Wireshark · Wireshark-dev: Re: [Wireshark-dev] PCAP-NG Block Formats

[Graham Bloice <graham.bloice@xxxxxxxxxxxxx>]

On 10 June 2016 at 13:19, Paul Offord <Paul.Offord@xxxxxxxxxxxx> wrote:

I’m writing some code that produces PCAP-NG files that I subsequently read with Wireshark.  However, WS throws an error when I open the resulting file and it’s complaining about there not being enough data left in an Enhanced Packet Block to satisfy the stated size of an option.  I’ve think I know what’s wrong but it’s led me to a fundamental question.

 

In the documentation in https://www.winpcap.org/ntar/draft/PCAP-DumpFileFormat.html section 3.3 the diagram shows that a variable length Options block can exist at the end of an Enhanced Packet Block.  I have assumed that if I don’t want to specify options I have to add an Option Code opt_endofopt and Option Length of zero at the point, which comes to four bytes 0x00000000.

That's obsolete documentation, try here: https://github.com/pcapng/pcapng 

I've no idea if that will explain the issue, but at least you'll then be using the same spec as the Wireshark code.

 

However, I’ve noticed that Wireshark generated traces don’t seem to do this.  All the examples I have looked at have two bytes of zeros, but this happens to align the end of the packet data on a 32-bit boundary and so I’m not sure if that’s the reason for the two bytes.

 

·        If I don’t want to add options to a packet, what should I add just prior to the trailing Block Total Length value?

·        Is this a Wireshark bug (writing incorrect format PCAP-NG files)?

 

Thanks and regards…Paul

 

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Graham Bloice