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Using CPAN Behind a Firewall

By Jeffrey Howard
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Occasionally, I've had trouble getting Perl modules from CPAN when using computers behind firewalls. Each of the many communication schemes fails after a lengthy timeout period. What's happening is that all of these methods of grabbing a file are all using the same basic protocol: FTP. FTP tries to open a separate connection for data, from the server to your client machine. If you're behind a firewall that doesn't allow incoming connections, the process will fail.

To fix the problem, try using FTP passive mode, which has your client machine initiate the connection for the data transfer. You can do this by setting the environment variable FTP_PASSIVE to 1. How you do this depends on your command shell. I use tcsh, so the command looks like:

setenv FTP_PASSIVE 1