Google Compute Engine FTP Firewall -

i have ftp working on localhost on google compute engine. added firewall rule allow port 21 still cannot connect world ftp.

any ideas?

ftp complex protocol uses multiple tcp connections, feature causing trouble. port 21 used command channel data transfer done using new tcp connection necessary endpoints (i.e. port numbers) gets dynamically allocated , communicated inside command channel.

this behavior makes impossible use securely simple firewalls, i.e. need have wide range of ports open time. more intelligent firewalls provide helpers watch command connection , dynamically open ports. these loose if use encryption, i.e. ftp tls (ftps), because can no longer inspect command connection.

thus if there direct connection between client , server without firewalls in between ftp works fine. if 1 side behind firewall or nat (like home users) might have success after playing around active or passive ftp mode. if both sides behind firewall or nat have lost.

instead of using ftp recommend use sftp. protocol based on ssh , uses single tcp connection. can configured firewalls. apart more secure, i.e. encrypted default. lots of clients filezilla or winscp support it.