Shulou(Shulou.com)06/01 Report--

Today, an error was encountered when setting up filezilla, resulting in the client being unable to communicate with the server segment normally.

Port

In active mode, after the command channel is established between client random port and server port 21, client tells server local data port through port command channel, and server actively connects the port provided by client from native port 20.

Pasv

In passive mode, after the command channel is established between client random port and server port 21, server informs client local data port through pasv command, and server local random port passively accepts client data request.

Switching mode: pasv mode is used by default, which can be switched through passive in command mode.

Analysis:

After testing, it is found that if the client connects to ftp and uses passive to switch to port mode, everything will be fine; on the contrary, if pasv mode is used, it will constantly prompt for timeout to break the connection.

WIN7 ftp test, 503error, linux ftp test, 227error.

The client has tried to change more than one PC, and the failure remains the same. It can be confirmed that the problem should be on the server side. In addition, during the connection process, only the data channel cannot be established, so there should be a policy blocking between the server and the client.

After inspection, it is found that the firewall on the server has set the FTP release policy, but the policy only releases the port mode, and the random port used by pasv is not in the policy, resulting in blocking.

Solution:

First, set the port range used in PASV mode in server, and then release the corresponding port in the firewall policy. Troubleshooting at this point.

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