Description:

FXP is part of the name of a popular Windows FTP client: 
http://www.flashfxp.com . This client has made the name "FXP" commonly used
as a synonym for site-to-site FTP transfers, for transferring a file
between two remote FTP servers without the transfer going through the
client's host. Sometimes "FXP" is referred to as a protocol; in fact, it is
not. The site-to-site transfer capability was deliberately designed into
RFC 959. More info can be found here: 
http://www.proftpd.org/docs/howto/FXP.html .


Implementation:

Actually pyftpdlib does not allow site-to-site transfers.
Because of a type of attack known as the "FTP bounce" attack: 
http://www.cert.org/advisories/CA-1997-27.html ...it will reject any connection 
from/to remote IP addresses which do not
match the client's IP address.

However, some site administrators may want to allow their servers to
support site-to-site transfers. 
pyftpdlib should provide a variable to allow administrators to disable
bounce protection for cases where FXP site-to-site transfer capability is
desired.