Edit :
vi /etc/sysctl.conf
net.inet.ip.forwarding=1

vi /etc/pf.conf
eth0 = "fxp0"
eth1 = "fxp1"

vi /etc/rc.conf.local
sendmail_flags=NO
pf=YES
check_quotas=NO
ntpd=NO
named_flags=""

Do command :
nat on $eth0 from $eth1 to any -> ($eth0)