FTP with NAT problem

[eokane]

Hi,

I have encountered a problem with FTP when going out through one router and coming back through another. Each router is running static NATs and has the same static NAT translations. However, if a connection goes out through one a router and back through another the FTP fails. Why should an IP address translation affect a higher layer protocol such as FTP? All other protocols, such as HTTP, work fine.

Cheers,

Eamon.

[pswarovsky]

Hi Eamon,

FTP uses two TCP Ports, one for the control session and a second one for data.

In your scenario, if the client is set up to make a 'passive open', the session will fail. Passive mode causes the server to send data to client containing the address and port number which the client should use to establish the data connection. This data does get translated only correct if the whole connection runs over the same NAT-router.

kind regards Peter