In
the FTP protocol, data transfers are made on a different
connection to the control connection - and a new connection is made for
each data transfer or directory listing.
Data connections
can be made in two different ways - the server initiating the
connection (active mode) or the client initiating the connection
(passive mode). For more discussion on connect modes, see
Active and Passive Modes.
The connect mode has certain implications for FTP'ing through firewalls. If problems are experienced
using one mode, the alternative mode should be tried.
Active
Mode
To use active mode,
set the ConnectMode property as shown below:
In active mode, the
client supplies the port number to which the server connects.
This is normally a random port, but a port range can be specified (for example a
permissable range that is configured in a firewall):
The port number being sent to the server can be found from
the log file (in DEBUG mode), looking for the PORT command, e.g.
PORT 151,134,10,195,240,68
The first four numbers are the IP address, and the last two form the port number. To calculate the port number
that the server will try to connect to, multiple the first port number by 256 (2^8), and then add the second port number.
In the example it will be 240*(2^8) + 68, yielding a port number of 61508.
Passive
Mode
To use passive mode, set the
ConnectMode property as shown below:
ftpConnection.ConnectMode = FTPConnectMode.PASV;
If
problems are being experienced with file transfers and directory
listings (both of which use a new data connection each time), it is likely that a
firewall is preventing the creation of the data connection. Try
swapping from active to passive modes, or vice versa.
