IP Proxy ARP Behavior

yuchenglai · ‎07-12-2012

Hi All,

I was wondering if anyone knows whether or not a router (enabled with proxy arp) will respond with its own mac-address for destinations that doesn't match any specifc routes in its routing table but matches only the router's default route.

Any insight would be extremely helpful.

Thanks,

David

Giuseppe Larosa · ‎07-12-2012

Hello David,

the Proxy ARP feature provides an ARP reply if the router knows how to route to a destination.

the router should answer even if the only matching route is the default route 0.0.0./0, because it is a valid entry in the IP routing table

Proxy ARP is intended to assist hosts that miss a default gateway in their TCP/IP stack configuration.

In the original RFC based Proxy-ARP suggested implementation (RFC 1027) ARP replies should have been provided only for destinations within the same major network of the device that performed the ARP request.

However, Cisco implementation of Proxy-ARP extended the answer to all known destinations.

Hope to help

Giuseppe

yuchenglai · ‎07-12-2012

Giuseppe,

What if I have 3 routers, A, B, and C. Router C sends an ARP request for a destination to Routers A and B. Router A sends an ARP reply to C for a specific route, but Router B later sends an ARP reply to C for a 0.0.0.0 route. Which ARP entry would C keep in its ARP table? I'm assuming Router C would keep the ARP entry that it received last from Router B. Is this understanding correct?

David

Giuseppe Larosa · ‎07-12-2012

Hello David,

relyiing on Proxy ARP for routing may lead to sub-optimal choices.

So between routers it is not recommended to rely on Proxy ARP.

The ARP reply provided by Proxy ARP is a regular ARP reply, I mean it does not contain any hint about how much specific is the route that is present in the IP routing table of the device that has answered.

So it becomes just a question of timing and of ARP implementation: depending on implementation the first answer is used and the later ARP reply is ignored or the later reply may override the first one, but the router that receives the ARP replies has no element to decide which answer is more specific, this level of detail is lost.

My guess is that modern implementations (with hardened ARP to avoid gratuitous ARP attacks) may prefer and use the first ARP reply received, but it is not an authoritative response.

Hope to help

Giuseppe

Ganesh Hariharan · ‎07-12-2012

David Lai wrote:

Hi All,

I was wondering if anyone knows whether or not a router (enabled with proxy arp) will respond with its own mac-address for destinations that doesn't match any specifc routes in its routing table but matches only the router's default route.

Any insight would be extremely helpful.

Thanks,

David

Hello David,

Proxy ARP is the method in which one host, usually a router, answers ARP requests intended for another host. The router accepts responsibility for routing packets to the destination so default route which is valid entry in routing table will be answered for proxy arp if it is enabled.

Hope to Help !!

Ganesh