Re: Fwd: ASN.1 dissector Wireshark

[Vincent Randal <vtrandal () gmail com>]

Typo: I wrote usable when I meant unusable in this sentence: "the ASN.1 was
usable by ans2wrs()"

On Mon, Apr 12, 2021 at 11:08 PM Vincent Randal <vtrandal () gmail com> wrote:

> Greetings everyone,
>
> Thank you (John) for delving into a nice description of the overall
> process. I do have a couple more questions for you and the group:
> 1. What is the meaning of "work is in progress to at least read all ASN1
> specifications" ???
> I'm trying to imagine what that implies. Does it imply some dissectors in
> ./epan/dissectors/asn1 have an .asn file that is not being read? That could
> make sense if the ASN.1 was usable by ans2wrs(). In that case the dissector
> is then manually created? Or extra work goes into the .cnf (or other files)
> to get the dissector completed?
>
> Background:
> I'm here to specify a new protocol using ASN.1 (in some sense I represent
> a standards body in your previous description). Because I believe Wireshark
> has good support for ASN.1 based dissector development I am hoping to get a
> dissector from the ASN.1 description that I will be writing.
>
> [Should the following question be a new separate question?]
> 2. How to add a new ASN.1 "foo" dissector to cmake [in
> ./epan/dissectors/asn1/CMakeLists]?
>
> I want to start with a simple ASN.1 example so I chose the "foo" protocol
> dissector mentioned here:
>
> https://www.wireshark.org/docs/wsdg_html_chunked/Asn1DissectorRequirements.html
>
> I created the 4 source code files (in foo subdirectory) from the content
> provided here:
>
> https://www.wireshark.org/docs/wsdg_html_chunked/SimpleASN1BasedDissector.html
>
> Problem: I don't know what else to do to get it to build. I want my
> dissector to be built like those in ./epan/dissectors/asn1 (i.e. not a
> plugin).
>
> I've tried adding "foo" in the list of directories in the parent
> CMakeLists.txt (but that does not affect build).
>
> By the way: The first thing I did was build wireshark-3.4.4 from source
> using cmake and that works.
>
> Vincent Randal
>
>
>
>
> On Sat, Apr 10, 2021 at 9:50 PM John Thacker <johnthacker () gmail com>
> wrote:
>
> > On Sat, Apr 10, 2021 at 11:15 PM Vincent Randal <vtrandal () gmail com>
> > wrote:
> >
> > > Greetings,  everyone. I’ve a question regarding:
> > >
> > > https://www.wireshark.org/docs/wsdg_html_chunked/Asn1DissectorRequirements.html
> > >
> > > I take this to mean Wireshark ASN.1 support might not be up to date
> > > going forward. Is that the meaning? ASN.1 has a long history. It makes
> > > sense periodically to freeze support for specific versions of ASN.1. Is
> > > that the meaning here (Changing the ASN1 file is being depreciated)? It
> > > says:
> > >
> > > The compiler needs 4 input files: an ASN.1 description of a protocol, a
> > > .cnf file, and two template files. The ASN.1 specification may have to be
> > > edited to work, however work is in progress to at least read all ASN1
> > > specifications. Changing the ASN1 file is being depreciated as this creates
> > > problems when updating protocols. The H.248 Binary encoding dissector is a
> > > good example of a dissector with relatively small changes.
> > No, I take that to mean:
> >
> > Many protocols (especially those associated with the ITU-T or ETSI) are
> > formally specified in ASN.1. Sometimes it is difficult for asn2wrs.py, the
> > ASN.1 to Wireshark dissector compiler, to translate the ASN.1 specification
> > into code that works optimally (and in rare cases, even at all) as a
> > Wireshark dissector. (Fairly trivial examples include protocols that define
> > certain fields as OCTET STRINGs of length 1, 2, 3, or 4, when in Wireshark
> > they would be better displayed as uint{8,16,24,32}, but much more
> > complicated examples exist.)
> >
> > In such cases, the preferred method is to edit the .cnf file or the
> > template files (packet-PROTOABBREV-template.[ch]), rather than edit the
> > ASN.1 description directly, as the latter comes from the official protocol
> > specification as defined by the standards body. While the ultimate goal is
> > for asn2wrs.py to at least successfully read all ASN.1 specifications and
> > produce usable output (even if the .cnf file or template files must be
> > adjusted), in rare cases it may fail to do so, and then it will be
> > necessary to edit the ASN.1 file itself. However, editing the file is
> > deprecated and should be viewed as a last resort, as any such changes would
> > have to be reapplied when the protocol is updated to a later released
> > version of the specification with a new official ASN.1 definition. Problems
> > sometimes result, as occurs anytime forked code has to be merged.
> >
> > John Thacker
> >
> > ___________________________________________________________________________
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