These are some notes on the FTP
protocol, and on the ftp command line client.
Active Vs Passive Mode
Two network connection are required for FTP transfers. In active mode, the client initiates a connection to the server's port 21, and the server initiates a connection back to the client's port 21. This incoming connection (from the client's point of view) is often blocked by firewalls, and/or not available in LANs that are NATed. To overcome this problem, in passive mode, the client initiates both connections to the server.
In the ftp CLI utility, the default mode is active. You can switch
between modes with the passive command:
[email protected] ~ $ ftp
ftp> open ftp.zindilis.net
Name (ftp.zindilis.net:marios):
331 Please specify the password.
Password:
ftp> passive
Passive mode on.
ftp> passive
Passive mode off.
ftp>
There is no active command, modes are toggled with passive.
ASCII Vs Binary Transfers
There are two transfer modes, ASCII and Binary. In ASCII, files are transferred as characters, some of which (like newlines) are converted to match the operating system of the client. This works well for text files, but is almost certain to corrupt any other type of file. In binary mode, files are transferred unaltered. This helps maintain the integrity of files, but increases the possibility of text files appearing broken when transferred between different operating systems.
In the ftp CLI utility, the commands ascii and binary switch
transfer modes:
[email protected] ~ $ ftp
ftp> open ftp.zindilis.net
Name (ftp.zindilis.net:marios):
331 Please specify the password.
Password:
ftp> ascii
200 Switching to ASCII mode.
ftp> binary
200 Switching to Binary mode.
ftp>