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How to see the FCS in Ethernet frames?

asked 2018-05-05 07:24:15 +0000

Student gravatar image

updated 2019-01-04 18:25:01 +0000

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 2018-05-05 08:08:38 +0000

NJL gravatar image

updated 2018-05-05 08:16:40 +0000

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:

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You can do this with an Intel NIC on windows, too:

Christian_R gravatar imageChristian_R ( 2018-05-05 20:05:08 +0000 )edit

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

NJL gravatar imageNJL ( 2018-05-05 20:18:15 +0000 )edit

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

cmaynard gravatar imagecmaynard ( 2018-05-07 14:52:40 +0000 )edit

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

Christian_R gravatar imageChristian_R ( 2018-05-07 14:58:12 +0000 )edit

answered 2018-10-04 15:42:17 +0000

Try this:

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: 2018-05-05 07:24:15 +0000

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Last updated: Oct 04 '18