PCAP-NG Block Formats

[Paul Offord <Paul.Offord () advance7 com>]

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.

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

Paul Offord FBCS CITP
Chief Technical Officer
Advance7

Phone:  01279 211 668
Mobile:  07764 931 431
Email:  paul.offord () advance7 com<mailto:paul.offord () advance7 com>
LinkedIn:  https://uk.linkedin.com/in/paulofford


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