netsniff-ng's known issues:
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Q: When I perform a traffic capture on the Ethernet interface, the PCAP file is
   created and packets are received but without 802.1Q header. If I use
   tshark, I get all headers but netsniff-ng removes 802.1Q headers. Is that
   normal behavior?
A: Yes and no. The way how VLAN headers are handled in PF_PACKET sockets by the
   kernel is somewhat problematic [1]. The problem in the Linux kernel is that
   some drivers already handle VLAN, others not. Those who handle it have
   different implementations, i.e. hardware acceleration and so on. So in some
   cases the VLAN tag is even stripped before entering the protocol stack, in
   some cases probably not. Bottom line is that the netdev hackers introduced
   a "hack" in PF_PACKET so that a VLAN ID is visible in some helper data
   structure that is accessible from the RX_RING. And then it gets really messy
   in the user space to artificially put the VLAN header back into the right
   place. Not mentioning about the resulting performance implications on that
   of /all/ libpcap tools since parts of the packet need to be copied for
   reassembly. A user reported the following, just to demonstrate this mess:
   Some tests were made with two machines, and it seems that results depends on
   the driver ...

    1) AR8131
        * ethtool -k eth0 gives "rx-vlan-offload: on"
        -> wireshark gets the vlan header
        -> netsniff-ng doesn't get the vlan header

        * ethtool -K eth0 rxvlan off
        -> wireshark gets twice the same vlan header (like QinQ even though
           I never sent QinQ)
        -> netsniff-ng gets the vlan header

    2) RTL8111/8168B
        * ethtool -k eth0 gives "rx-vlan-offload: on"
        -> wireshark gets the vlan header
        -> netsniff-ng doesn't get the vlan header

        * ethtool -K eth0 rxvlan off
        -> wireshark gets the vlan header
        -> netsniff-ng doesn't get the vlan header

    Even if we would agree on doing the same workaround as libpcap, we still
    will not be able to see QinQ, for instance, due to the fact that only /one/
    VLAN tag is stored in this kernel helper data structure. We think that
    there should be a good consensus on the kernel space side about what gets
    transferred to the userland.

    [1] http://lkml.indiana.edu/hypermail/linux/kernel/0710.3/3816.html

    Update (28.11.2012): the Linux kernel and also bpfc has built-in support
    for hardware accelerated VLAN filtering, even though tags might not be
    visible in the payload itself as reported here. However, the filtering
    for VLANs works reliable if your NIC supports it. bpfc example for filtering
    for any tags:

	_main:
	 ld #vlanp
	 jeq #0, drop
	 ret #-1
	drop:
	 ret #0

   Filtering for a particular VLAN tag:

	_main:
	 ld #vlant
	 jneq #10, drop
	 ret #-1
	drop:
	 ret #0

   Where 10 is VLAN ID 10 in this example. Or, more pedantic:

	_main:
	 ld #vlanp
	 jeq #0, drop
	 ld #vlant
	 jneq #10, drop
	 ret #-1
	drop:
	 ret #0

Q: When I start trafgen, my kernel crashes! What is happening?
A: We have fixed this ``bug'' in the Linux kernel under commit
   7f5c3e3a80e6654cf48dfba7cf94f88c6b505467 (http://bit.ly/PcH5Nd). Either
   update your kernel to the latest version, e.g. clone and build it from
   git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git or don't
   start multiple trafgen instances at once resp. start trafgen with flag -A
   to disable temporary socket memory tuning! Although trafgen's mechanism is
   written in a correct manner, some probably Linux internal side-effects
   cause the tigger of the BUG macro. Why tuning? In general, if not otherwise
   specified, the netsniff-ng suite tries to get a good performance on default.
   For instance, this includes things like tuning the system's socket memory,
   enabling the BPF JIT compiler, migrating the NIC's interrupt affinity and
   so on. If you don't want netsniff-ng to do this, look at the relevant cmd
   line options that disable them with ``--help'' and explicitly specify them
   on the program start.