Wireshark-dev: Re: [Wireshark-dev] The IPv6 value "24" in DLT_NULL causes Npcap's BPF filter no
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From: Guy Harris <guy@xxxxxxxxxxxx>
Date: Wed, 14 Sep 2016 20:20:22 -0700
On Sep 13, 2016, at 10:03 PM, Yang Luo <hsluoyb@xxxxxxxxx> wrote:

> But the problem is, libpcap  (which is used by Npcap) only supports the value 23 in the BPF filter code.

This is a problem in more ways than one.

For example, if you're running on FreeBSD, and are reading a pcap or pcapng file from NetBSD, and it has loopback packets, a filter checking for IPv6 packets won't work, because it'll be looking for the FreeBSD AF_INET6 (28), but IPv6 packets will have NetBSD's AF_INET6 (24).

So, when reading a capture file, the filter should check for *all* of the possible AF_INET6 values, because there's no way to determine, from the file, which AF_INET6 value was used.

When doing a live capture, it should, if possible, just use whatever the capture mechanism provides, to avoid doing other checks.

I'll fix this up; for now, just keep using 24 - it's not as if any code that wants to read an AF_NULL pcap file can rely on its OS's AF_INET6 being the packet type value *anyway*, it has to check for all platform values (which both tcpdump and Wireshark do).