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

[Paul Offord <Paul.Offord@xxxxxxxxxxxx>]

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@xxxxxxxxxxxx
LinkedIn:  https://uk.linkedin.com/in/paulofford

 

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