Slightly stricter

/etc/pf.conf

tcp_services = "{ ssh, smtp, domain, www, pop3, auth, pop3s }"
udp_services = "{ domain }"

block all
pass out proto tcp to any port $tcp_services 
pass proto udp to any port $udp_services 

OpenBSD 4.0 and earlier needs keep state added to these rules

- load your new rules

peter@skapet:~$ sudo pfctl -f /etc/pf.conf

for syntax check only:

peter@skapet:~$ sudo pfctl -nf /etc/pf.conf

Note: only valid rule sets load, flushing rarely makes sense