rdr on $INT_IF inet proto { tcp, udp } from $INT_IF:network to any port 53 
-> $DNS_SERVER port 53
-------------------------
# iptables -t nat -A PREROUTING -p tcp -i $INT_IF --dport 53 -j DNAT --to-destination $DNS_SERVER:53
# iptables -t nat -A PREROUTING -p udp -i $INT_IF --dport 53 -j DNAT --to-destination $DNS_SERVER:53
# iptables -t nat -A POSTROUTING -p tcp -o $EXT_IF --sport 53 -j SNAT --to-source $SNAT_IP
# iptables -t nat -A POSTROUTING -p udp -o $EXT_IF --sport 53 -j SNAT --to-source $SNAT_IP
-------------------------
rdr on $INT_IF inet proto tcp from $INT_IF:network to any port 25 -> $SMTP_HOST port 25
-------------------------
# iptables -t nat -A PREROUTING -p tcp -i $INT_IF --dport 25 -j DNAT --to-destination $SMTP_HOST:25
# iptables -t nat -A POSTROUTING -p tcp -i $EXT_IF --sport 25 -j SNAT --to-source $SNAT_IP
