monitoring IDS in a NATTed environment

| 68.33.256.x
|  <--DMZ ( network). IDS resides here
internal router
internal network

If I have someone on the internet attacking me, wouldnt the source IP be 192.168.1.x
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As far as I know NAT should work somthing like this:

Outgoing Packet:

before router-   src:192.168.1.x:dest:<internet IP>
after router-     src:<public IP of modem>:dest:<internet IP>

Incoming packet:

before router-     src:<internet IP>:dest:<public IP of modem>
after router-        src:<internet IP>:dest:192.168.1.x

So any attacks coming from the internet will show the actual IP of the attacker.

Also ive noticed that some "hackers" try to spoof and internal IP address, to fool the router into thinking they are on your network... but a good NAT router should
detect that and stop it.

If anyone wants to correct me, feel free.

Hope this helps


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>If I have someone on the internet attacking me, wouldnt the source IP be 192.168.1.x

No, the source IP does not change as it is routed to your site,  it is the destination IP that will change. So the source will be the public ip.
Hopefully your security policy will mitigate the attack and not allow him access. If your boxes are compromised in the DMZ then traffic could originate/source
 from 192.168.1.x.


dissolvedAuthor Commented:
I got ya. So only my internal traffic is modified at layer 3. Public traffic isnt modified at layer 3
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Im pretty sure thats how it works.
Yeah....normally IDS or IPS will be placed in a place where it can "lsiten" to the most the traffic...
Added to Dr_Binks & harbor...

just my 2 cents..Its better to have a monitoring @ Firewall..Some times the attacker may succesfully bypass the routers n stuff n come in to the internal network..:)
dissolvedAuthor Commented:
thanks fellas
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