Why is the connection being closed?

Question

FTP

This message comes up very often in FileZilla.

23:36:58 Error: Connection closed by server
23:36:58 Error: Unable to connect to server

Then it reconnects itself and uploads files normally.
The error can occur every minute or every 5-15 seconds, there is no exact time.
But it appears every time files are sent, no matter 10 or 100000.
I just can’t figure out who is messing up.
On the server "like" there are no restrictions.
But maybe I missed something, Centos 6.5.

Answer 1

Look towards keep-alive connection support + operating modes (passive / active).