CPAN is an essential tool for Perl developers.  With the “cpan” command, you can install whole Perl libraries effortlessly.  However, on many default installations of cygwin, cpan simply hangs:

cpan> install LWP
CPAN: Storable loaded ok
Going to read /root/.cpan/Metadata
Database was generated on Sat, 26 Feb 2005 19:48:34 GMT
LWP not available
LWP not available
CPAN: Net::FTP loaded ok
Fetching with Net::FTP:
ftp://ftp.sonic.net/mirrors/CPAN/au…01mailrc.txt.gz

As it turns out, Net::FTP defaults to non-passive FTP, which conflicts with many firewalls since it requires a connection from the FTP server back to the FTP client.

How can you fix this?  Set the FTP_PASSIVE environment variable to 1:

BASH>  export FTP_PASSIVE=1 
DOS> set FTP_PASSIVE=1

This allows cpan to successfully retrieve files.

If you are creating a Net::FTP object in Perl, you can use:

$ftp=Net::FTP->new(HOST,[your options,]Passive   => ‘1’)

This will force the FTP connection to be passive and work through firewalls.