How can i get this command to restart at boot and in the background even if i log off

I don't know if there's a snort command line option to have it fork, but you should be able to use:

snort -c snort.conf -i eth0 -D &

Which should fork it to another process and when you logoff detach from yours as the parent and keep running.

To have it run at boot, put the same line above in /etc/rc.d/rc.local (on a linux system).