proftpd dropping connections

[EmBee]

I'm having issues with proftpd dropping connections after a very short amount of time.

All timeout settings in the configuration file and the router have been modified to prevent this from happening. For large files, the data connection stays alive, but the control connection dies after x minutes. After the data file finishes, the client just sits there not knowing what to do because it doesn't receive the proper response from proftpd on the control connection indicating that the transfer has completed.

I am being forced to perform PASV connections due to firewall restrictions on the server side.

Is this proftpd's fault, or is it the ISP's fault? How can I find out?

Thanks.

[schumaku]

If I understand you correc,t the ftp-control session is dropped, while the ftp-data connection remains transfering data?

Any router doing ftp-ALG in between? Be aware that many simple NAT routers are dropping the oldest sessions if the NAT pool or connection table is exhausted due to intensive traffic for example. What router is in place?

What says the cliente, i.e. FileZilla?

Plain ftp, or ftps/ftpes ? If it's ftpes, please check viewtopic.php?f=182&t=41148

What file sizes are in transfer?

If plain text ftp, capture the ftp-control and ftp-data traffic on the ftp client, on the NAS side, or if feasible on the security appliance...