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FTP Passive Mode on Different Port

Posted on 2005-03-09
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Hey everyone! I'm having a little problem and was wondering if anyone can help.

Here's what I have set up, i have an FTP on port 21 that works excellent. Its Passive, so after connecting, it connects to a port in the 24XXX range. I wanted a seperate FTP to run off of port 1773. When I connect to that port, it logs in and everything, but then creates a PASV command using a LOCAL ip.

It connects and authenticates to 267.282.274.43:1773 then says this:

COMMAND:>      PASV
            227 Entering Passive Mode (192,161,0,9,13,206).
COMMAND:>      LIST
STATUS:>        Connecting FTP data socket 192.161.0.9:3534...

How can i get it to create a passive connection using the EXTERNAL ip? Again, the FTP on port 21 creates a passive connection (using PASV command) using the EXTERNAL ip. Why does it change when i try to change the port?

Thank you!
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Question by:bick2000
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Dave_Dietz earned 2000 total points
ID: 13502781
I think you forgot to tell us you have a NAT device of some sort in front of the FTP server.....  ;-)

IIS 6.0 FTP will *always* respond to a PASV command with the IP address of the interface that it received the request on.  If it receives the request on IP 10.0.0.5 it will send 10.0.0.5 back in the 227 response (shows up as 10, 0, 0, 5, high, low).

Your NAT device is watching traffic on port 21 and properly translating the IP in the 227 response to the external IP address and everything is working properly.  Unfortunately unless you can configure your NAT device to also expect FTP command channel traffic on port 1773 it will not know to watch for 227 responses and will not translate them properly.

I would be willing to bet money that if you move the 'working' site to a different port for testing it will show the same symptoms as the one currently failing.

Short answer - you're probably out of luck unless you have or are going to buy an expensive NAT/Firewall device that you can configure to watch for FTP on port 1773 and translate it accordingly.

Dave Dietz
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by:bick2000
ID: 13511110
Ok, I guess that makes sense... Is there no way around this?

Thanks,

Bick
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by:Dave_Dietz
ID: 13513855
Only ways around it would be to use a different non-RFC compliant FTP serverthat allows you to hardcode the 227 response or to get a different firewall.

Sucks, but this is happening more and more often with firewalls and NAT devices springing up everywhere.....

Dave Dietz
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by:bick2000
ID: 13514195
ok thanks for the help

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