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[lwip-users] arp question

From: Jim Gibbons
Subject: [lwip-users] arp question
Date: Thu, 10 Feb 2005 15:58:36 -0800
User-agent: Mozilla Thunderbird 1.0 (Windows/20041206)

I have recently encountered an ultra wide band radio bridge for ethernet packets that presents problems for etharp.  The radio bridge pair modifies the source ethernet address of the packets that it bridges.  This seems to present problems for etharp_ip_input, which calls update_arp_entry with incoming source IP address and source ethernet address pairs.  Even if a correct ethernet address has previously been obtained from an ARP reply, this update_arp_entry call will overwrite it with the ethernet address that has been modified by the bridges.

The radio bridges seem to be a disservice by altering the source ethernet address as they do.  Nevertheless, they are able to work with Windows workstations, which apparently do not modify their arp entries based on the incoming IP traffic.

Who's right here?  Is it legitimate to glean ethernet source addresses from incoming IP packets, or should one rely exclusively on the information presented in ARP replies? 

I haven't been able to find any reference to the gleaning method in RFCs or my elderly copy of Stevens.  If gleaning is wrong, the solution is would seem to be as simple as deleting one's calls to etharp_ip_input.

Jim Gibbons
Gibbons and Associates, Inc.
TEL: (408) 984-1441
900 Lafayette, Suite 704, Santa Clara, CA
FAX: (408) 247-6395

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