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capture filter "vlan and stp" showing nothing

asked 2019-03-06 00:50:31 +0000

surenets gravatar image

What am I doing wrong? I have a dot1q trunk and am trying to capture stp traffic only. I use the filter "vlan and stp" and get nothing. but if I capture traffic with no capture filter and use "stp" as a display filter I see all the stp packets with their 802.1q tags.

all the traffic on this trunk is 802.1q tagged ...

thanks for any help ... I'm a noob. ;-)

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  1. vlan part of the filter works ... I sniffed without tags and capture of stp still doesn't work
  2. stp packets I'm seeing are pvst stuff, display filter recognizes them
  3. the BPF code lets me know that the capture filter doesn't recognize this form of stp:

(000) ldh [12]

(001) jgt #0x5dc jt 5 jf 2 (checks that the packets are 1500 bytes or less)

(002) ldb [14]

(003) jeq #0x42 jt 4 jf 5 (spanning tree SAP)

(004) ret #262144

(005) ret #0

but the packets for RPVST have SNAP PID of 0x010B

I'm guessing, but is there a way to capture filter based on the SNAP PID?

Apparently the display filtering system knows how to do it, but the capture filtering doesn't.

surenets gravatar imagesurenets ( 2019-03-06 03:34:45 +0000 )edit

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answered 2019-03-06 11:25:26 +0000

Guy Harris gravatar image

libpcap's BPF compiler doesn't generate code to test for (R)PVST+, as per libpcap issue #678. There isn't a convenient way to test for that; you'd have to use the expr relop expr mechanism in the pcap filter language to test for it - and you'd have to take the VLAN header's presence into account.

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thank you ... I added a filter "vlan and ether[18:2]=0xaaaa" and limited my capture to SNAP and that's good enough for what I'm doing.

surenets gravatar imagesurenets ( 2019-03-06 18:51:52 +0000 )edit

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Asked: 2019-03-06 00:50:31 +0000

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Last updated: Mar 06