[dpdk-dev] tcpdump support in DPDK 2.3

[Bruce Richardson]

On Mon, Dec 14, 2015 at 05:36:13PM -0500, Matthew Hall wrote:
> On Mon, Dec 14, 2015 at 04:29:41PM -0500, Kyle Larose wrote:
> > I've seen lots of ideas and options tossed around which would solve
> > some or all of the above items, but nobody actually committing to
> > anything. What can we do to actually agree on a solution to go and
> > implement? I'm relatively new to the community, so I don't really know
> > how this stuff works. Do people typically form a working group where
> > they go off and discuss the problem, and then come back to the main
> > community with a proposal? Or do people just submit RFCs independently
> > with their own ideas?
> > 
> > Thanks,
> > Kyle
> 
> I am getting the impression of a misplaced sense of urgency / panic. I don't 
> think anybody came up with a reason why we have to answer all these questions 
> tremendously quickly. It will take some more time, particularly with the 
> holidays, for the developers to finish the last bug fixes on the current 
> release before they have time to discuss 2.3 features.
> 
> When that happens, someone working on DPDK full time will be identified as the 
> leader for the feature, that will lead the effort on PCAP, and help us 
> formulate the plan. Until then, what we really could use at this point is not 
> necessarily more writings and speculation, but an answer on some key tech 
> questions, particularly from some kernel guys:
> 
> 1) How do we get the pcap filter string and/or BPF opcode vector from libpcap 
> / tcpdump / tshark / wireshark, into the DPDK application? There we can 
> compile it using the user-space bpfjit, so we can filter the packets at very 
> high speeds and not end up breaking everything doing a ton of stupid copies 
> when somebody does a capture of one flow on his i40e device or such. libpcap 
> is crappy about this, as it sends it all over syscalls which are always 
> assuming the kernel is on the other end, which is a bad assumption on their 
> part but many decades old and not so easy to fix.
> 
> 2) How do we get the matched packets back out to the extcap or libpcap? From 
> what I saw extcap is tshark / wireshark only, which are 1) GPL licensed in 
> various ways, 2) not as widely used as libpcap. So using only extcap might be 
> kind of crappy.
> 
> 3) For libpcap to work, maybe it will help if some of our kernel guys can help 
> us find out how to "detect" the kernel put a BPF capture filter onto a TUN / 
> TAP interface, and copy that filter to the DPDK app. Then, take any matched 
> packets and write them back onto the TUN / TAP. This would also be super 
> efficient and work with more off-the-shelf tools besides just tshark / 
> wireshark.
> 
> If we don't find the answers for these items I don't think we have a path to a 
> working solution, forgetting about all the nice-to-have points such as UX 
> issues, troubleshooting, debugging, etc.
> 
> Matthew.

Hi,

we are currently doing some investigation and prototyping for this feature.
Our current thinking is the following:
* to allow dynamic control of the filtering, we are thinking of making use of
  the multi-process infrastructure in DPDK. A secondary process can attach to a
  primary at runtime and provide the packet filtering and dumping capability.
* ideally we want to create a generic packet mirroring callback inside the EAL,
  that can be set up to mirror packets going through Rx/Tx on an ethdev.
* using this, packets being received on the port to be monitored are sent via
  an rte_ring (ring ethdev) to the secondary process which takes those packets
  and does any filtering on them. [This would be where BPF could fit into
  things, but it's not something we have looked at yet.]
* initially we plan to have the secondary process then write packets to a pcap
  file using a pcap PMD, but down the road if we get other PMDs, like a KNI PMD
  or a TAP device PMD, those could be used as targets instead.

This implementation we hope should provide enough hooks to enable the standard
tools to be used for monitoring and capturing packets. We will send out draft
implementation code for various parts of this as soon as we have it.

Additional feedback welcome, as always. :-)

Regards,
/Bruce