsWorldcom\tmp> test.pl
0400010005f00000a40000000000012c0000000000000000ac140428
ac140428
172.20.4.40

    [download]
[reply]
[d/l]
[select]
      Instead of qx, echo and base64, you can use MIME::Base64 (in core since 5.8) and its decode_base64 function. 
      ($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

      [download]
[reply]
[d/l]
[select]

        Obviously a better solution than my backticks and base64 system command. A quick Google turned up Mime::Base64 for base64 in Perl, I did *not* go a step further to see if it was in core and I knew it would not be CPAN module I had downloaded so didn't even try, assumed I didn't have it and rolled something quick.

        Thanks for the tip! Next time I'll check 'corelist' before backtick-ing out.

[reply]
      We got a winner!

      I need to read it a bit more carefully to understand what you just did, but it works ;-)

      Thanks!

[reply]

        Combining all the responses:

        #!perl

use strict;
use warnings;
use Socket;
use MIME::Base64; # in core, thanks choroba

my $blob = "BAABAAXwAACGAAAAAAABLAAAAAAAAAAArBQEKA==";

$blob = decode_base64($blob);

my (
    $dataLength,    # 2 bytes
    $type,          # 2 bytes
    $version,       # 1 byte
    $rank,          # 1 byte
    $flags,         # 2 bytes 
    $serial,        # 4 bytes 
    $ttl,           # 4 bytes 
    $reserved,      # 4 bytes 
    $timestamp,     # 4 bytes
    $data ) = unpack( 'S S C C S L N L L a*', $blob );
# http://www.perlmonks.org/?node_id=1152688 see Tux comment

print $dataLength, "\n";
print "$type\n";
print "$version\n";
print "$rank\n";
print "$flags\n";
print "$serial\n";
print "$ttl\n";
print "$reserved\n";
print "$timestamp\n";
print inet_ntoa($data) . "\n";

        [download]
[reply]
[d/l]
Re: unpack blob
by 1nickt (Canon) on Jan 12, 2016 at 22:06 UTC

    Edit: See this answer.

    Try double quotes: 
    unpack( "S S C C S L N L L a*", $blob );

    [download]
    The way forward always starts with a minimal test.
[reply]
[d/l]
      hi, thanks, unfortunately same result.
[reply]

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