Monday, 23 January 2012

Using Capture Filters with Encapsulated Packets

One of the most annoying things I found when I started working on carrier networks was that while Wireshark's display filters worked perfectly, the capture filters frequently did not. I would regularly set up a capture filter only to find that no packets at all were saved - that's a real pain if you want to pull a few easily described packets out of a 50 Mbps stream across a period of 20 minutes.

After a while I realised that my problem was related to encapsulation. Unlike the hierarchical and detailed display filters, capture filters have to be really fast - that basically means using bit masks and comparing values at fixed offsets. With plain old untagged Ethernet frames the filters work fine, however as soon as you add 802.1Q tags, PPP or MPLS suddenly all the offsets are no longer valid and anything you match will be purely coincidental.

Luckily there are filter keywords to handle that situation. All of the following adjust the offsets for you each time they are used:

vlan [x] - matches a single VLAN tag, the ID of which may optionally be specified by the user
pppoes - matches a PPPoE session header
mpls [x] - matches a single MPLS label, the number of which may optionally be specified by the user

These are very flexible - for example if you are capturing QinQ traffic, you could match all the SMTP packets using:

vlan && vlan && tcp port 25

If you know the VLAN IDs (or MPLS labels) in use, you can narrow the selection based on those. To show all the IGMP passing over a particular MPLS pseudowire with VLAN ID 200, you could use:

mpls 131066 && mpls 131068 && vlan 200 && pppoes && ip proto 2

For a long time I was using makeshift capture filters along the lines of "ether[39] = 2" to match pertinent bytes in the packet (see my next blog post for info on that) however you will probably agree this is much simpler. These filters are equally applicable to Wireshark, Tshark and tcpdump so they may be useful even when forced to capture using some really obscure UNIX box. For Tshark and tcpdump don't forget to put quotes around any expressions that use the ampersand (&).

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