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FTP Client and FTP Server display a different PASV response when using Windows Firewall

There are cases where the Windows Firewall changes the outbound passive (PASV) mode response from Serv-U to an FTP client. The Serv-U log shows the proper PASV port specified under Server > Settings > Advanced. The client's log, however, indicates a different port.

First published date

10/15/2018 6:42 PM

Last published date

6/15/2020 1:30 PM

Overview

There are cases where the Windows Firewall changes the outbound passive (PASV) mode response from Serv-U to an FTP client. The Serv-U log shows the proper PASV port specified under Server > Settings > Advanced. The client's log, however, indicates a different port. 

Use the following formula to calculate the PASV port:

Note: The port is specified in the last two numbers in a PASV response.

((The second to the last set of numbers x 256) + port number)

 

For example:

227 Entering Passive Mode (127,0,0,1,195,80).

((195 x 256) + 80) =50,000

Product section

Serv-U Managed File Transfer & Serv-U FTP Server

Cause

The issue is caused by the default FTP Server rule included in the firewall for port 21, which is designed for the Windows FTP service.

Resolution

  1. Open the Windows Firewall configuration page through Control Panel > Windows Firewall.
  2. Click Advanced.
  3. Click Network Connection Settings > Settings.
  4. Highlight the appropriate network interface. If more than one shows in the list, select the network interface first.
  5. Clear the FTP Server option from the Services list.

The Windows Firewall should no longer modify Serv-U's PASV response.