[atlas] Problems with DNS measurements and NSID

On 21/05/14 10:07, Klaus Darilion wrote:

    Thanks. Meanwhile I managed to iterate over the JSON list and print the
    nsid:

    …

    But the problem is if there was an error during the measurement (eg
    timeout), then my script terminates:

    …

    What is the suggested way to find out if a result is valid and exists?

First of all, i should say thanks for working with us on this. The
trouble with a parsing library is that it’s supposed to be able to
handle all of the edge cases, but finding them independently is quite
difficult. When you post examples to the list of result blobs that act
in unexpected ways, this really helps in polishing the parser and making
it more intuitive.

Given that, and other comments from John regarding error handling for
DNS results, I’ve added a little more code to better handle errors, so
if you update Sagan to 0.1.12 (or just |git pull|), and re-run your code
you’ll find that |result.is_error| is now set to |True| in your case.
Additionally, if you pass |on_error=Result.ERROR_FAIL| to the parsing
argument, it’ll explode with a |ResultParseError| which you can handle
any way you please.

So given your example, you could do something like:

|    result = DnsResult(d)
    if not result.is_error:
        # Do stuff
|

However, there’s likely cases where you might see results that /aren’t/
errors, but still don’t have any responses, or where |edns0.options| is
an empty list. Unfortunately, you have to write your code to account for
these, since they’re perfectly valid parsings:

|    result = DnsResult(d)
    if not result.is_error:
        for response in responses:
            if response.edns0:
                for option in response.edns0.options:
                    print(option.nsid)
|

Note that I’m not really checking that much here, just looping over
(potentially empty) lists. This keeps your code free of checks, and will
make sure that it accounts for cases where the number of responses is >
1 or the number of edns0 options is > 1.

But please, if you find another result blob that isn’t performing the
way you’d expect, please feel free to post it here or send it to
atlas-bugs at ripe.net so we can look into it and make sure that Sagan has
complete coverage.

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