User A at Company A is trying to transfer files to Company B, using FTP.
User A can connect to the FTP server at Company B correctly, but User A cannot get a directory listing or upload files.
The session hangs.
What are two possible causes for this problem? (Choose two.)
Click on the arrows to vote for the correct answer
A. B. C. D. E.AC.
The issue is that User A is not able to get a directory listing or upload files while transferring files from Company A to Company B via FTP. The session hangs.
There can be several reasons for this issue. Two possible causes are:
A. Active FTP is being used, and the firewall at Company A is not allowing the returning data connection to be initiated from the FTP server at Company B.
FTP uses two separate connections: a control connection (TCP port 21) for sending commands between the client and the server, and a data connection for transferring files. Active FTP mode is a mode where the client initiates both connections to the server, whereas in passive FTP mode, the server initiates the data connection to the client.
In active FTP mode, the client initiates the data connection to the server on TCP port 20. However, due to the stateful nature of firewalls, the firewall at Company A may not be allowing the returning data connection to be initiated from the FTP server at Company B. This can result in the session hanging.
B. Passive FTP is being used, and the firewall at Company A is not allowing the returning data connection to be initiated from the FTP server at Company B.
In passive FTP mode, the client initiates the control connection to the server on TCP port 21 as usual, but the server initiates the data connection to the client on a random high port. The client then connects to this high port on the server to receive the data. However, the firewall at Company A may not be allowing the returning data connection to be initiated from the FTP server at Company B, resulting in the session hanging.
C. At Company A, active FTP is being used with a non-application aware firewall applying NAT to the source address of User A only, is not a possible cause.
D. The FTP server administrator at Company B has disallowed User A from accessing files on that server, is also not a possible cause since User A can connect to the FTP server.
E. Passive FTP is being used, and the firewall at Company B is not allowing connections through to port 20 on the FTP server, is also not a possible cause since in passive FTP mode, the data connection is initiated from the server to the client on a random high port, and not on port 20.
In summary, the possible causes for the issue could be that the firewall at Company A is not allowing the returning data connection to be initiated from the FTP server at Company B, whether the FTP mode is active or passive.