Linux – Bridge between two ports on the same network interface on a Linux server

I guess there are many ways to accomplish this. Keeping in mind what you want to do, I would:

 hostA on netA
     |
 --------------------
   IDS iface on netA
    IDS Host
   IDS iface on netB
 --------------------
     |
 hostB on netB
     |
    inet

hostB would have a default gateway which as it should already be set (a router on your ISP provider), and you should add a route to netA through IDS host (on hostB):

ip route add <netA> via <IDS Host on netB>

IDS Host should forward all traffic (or just the one you want):

echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -P FORWARD ACCEPT
iptables -I FORWARD 1 -j ACCEPT

This will enable forwarding in the kernel, and allow all traffic to go throuth this box. Last, hosts on netA should have as default gateway IDS host on netA (on any host on netA):

ip route add default via <IDS on netA>

So, all you need to do is, on your IDS, have a network interface on vmnetA and another one on vmnetB.

Use the tool called "socat", it is great tool for such things and it is already packaged in many linux distribution. Read about it here : http://www.dest-unreach.org/socat/doc/README

Port forwarding example with socat :

socat TCP4-LISTEN:80,fork TCP4:www.yourdomain.org:8080

This redirect all TCP connections on port 80 to www.yourdomain.org port 8080 TCP.