Re: Parsing partial IPv6 prefix - newbie question

[Alexis La Goutte <alexis.lagoutte () gmail com>]

Hi Tomek,


On Wed, Jul 13, 2016 at 11:20 AM, Tomek Mrugalski <
tomasz.mrugalski () gmail com> wrote:

> Hi!
>
> Long time user, first time trying to extend the code and would like to
> ask for a bit of guidance. I'm trying to extend DHCPv6 (packet-dhcpv6.c)
> dissector to support RFC7598. Couple options in that RFC uses new option
> format that stores IPv6 prefix as two fields:
>
> 1. prefix-len (1 octet, specifies length in bits)
> 2. prefix (minimum number of octets necessary to store the most
> significant bits)
>
> The prefix field can be of any length between 1 and 16.
>
> This is the recommended way for new options (see RFC7227, Section 5.3),
> so we will see more of this formatting in the future.
>
> For specific example, see S46 DMR option here:
> https://tools.ietf.org/html/rfc7598#section-4.3
>
> How can I parse and display this in wireshark?
I'd like to display it as human readable prefix, e.g. 2001:db8::/32. I
> could create a separate 16 bytes long buffer, then copy first
> (prefix-len) bytes and pad them with zeros, but I don't know how to
>
> Do you have look icmpv6.c dissector ? if i remember there is the same case
for add prefix (like /XX), you can use proto_item_append_text(ti, "/%u",
prefix_len); for example


> So far I came up with the following code: http://pastebin.com/wsFPF4Kp
> (see lines 1987 to 1999). It sorta works, but displays prefix len as
> separate field and part of the prefix as hex. Although that matches
> on-wire format, that's not really user friendly way.
>
> I've made the traffic capture available in case it is useful:
> http://git.kea.isc.org/~tomek/lw4over6/dhcpv6-lw4over6-rfc7598.pcapng
> See packet 6 or 8.
Can you push directly on Gerrit ( https://code.wireshark.org/review  (you
can set WIP on commit log), it is more easy for review and try...
and push also the pcap on BugTracker ( https://bugs.wireshark.org )

> Not sure if it changes anything, but I'm more than willing to contribute
> the code once it's functional. Finally, there's a bunch
> of DHCPv6 options that were published, but are not supported yet. Do you
> prefer to get them all in one patch or should I split them up into
> separate patches?
If it is options from the same RFC, you can push only a patch with all
options.

Cheers,

> Thanks in advance,
> Tomek


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