Ok then, I've just test again the Ulimit command with the correct syntax and it's working perfectly.
It was my bad, the Debian Distrib need the following syntax: ulimit [Options] Amount of FD BUT in fact the command is ulimit -SH -n 256000 instead of the ulimit -SHn 256000 as we could believe it if we respect the syntax exemple. Many thanks SQUID users and team ;)
