Well you can’t run it as a daemon since there is no method for tcpdump (http://www.daemon-systems.org/man/tcpdump.8.html), but you can do a little trick.

You can call nohup utility which allows to run command./process or shell script that can continue running in the background after you log out from a shell. In case of any errors you need to specify where to write them so I wrote 2 >/dev/null (yes it means write them nowhere).

Final command :

nohup tcpdump -n -i eth0 -w xyz.pcap 2>/dev/null