Here is the simplest form of what I now have.
int_if = "rl1" ext_if = "rl0" log_if = "fxp0"
pass in on $ext_if dup-to $log_if all
pass out on $ext_if dup-to $log_if all
I've not used dup-to for a while, but you should try to add the address of your log host, to allow PF to know the MAC addres of your log host, for example.