Double data socket connection when FTP passive through NAT

[bripatand]

When transferring files through FTP passive mode from a Windows 7 64 bits Guest in NAT network mode on a Windows 7 64 bits Host to a FTP server, an extra FTP data connection seems to be created by Virtualbox for each file transfer. For instance, if the TCP data connection is created from FTP Client in Guest port N to port S on FTP server, we can see 2 TCP data connections are created in the Host:

• Host port M to port S on server (N is mapped to M by the NAT)
• Host port M-1 to another random port T on server.

Most of the time there is no effect as no process is listening to port T on the server but randomly T = S which mess up the FTP data connection resulting in zero file size being created on the server without any error or the connection is reset. This was seen with both a Filezilla client or a .Net client.

In attachment, you can see the Wireshark trace for Guest, Host, and Server. After each frame in FTP Cmd connection "Entering Passive mode", you can see this extra TCP Data connection in the Host (and server) which is not found in the Guest.

[Valery Ushakov]

This should have been fixed in 4.3.12

[Valery Ushakov]

Marking as fixed. Please, reopen if necessary.