Proxy arp to a Loopback address within the same subnet?

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Dec 14th, 2007
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I have a problem that might require some special form of proxy arp:

We have a big subnet (/20) with a lot of servers that are configured to use 2 DNS servers in the same subnet.

We have a new DNS appliance in that same subnet (through its LAN1 interface), and we want to migrate the addresses of the existing DNS servers on this appliance. But unfortunately this appliance does not support secondary addresses on its interfaces, but only Loopback addresses.

So we migrate the existing DNS servers' IP addresses to Loopback addresses on the new appliance and then update the subnets gateway with static host routes to the Loopback addresses via the appliance LAN1 interface.

All servers outside the subnet can now reach the DNS service on the Loopbacks.

But all servers within the subnet cannot!

So we configure proxy-arp for those Loopback addresses on the router.

But this does not work either because the router does not answer the ARP request when the destination (appliance LAN1) is on the same subnet as the answer came from!

Is there a way to force the router to answer the arp request?



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Edison Ortiz Fri, 12/14/2007 - 07:56
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> Is there a way to force the router to answer the arp request?

I'm afraid not. The only solution I may suggest and I'm afraid it may cause other side-effects, it's changing the subnet mask on the servers trying to read the DNS w/ loopbacks.

Changing the subnet masks will force the server(s) to reach the router if the DNS IP isn't on the same subnet range.

Francois Tallet Fri, 12/14/2007 - 09:47
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If your IOS supports the "ip local-proxy-arp", that should do it. This command was introduced for private vlan, so I'm not sure it's going to be available on your platform.




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