Decoding payload as ASN.1 DER/BER

asked 2020-06-30 13:49:45 +0000

schmittlauch
3 ●1 ●1 ●4

updated 2020-06-30 14:53:54 +0000

I designed a custom UDP-based protocol and serialise the payload as ASN.1 DER. While Wireshark claims to have BER (the non-canonical version of DER) dissecting support, I cannot find BER, DER or ASN.1 in the list of protocols in the "Decode as…" window.

An old mailing list post from 2011 claims that Wireshark used to be able to decode arbitrary payloads as ASN.1 BER and that, while the functionality was lost, it was trivial to re-introduce that. But it looks like the progress on that has stalled.

Am I missing the right way for decoding stuff as BER or is this really not possible anymore?

I am using Wireshark-qt Version 3.2.3 (Git commit f39b50865a13) on NixOS

Thanks.

answered 2020-06-30 14:20:09 +0000

Anders

5023 ●97 ●58

There is this code in packet-ber.c

    /* allow the dissection of BER/DER carried over a TCP transport
   by using "Decode As..." */
dissector_add_for_decode_as_with_preference("tcp.port", ber_handle);

You cound try it out by hand crafting a TCP packet with your BER as payload. If it works we could easily add UDP. The prefered metod of decoding asn1 coded protocol is to use the asn1 decription(s) and auto generate a dissector using the asn2wrs script.

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@Anders I just crafted a TCP packet with my ASN.1 BER payload, and the ASN.1 dissector works totally fine on it. It'd be very useful to enable that dissector for UDP as well. While I'll probably take a look at asn2wrs at some point, the raw decoded ASN.1 already is a good statring point for investigations before taking the additional effort of asn2wrs.

How to proceed? Shall I open a bug about this? I can provide dumps of both TCP and UDP example packets.

schmittlauch ( 2020-06-30 15:28:45 +0000 )edit

pushed a patch

Anders ( 2020-07-01 06:42:34 +0000 )edit

Great, I just compiled 3.2.3 with that patch and it's working fine. Thank you.

schmittlauch ( 2020-07-01 17:12:53 +0000 )edit

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