Re: New pcap-ng block requires a rescan

[Guy Harris <guy () alum mit edu>]

On Jun 8, 2018, at 10:10 AM, Paul Offord <Paul.Offord () advance7 com> wrote:

> proto_register_diameter(void) contains:
>
>                /* Delay registration of Diameter fields */
>                proto_register_prefix("diameter", register_diameter_fields);
>
> I wasn’t aware of this function.

It might not help in your case.

What it *does* support is "lazy" registration of fields, where the field registration is delayed until a field with a 
given prefix needs to be used (e.g., in a custom column or in a packet-matching expression).  The callback specified as 
the second argument to proto_register_prefix() is called in that case.

This lets you use Diameter fields in custom columns, read filters, and coloring rules.

This works for Diameter because the only reason to delay registering Diameter fields is that it's time-consuming - it 
has to read the entire Diameter dictionary and process it.

In your case, where the file *itself* contains information that's used to define fields, there's no callback you can 
supply.

In order to allow your fields to be used in custom columns, read filters, and coloring rules, you will have to

        1) require that *all* TDBs in a pcapng file section appear before *any* block type that *might* (not *will*, 
but *might*), have a custom column, be affected by a coloring rule, be filtered by a display filter, or be discarded by 
a read filter - this presumably includes all packet blocks and TRBs;

        2) arrange that there be a phase in the capture-file reading process that comes *after* the file is opened but 
*before* the main record-reading loop occurs, and *before* the custom columns, read filter, and coloring rules are used.

This will require a significant API change in libwiretap and significant changes to the file reading process in 
programs that use it, so this will only work in Wireshark 3.0.
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