vsftpd log change process

[lists at dwsasia.com]

Hi,

I have written an daemon that is monitoring FTP uploads. I am using  
FAM to monitor the /var/log/vsftpd.log

In general it is working as it should, but there is one step were  
vsftpd is swapping the old FTP file to history like:

vsftpd.log.1
vsftpd.log.2
...

The problem is that if there is no activity on the FTP for a longer  
period (lets say 5 days), the file will be swapped at the current  
write to the log file remaining only the last log line in the log file.

What daemon is writing these log files? And how it is working? Are the  
files swapped? Renamed? Piped?

/Peter