[riot-notifications] [RIOT-OS/RIOT] sock_dns: correctly report too short messages (#10896)

Martine Lenders notifications at github.com
Tue Jan 29 19:17:45 CET 2019

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### Contribution description
While writing the tests I promised in #10740 I noticed this bug. `sock_dns_query()` returns the length of a too short DNS response instead of just reporting that it is wrong. While this is not severe, it can lead to confusing behavior, as the function does not report an error but just a number `>0`. This basically means, `sock_dns_query()` will tell the user that the address is whatever currently is on the stack ;-).
Put here the description of your contribution:
- describe which part(s) of RIOT is (are) involved
- if it's a bug fix, describe the bug that it solves and how it is solved
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### Testing procedure
After the interface configuration described in the README of `tests/gnrc_sock_dns` start the application (but not the `dnsmasq`) and the following command in a separate terminal

while sleep 1; do echo AACBAAAACpoAAAAA | base64 -d | sudo socat fd:0 udp6-sendto:'[<native-lladdr>%tap0]':49152,bind="[2001:db8::1]:53",sourceport=53; done

Without this PR the response the output will be

example.org resolves to ::

With it correctly reports

error resolving example.org

Details steps to test your contribution:
- which test/example to compile for which board and is there a 'test' command
- how to know that it was not working/available in master
- the expected success test output

### Issues/PRs references
None, but found while working on https://github.com/miri64/RIOT/tree/tests/enh/gnrc_sock_dns/tests/gnrc_sock_dns
Examples: Fixes #1234. See also #5678. Depends on PR #9876.

Please use keywords (e.g., fixes, resolve) with the links to the issues you
resolved, this way they will be automatically closed when your pull request
is merged. See https://help.github.com/articles/closing-issues-using-keywords/.

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-- Commit Summary --

  * sock_dns: correctly report too short messages

-- File Changes --

    M sys/net/application_layer/dns/dns.c (12)

-- Patch Links --


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