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BER decoding fails during Varbind packet extraction due to noSuchInstance on some #3900

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nathanIL opened this issue Feb 14, 2023 · 2 comments · Fixed by #4069
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BER decoding fails during Varbind packet extraction due to noSuchInstance on some #3900

nathanIL opened this issue Feb 14, 2023 · 2 comments · Fixed by #4069
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@nathanIL
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nathanIL commented Feb 14, 2023 •
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Brief description

Any OID that has noSuchInstance (as shown in in tcpdump) fails with:

Exception has occurred: BER_Decoding_Error
Unknown prefix [81] for [b'\x81\x00']
### Already decoded ###
None
### Remaining ###

this in turn causes the whole SNMPResponse packet to be dropped/ignored.

I'm parsing a pcap file with sniff(...), no filters, doing nothing special.

Scapy version

2.5.0

Python version

3.11

Operating system

Linux x86_64

Additional environment information

No response

How to reproduce

pick some pcap where there are noSuchIsntace responses.

Basically, run tcpdump on some device and ask/get non existing OIDs

Actual result

The whole SNMP response packet is ignored

Expected result

At least return the packet with the right / extracted varbinds and ignore the noSuchInstance ones, or create new relevant type NONE ASN1_Tag doesnt handle that.

Related resources

No response
@nathanIL nathanIL changed the title BER decoding fails during Varbind packets due to noSuchInstance BER decoding fails during Varbind packet extraction due to noSuchInstance on some Feb 14, 2023
@gpotter2
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Hi, could you provide a pcap? (or something reproducible?) Thanks

@nathanIL
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Hi, could you provide a pcap? (or something reproducible?) Thanks

sure, i will generate one and attach asap

@christian-sahlmann christian-sahlmann mentioned this issue Jul 13, 2023
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gpotter2 pushed a commit that referenced this issue Jul 23, 2023
@christian-sahlmann @gpotter2
add support for exceptions in SNMP varbind responses
5ac3c28 
[RFC 3416](https://datatracker.ietf.org/doc/html/rfc3416#section-3)
```
   VarBind ::= SEQUENCE {
           name ObjectName,

           CHOICE {
               value          ObjectSyntax,
               unSpecified    NULL,    -- in retrieval requests

                                       -- exceptions in responses
               noSuchObject   [0] IMPLICIT NULL,
               noSuchInstance [1] IMPLICIT NULL,
               endOfMibView   [2] IMPLICIT NULL
           }
       }
```

Fixes #3900
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add support for exceptions in SNMP varbind responses
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@nathanIL @gpotter2