Odd Issue With Office Internet And WinSCP

WinSCP,

I have an odd issue which I do not think is a WinSCP issue but I have no idea how to resolve it. I was hoping you could lead me in the right direction. We are using the latest version of WinSCP.

I have a user who is on a laptop and uses WinSCP to connect to a FTP server. The office network infrastructure consists of a WiFi router which connects to a switch. The switch connects to a gateway server run by the landlord and from there goes out to the Internet. The protocol is FTP with TLS/SSL Implicit encryption. It uses a certificate for verification.

When I provided the laptop it was running Windows 10 and WinSCP worked fine everywhere including the office. At some point the user's laptop updated to Windows 11 and I was not notified so the roll back period ended. The user is not sure but sometime before or after the update WinSCP stopped working all the time in the office.

When using WinSCP in the office it goes through the connection process. It successfully loads the client certificate. It connects to the server. A TLS connection is established. It says connected and then starting the session. Finally it reaches Reading the remote directory and just hangs there for a bit. After that it gives an error message:

Server sent passive reply with unroutable address X.X.X.X, using host address instead.
Timeout detected. (data connection)
Could not retrieve directory listing
Error listing directory '/'.

Here is where it gets odd. If the user takes the laptop home and uses it on her home Internet it works fine. In addition we tried it with a Verizon Jetpack hotspot and it also works fine. Further the user has a Windows 7 desktop in the office (which she should not be using) with WinSCP configured the same way as the laptop and that also works fine.

I thought the issue may be the office WiFi connection so I disconnected the RJ-45 from the desktop and plugged that into the laptop. I tested to make sure it was using the cable connection before trying WinSCP again. WinSCP still got stuck at the same point with the same error message.

I then thought it may be because of the Windows 11 update. So I got a brand new laptop out of the box which still had Windows 10 on it. I set it up with WinSCP at home and it worked fine. I took it into the office and it had the same exact issue where it hung at reading the remote directory. I also tested with the cable plugged in and it also did not work.

I went through some settings to see if that would work:

Turning off passive mode did not work at all.
I used WhatsmyIP to find the external IP address and put that in under Network > Use the following external IP address which did not work.
Under Advanced site settings I changed Force IP address for passive mode connection to on which did not fix it.
Under Advanced site settings I changed Force IP address for passive mode connection to off which got rid of the passive mode part of the error message above but still got

Timeout detected. (data connection), Could not retrieve directory listing, and Error listing directory '/'.

I turned off Use MLSD command of directory listing and it did successfully connect but there were no files or directory showing on the site I was connecting to. Then it timed out after a bit.

All my Google searches did not yield any results nor did looking at the issues on this forum.

Now clearly WinSCP is working on the laptops but I am at a total loss as to why it won't work in the office using either WiFi or direct cable for both laptops, yet it still works fine with the desktop using the same connection. The only thing I can think of at this point is there is an issue with open ports on the landlord's gateway, but then why would the desktop still work?

If you would be so kind as to point me possibly in the right direction I would greatly appreciate it.

It looks like the office firewall or NAT blocks the data ports of the FTP server.

Thank you much! I will contact the landlord when he gets back and see what we can do.

WinSCP,

This is a follow-up to my previous post.

We found out the landlord had a new firewall installed in February 2022. Though they used the same configuration as the old firewall there was one difference. For outbound security they changed it from packet filtering to proxy.

I had them change it back to packet filtering and WinSCP worked without issue.

So that was the cause of the issue.

Thank you for all your help!

Thanks for your feedback.