Why can't I see traffic between another computer and its Ethernet peripheral?

asked 2022-08-30 12:26:30 +0000

updated 2022-08-30 13:36:56 +0000

flag of United Kingdom of Great Britain and Northern Ireland

23835 ●4 ●976 ●227 https://www.wireshark.org

I have a Windows 11 PC A connected to an Ethernet switch. Also connected to the switch is a CAN-bus-to-Ethernet bridge. PC A sends and receives TCP packets (mostly) successfully to the CAN-Ethernet bridge.

For some reason I don't understand I can't run npcap on computer A so I have also connected two other computers B and C to the same Ethernet switch both running Wireshark.

I can ping the CAN-Ethernet bridge form PCs B and C. But why can't I see any TCP packets going between PC A and the CAN-Ethernet bridge?

If I ping the CAN-Ethernet bridge, I can see the pings in Wireshark but only on the laptop doing the pinging. I don't seem to be able to capture any packets going between PC A and the CAN-Ethernet bridge (although they are communicating with each other) with WireShark on computers B or C.

If, however, I run the application on PC B or C as well as WireShark, then that computer can see the traffic between the application and the CAN-Ethernet bridge.

What do I have to do to be able to able to see the TCP packets on a computer which isn't communicating with the CAN-Ethernet bridge?

Thanks.

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i.e. it's a switch, you will normally see only traffic that traverses the switch ports you're capturing on, so neither PC B or C will see the traffic between PC A and the CAN Ethernet bridge.

grahamb ( 2022-08-30 13:39:04 +0000 )edit

Ah ok, thanks @grahamb, I didn't realise that; I'm sure I had this working many years ago so I started thinking it should work. I have just looked it up again and found that it used to work but now is generally not possible so thank you for pointing me in the right direction.

Magicbean ( 2022-08-30 15:22:54 +0000 )edit

You need to create a span port on your switch. (OR whatever it is called by your gear in use.)

hugo.vanderkooij ( 2022-08-31 05:59:47 +0000 )edit

Any broadcast type traffic should still be visible.

ajaznawaz ( 2022-09-01 08:46:20 +0000 )edit

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