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hi guys,

I'm looking at a UDP conversation and i do not understand the following. The conversation is between IP 128.x.x.x and 166.x.x.x. In the first packet I see the src ip 128.x.x.x and the src mac (let's say Vmware_00-_00_00) dst ip = 166.x.x.x dst mac (IETF-VRRP-VRID_01). no when I look on the packet sent from 166.x.x.x to 128.x.x.x the source MAC is different than IETF-VRRP-VRID_01 but the destination MAC address is the same used as the source in the first packet.

can someone please help me with this ?

thank you and best regards


asked 25 Sep '15, 02:28

adasko's gravatar image

accept rate: 0%

edited 25 Sep '15, 14:40

Guy%20Harris's gravatar image

Guy Harris ♦♦

That was my assumption as well. BUT what I don't understand, is why in the packet sent back (2nd packet) the dst mac is set to the MAC address of the 128.x.x.x host. I mean if both are behind a router shouldn't the dst mac address in the second packet by the MAC of the router ?

(25 Sep '15, 04:21) adasko

The unexpected mac could be the physical address of the router.

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answered 25 Sep '15, 04:37

Christian_R's gravatar image

accept rate: 16%

edited 25 Sep '15, 04:39

So when I start the conversation from A - B and I receive the response from B - A the MAC destination address in the packet from B - A is the MAC address of the source (A) device so this cannot be the physical address of the router. alt text

second packet alt text

you see now what i mean? So the src mac from first frame = dst mac in the second frame , but if there is a router in between, the dst mac in the 2nd packet should be set the routers interface

(25 Sep '15, 04:54) adasko

So, from my point of view it looks like expected. So the question may be, where has been the capture Point.

(25 Sep '15, 05:22) Christian_R

But if there is a router between (both look to me, to be not on same sub network). So how can the second packet have the dst mac set to the mac of host A if it's not in hosts B network ?

(25 Sep '15, 05:28) adasko

Because you are tracing in the subnet of the VMware host.

(25 Sep '15, 06:42) Christian_R

It starts with a packet MAC(A) to MAC(V), where MAC(V) is actually a virtual router address. It does get forwarded to the server by one of the actual routers in your net.

When the response packet comes back from the server this packet then gets forwarded via a router i your net and this router uses it's own MAC(R), not the virtual router MAC(V), as source MAC.

(25 Sep '15, 07:01) Jaap ♦

Jaap, i think i know what i was doing wrong. I took the capture at Server A that is initiating the conversation. When looking at the second packet in the conversation I was (for any reasons) looking and the frames from perspective of the Server B, but as I'm capturing on device A I will see the frame addressing from router to Server A. Am I now correct ?

(25 Sep '15, 07:28) adasko

Yes now you are correct. That is what I have meant with the point of capturing.

(25 Sep '15, 07:31) Christian_R
showing 5 of 7 show 2 more comments

From the MAC address you listed it seems that you're talking to a server via a router using Virtual Router Redundancy Protocol.

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answered 25 Sep '15, 03:03

Jaap's gravatar image

Jaap ♦
accept rate: 14%

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question asked: 25 Sep '15, 02:28

question was seen: 3,268 times

last updated: 25 Sep '15, 14:40

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