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Linksys router as "poor mans" DMZ

Posted on 2006-06-14
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Last Modified: 2013-12-23
I recently upgraded the firmware on my Linksys router which I'm using in sort of a poor man's DMZ. It sits off of a 3rd NIC on my ISA Server behind our cisco router. The 192.168.... subnet used by the linksys router is completely separate from our internal network addressing scheme.

My mistake was not jotting down all the settings before the upgrade because they did not all port over (my bad). The part I'm struggling with is the static IP address. Because it sits behind our router I assume I can't leave it as 'obtain IP address automatically'. I'm pretty sure it was set to 'static IP address' before the firmware upgrade. I just can't remember what ip address, subnet mask and default gateway I'm supposed to use.

I tried using the IP address of the router itself (192.168.1.1) but with no luck (I was also using that same address as the default gateway). I tried using the IP address of the external interface of ISA. BIG MISTAKE. I had a major IP address conflict and no one could get to the internet fro a bit.

Somehow I've got to tell the router where the Internet is. Just can't recall how to do it. Anyone have the same problem? Any solutions?

Thanks.
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Question by:Dave_O
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Bing CISM / CISSP earned 500 total points
ID: 16927133
1. if you can access the configurations of that ISA and cisco router, get the realted IP subnet number and mask from there.
2. if you can not, run a sniffer program to learn the IP subnet number and mask by listening to the subnet connecting with ISA or cisco router.
3. the default gatway of that Linksys ought to be cisco's LAN port if the Linksys was connected to the cisco directly. otherwise, it ought to be one of the ISA's LAN IP it directly connected.
4. as for the router's previous IP, commonly, it might be .1, .2, or .254 (the last one of its subnet, depending on the value of mask)

hope it helps,
bbao
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by:Dave_O
ID: 16934283
Thanks bbao,

I finally got it working over the weekend utilizing a couple of the items you mention in your reposnse.

I was finally able to get a list of our external IP addresses from our provider and back-tracked through ISA to get the default gateway IP address as well. After a few 'trial and error' entries on the linksys router I hit the right combination and got it talking to the outside world again.

Thanks for posting. It looked like no one wanted to touch this one. Thanks for stepping up. The points are yours.

Thanks,
Dave_O  
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by:Bing CISM / CISSP
ID: 16939408
glad to help, Dave_O. :)
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