Summary: I have a group of CGIs that check a license at runtime. The license is based on the IP of the server and it's stored in an external file. This model has been useful for a while, but I have encountered problems reading the real IP address of the machine because (I believe) the way I read the IP address is based on the hostname. Let me elaborate.

The license string is generated doing an hexdigest of the IP addres and a predefined number. I also check that the IP address is not an internal network IP (such as 10.*.*.*) or the loopback address 127.0.0.1.

Basically the license creation is done like this: 

use Sys::Hostname;
use Socket;
use Digest::MD5;
my $hostname = hostname();
my $address  = gethostbyname($hostname);
my $ip = inet_ntoa($address);
my $key = "1234567"; # arbitray number
my $md5 = Digest::MD5->new;
$md5->add($key, $ip);
my $license = $md5->hexdigest;

[download]

For checking the license is just the reverse of the above.

My methodology has some problems because sometimes the /etc/hosts resolves the hostname to 127.0.0.1 or to an internal IP instead of the "real" external IP. I obviously don't want to give licenses to internal IPs or the loopback address.

I wish there was a way to know the "real" IP address that the machine has for the exterior world. Question 1: Is there a better way to read the IP address?
Question 2: Have any of you implemented a similar licensing model, what other models have you used?

P.S. I have encrypted my code so the license thing is not easy to read. Although the only way to hide the source code is by deleting it :-) (FAQ).

Pax vobiscum.
-Andrés

Update: Thanks for your comments. Yes, I am aware that there is no way to be 100% sure that the customer cannot read the code. But as I mentioned before, this protection is good enough for this specific case.

In reply to Licensing model for CGIs using IP addres by amonroy

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