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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.




em0 (used to manage the box)
em1 (outside address same lan as outsidehost)
em0_alias0 (inside address same lan as hosta)

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

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
  • change source address to
  • redirect traffic for to
  • the packet leaves with src goes to then gets sent back to translates back to whatever the source addy was in this case

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?

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1 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 -d \
         -j DNAT --destination-address 
# define the source and destination of the traffic allowed through.
# Change the dest address to our inside host. 

iptable -t nat -A INPUT -s -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 \
         -j SNAT --source-address
# 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. 
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