I'm seeing an issue with a computer (running an embedded OS) transmitting over a network to an XP embedded machine.

The computer has 10 physical ethernet ports, 9 of which are on this network. I have assigned in the application, each eth port to be a unique address, and have verified when I do an "ifconfig" that each physical port has a unique mac address as well.

When I do a wireshark capture, I see that all the packets going out of the computer correctly transmit from 9 different IP addresses. However, they all have the same exact MAC address when sending. On the other hand, when the XP embedded machine responds with TCP acks, each ack packet is correctly paired with a unique IP and unique MAC address.

In summary: 9 packets sent from 9 different/unique physical ethernet ports on the same computer. They all have the same source mac address. 9 acks come back, each addressed to the unique 9 IP addresses and unique 9 MAC addresses.

Is this common? From what I can tell, the application is not doing this. And I doubt it is being done in the driver because another machine running the same driver with the same cards does not have this behavior.

Any insights?

migrated from stackoverflow.com Sep 5 '12 at 3:58

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  • 1
    Is there any device between those two machines? How the routing is setup? – Nikolai N Fetissov Aug 9 '12 at 16:19
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    I'm not sure this question belongs here. Try ServerFault.com – ewok Aug 9 '12 at 16:21
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    Where are you capturing the traffic? On the XP embedded machine, the other machine or in between? – Flanfl Aug 10 '12 at 12:09

This is why that happens!


pretty fascinating.

  • Probably not, actually. Microsoft doesn't have much to do with Apple's OSX. Also, the OP described unexpected behavior only in sending packets, not in sending and receiving. – HopelessN00b Sep 5 '12 at 20:25
  • I wouldn't discount this so quickly. I've seen this kind of behavior on various *nix OSes, most recently FreeBSD (in the form of an Isilon NAS). In my case, the real bummer was that the Isilon had interfaces on multiple networks. The documentation and the UI suggested that if you sent packets to one interface, you'd get them back out on that interface. In practice, it just chose its default route, which happened to be on the other interface. My firewall became sad with it. – Dan Pritts Jun 18 '13 at 3:53

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