Re: Question about asn2wrs

[Pascal Quantin <pascal.quantin () gmail com>]

Hi Craig,

Le jeu. 8 nov. 2018 à 19:44, Craig Jackson <cejackson51 () gmail com> a écrit :

> I'm working on a decoder for the NISO Z39.50 protocol. This is an
> ASN.1/BER protocol used in the library automation community.
>
> There some things I'm having trouble figuring out how to configure, and
> there are also a bunch of things which have never been documented in
> asn2wrs. I'm wondering if one or more of the undocumented things could help
> me.
>
> Here's a problematic type definition:
>
>    AttributeElement  ::=  SEQUENCE{
>       attributeSet    [1]       IMPLICIT AttributeSetId OPTIONAL,
>       attributeType   [120]  IMPLICIT INTEGER,
>       attributeValue      CHOICE{
>                      numeric    [121]   IMPLICIT INTEGER,
>
>         complex    [224] IMPLICIT SEQUENCE{
>                           list     [1] IMPLICIT SEQUENCE OF
> StringOrNumeric,
>                    semanticAction  [2] IMPLICIT SEQUENCE OF INTEGER
> OPTIONAL}}}
>
> The issue is that the "attributeType" values are actually enumerations
> chosen based on the value of "attributeSet". I see how I can supply a fixed
> set of strings using FIELD_ATTR (poorly documented), but I can't figure out
> how to get control so I could specify different sets of strings. (This
> might be done by specifying alternate hf values, perhaps.)
>
> I think I could hard-code the type specifying my own choice array, but I'm
> wondering if there's a better way.

Several ASN.1 dissectors have fields with a dissection depending on another
field. See for example s1ap.cnf file and the code for HandoverType and
Source-ToTarget-TransparentContainer fields.
So you could store the attributeSet value in the packet (using the
p_add_proto_data() function) and retrieve it when decoding the
attributeType value (thanks to the p_get_proto_data() function) and select
the right value_string (by selecting the relevant hg value, taht could be
manually defined in your template file - like what is done in
packet-s1ap-template.c). the PER dissector allows you to define a hf value
to -1 to avoid getting the item added to the tree automatically and
retrieving the field so as to manually select the right hf to use. I'm not
playing with BER but presumably either is it also supported, or could
probably be added.

I agree with you that asn2wrs is not well documented. But ther are plenty
of ASN.1 based dissectors that you can look at to take ideas or check how
things were done (this is how I learned myself). Looking at the python
script itself also helps ;)

Best regards,
Pascal.

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