Firewalls and FTP external IP address for PASV

Yesterday we came across what, at first, seemed to be a pretty odd case, and we think it’s worth sharing it with our users.

Most firewalls (we’d say all the ones we know) have NAT/PAT capabilities, and many are able to perform protocol-level inspection when the connection is not encrypted. SSH (and SFTP) are always encrypted, but FTP can be either encrypted or not; yet, theoretically protocol inspection should only prevent protocol-related attacks, not modify client requests or server responses.

Yet, yesterday a customer with a perfectly configured instance of Syncplify.me Server! was experiencing a weird behavior: FTPS/FTPES (encrypted) sessions were working perfectly, while plain FTP sessions were dropped upon every attempt to open a data connection to transfer files. Continue reading