There used to be a message on the atlas.ripe.net frontpage, I retrieved it from google cache:

Due to an unfortunate bug in a recent probe firmware (version 4760), TCP based DNS measurements are not providing correct results. This affects all ongoing and one-off measurements.

The issue also affected DNSMON measurements between 14-23 March. On 23 March we deployed a corrected firmware on RIPE Atlas anchors (version 4770), and in the near future we'll deploy it on regular probes as well.

So any probe that is still on version 4760 will provide timeouts for TCP DNS lookups.

From: ripe-atlas <ripe-atlas-bounces@ripe.net> on behalf of Davey Song(宋林健) <ljsong@biigroup.cn>
Sent: Tuesday, May 9, 2017 10:03 AM
To: ripe-atlas@ripe.net
Subject: [atlas] Many timeout-failures for DNS over TCP

Hi folks,

I setup measurements for DNS queries over both TCP and UDP. I found the timeout and failures of TCP is far more than UDP.

For example :

DNS over TCP: https://atlas.ripe.net/measurements/8552847/#!probes

DNS over UDP: https://atlas.ripe.net/measurements/8552846/#!probes

Is it normal ? I mean is there any special timeout routine/parameters of atlas probes from normal resolver?

Best regards,

Davey