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TCP traffic and OpenvSwitch

asked 2022-02-06 16:59:00 +0000

devrandom gravatar image

updated 2022-02-06 18:02:29 +0000

I'm having an issue with OpenvSwitch whereby I have ping traffic passing but am unable to pass any tcp traffic (e.g. SSH).

I've attached two pcap files which show the following:

  • Host 1 ( ping -c 3 && ssh
  • Brief Pause
  • Host 2 ( ping -c 3 && ssh

As you can see, ping passes but ssh doesn't.

This seems to be in contradiction to my OVS flow config which is simple as as follows (these are the only flows configured on this OVS bridge) :

ovs-ofctl add-flow lon2local dl_type=0x800,nw_src=,nw_dst=,actions=output:73
ovs-ofctl add-flow lon2local dl_type=0x800,nw_src=,nw_dst=,actions=output:76
ovs-ofctl add-flow lon2local arp,nw_dst=,actions=output:73
ovs-ofctl add-flow lon2local arp,nw_dst=,actions=output:76

Hopefully somebody here is familiar with OVS, has seen these sort of flow issues before and can suggest how to fix it ?


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Put the capture files on a public share and then post a link to them by editing your question.

grahamb gravatar imagegrahamb ( 2022-02-06 17:34:24 +0000 )edit

thanks @grahamb !

devrandom gravatar imagedevrandom ( 2022-02-06 18:02:42 +0000 )edit

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answered 2022-02-07 08:41:06 +0000

SYN-bit gravatar image

I took a quick look at your capture files. It seems your hosts are configured with an MTU of 9000, but the ethernet frames with a payload of >1500 don't seem to get forwarded. Are you sure your intermediate network also has jumbo frames enabled?

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Thank you for your observation @SYN-bit. As far as I know jumbo frames are enabled, but I will check this today and come back to let you know.

devrandom gravatar imagedevrandom ( 2022-02-07 10:00:39 +0000 )edit

Were you able to check the jumboframe setting and pinpoint the issue @devrandom?

SYN-bit gravatar imageSYN-bit ( 2022-02-10 09:16:13 +0000 )edit

Hello @SYN-bit. At present I've switched all hosts back to using standard MTU (i.e. 1500) because it works. I did a bit of further investigation, and after a lot of internet searching, I eventually found a one-line comment on a random mailing list that explained that versions of OpenvSwitch older than X had a known bug where custom MTU settings were not actioned (i.e. no matter what the config said, it would always be 1500). So basically I need to find a maintenance window to get the switch software updated. Then (fingers crossed !!) custom MTU values should magically start working.

devrandom gravatar imagedevrandom ( 2022-02-10 11:18:20 +0000 )edit

I'll keep my fingers crossed for the upgrade too. For now, glad things are working on MTU=1500 for all systems!

SYN-bit gravatar imageSYN-bit ( 2022-02-10 11:53:13 +0000 )edit

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Asked: 2022-02-06 16:57:57 +0000

Seen: 471 times

Last updated: Feb 07 '22