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How to see the FCS in Ethernet frames?

asked May 5 '18

Student gravatar image

updated Jan 4 '19

cmaynard gravatar image

When using wireshark to sniff ethercat frames (while a Twincat project is working), I can see all but the FCS

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answered May 5 '18

NJL gravatar image

updated May 5 '18

Depending on your OS and/or how you captured your traffic it's highly likely it's not possible.

Personally I've never seen it even when capturing using passive fiber TAPs (via a Network Packet Broker).

See this post for more discussion (and possible solutions) to the problem: https://osqa-ask.wireshark.org/questi...

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You can do this with an Intel NIC on windows, too: https://www.intel.com/content/www/us/...

Christian_R gravatar imageChristian_R ( May 5 '18 )

@Christian_R: Nice! Didn't know about this. Thanks!

NJL gravatar imageNJL ( May 5 '18 )

@Christian_R, that Intel article talks about VLAN tags, not Frame Check Sequences. What am I missing?

cmaynard gravatar imagecmaynard ( May 7 '18 )

@cmaynard 1—enabled (Store bad packets. Store CRCs. Do not strip 802.1Q vlan tags)

Christian_R gravatar imageChristian_R ( May 7 '18 )
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answered Oct 4 '18

Try this: https://stackoverflow.com/questions/2...

ethtool -K eth0 rx-fcs on #1 above: give us the FCS field ethtool -K eth0 rx-all on #2 above: even receive bad packets

and then Edit -> Wireshark -> Preferences -> Protocols -> Ethernet -> Assume packets have FCS

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Asked: May 5 '18

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Last updated: Oct 04 '18