Inserting a nat solution into a network while it's not the gateway

Question

I have tried this a few different ways. Currently trying with pf on freebsd 8.2

I am trying to insert a nat solution into an existing network that will redirect traffic from an outside ip address to an inside ip address on all ports (static nat) but I also want to translate the source address.

Current network.

hosta
192.168.1.2/24 

gw
192.168.1.1/24

outsidehost
10.0.0.1/24 

natbox
em0 192.168.1.3/24 (used to manage the box)
em1 10.0.0.2/24 (outside address same lan as outsidehost)
em0_alias0 192.168.1.4/24 (inside address same lan as hosta)
route 192.168.1.0/24 192.168.1.1
route 0.0.0.0 0.0.0.0 10.0.0.1

I want outsidehost to be able to telnet to 192.168.1.3 by telneting(sp) to 10.0.0.2

For this to work I assume I will have to change the source of the packet as it leaves em0 or it will get lost on the way back to em1.

So the flow goes like this:

• from outsidehost telnet 10.0.0.2
• change source address to 192.168.1.4
• redirect traffic for 10.0.0.2 to 192.168.1.2
• the packet leaves with src 192.168.1.4 goes to 192.168.1.2 then gets sent back to 192.168.1.4 translates back to whatever the source addy was in this case 10.0.0.1

I keep thinking this can be done with

binat and rdr but I can't figure out the syntax.

How can I get this done?

Answer 1

I ended up going with iptables under linux to accomplish this.

For, IP forwarding needs to be turned on:

echo net.ipv4.ip_forward=1 >> /etc/sysctl.conf

And set the following rules:

iptables -F -t nat
# flush the NAT Table.
iptables -t nat -P INPUT DROP
# set the input chain on the NAT table to DROP by default. 
# This way any traffic not allowed by defining a source address gets dropped.
# If you don't provide a -s address below it will allow all hosts from anywhere
# to reach the inside address via the outside ip. 

iptables -t nat -A PREROUTING -s 10.0.0.1 -d 10.0.0.2 \
         -j DNAT --destination-address 192.168.1.3 
# define the source and destination of the traffic allowed through.
# Change the dest address to our inside host. 

iptable -t nat -A INPUT -s 192.168.0.0/24 -J ALLOW
# Drop all traffic on sourcing from inside subnet. 
# This won't apply to traffic that matches the rule above
# as the source address will change in the next rule. 

iptables -t nat -A POSTROUTING -d 192.168.1.3 \
         -j SNAT --source-address 192.168.1.4
# here is the insert magic. Change the source address of any traffic destined
# for our inside host to our vip or owned inside address.
# This way the traffic is routed back to us at the FW.