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Possible Corner Case Bug in RIPE Atlas NTP Measurement
by Marco Davids 22 Jul '25

22 Jul '25

Dear RIPE Atlas team, I believe we may have discovered a corner case bug in the way RIPE Atlas handles NTP measurements. The issue surfaced during experiments with my NTP simulator — a small tool that mimics an NTP server and allows for controlled manipulation of its behavior. In one particular scenario, the simulator randomly varies the poll value in each response. While this doesn’t occur frequently in real-world implementations, it is technically valid behavior. It appears, however, that either the probes or the measurement system itself do not expect the poll value to change across responses. When initiating a measurement with the default setting of 3 packets, and each response has a different poll value, the result is that the measurement ends up reporting nothing. I first described this on the RIPE Atlas mailing list: https://mailman.ripe.net/archives/list/ripe-atlas@ripe.net/thread/33KNP7KPX… A detailed analysis can be found here: https://mailman.ripe.net/archives/list/ripe-atlas@ripe.net/message/OSRNUJ2Y… The NTP simulator is publicly reachable at: ntp1.testdns.nl I'd appreciate it if you could take a look. Thank you in advance for your time and consideration! Kind regards, -- Marco Davids SIDN Labs

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Re: Atlas probes not reporting NTP results on ntp0.testdns.nl ?
by Marco Davids 22 Jul '25

22 Jul '25

Hi Joachim, Thanks again for your work and the promising findings! I’ve updated the settings of the NTP simulator (now running at ntp1.testdns.nl) to keep the poll value fixed, and I’ve created a new measurement with 50 probes, each sending 10 packets per interval: https://atlas.ripe.net/measurements/118724341 And indeed — things already look a lot better! So... did we just find a bug? Best, -- Marco -------- Original Message -------- *Subject: *[atlas] Re: Atlas probes not reporting NTP results on ntp0.testdns.nl ? *From: *Joachim Kroß *To: *"Marco Davids (SIDN)" , ripe-atlas(a)ripe.net *Date: *Thu, 17 Jul 2025 15:30:35 +0200 > When multiple packets are sent per sample (default is to send three), > those are currently sent back to back. I.e., unless there is packet > loss, the second and third request are sent as soon as the response to > the previous request has been received. As consequence, the values of > the majority of fields in the responses are typically the same, e.g., > ref-id, poll, precision, stratum, version, mode, even ref-ts. As such, > they are at the top level of the JSON object. And the "result" structure > only has the fields that vary between the packets, i.e., the timestamps > related to the actual packet transmissions (and derived values). > > With the explicitly misbehaving server, however, some of the fields that > would typically not change between back-to-back responses, or rather > rarely only, _do_ change from one response to the next. E.g., the poll > and precision fields. In those cases, there still is an entry at the top > level of the JSON object, I guess perhaps derived from the first packet. > But then, the result structure would additionally have entries for those > types of fields where the value differs from the one at the top level.

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Atlas probes not reporting NTP results on ntp0.testdns.nl ?
by Marco Davids (SIDN) 18 Jul '25

18 Jul '25

Dear list, If anyone knows, or is able to figure out, why most (but not all) Atlas probes don’t report results for NTP measurements to ntp0.testdns.nl, I’d appreciate your insights. Examples: https://atlas.ripe.net/measurements/116922622/ https://atlas.ripe.net/measurements/116946257/ https://atlas.ripe.net/measurements/116830921/ Ping works: https://atlas.ripe.net/measurements/116921717/ Thanks in advance. -- Marco

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Re: Atlas probes not reporting NTP results on ntp0.testdns.nl ?
by Marco Davids (SIDN) 17 Jul '25

17 Jul '25

Hi Joachim, Thank you for looking into this! On Thu, 17 Jul 2025 15:30:35 +0200 Joachim Kroß wrote: > It seems the explicitly misbehaving server has been replaced by a more > sane one (albeit apparently not synchronized) Oh, yes that's correct - sorry for that. I started doing some tests with an NTPv5 enabled Chrony server. The NTP simulator is still available on another server though, albeit IPv6 only (because such VM's are so much cheaper): ntp1.testdns.nl. For those interested; my simple NTP simulator is here: https://github.com/mdavids/ntptools/tree/main/go/fake-ntp-server A version that allows to simulate jitter and clock drift is here: https://github.com/mdavids/ntptools/tree/main/go/fake-ntp-server-2 Just a few simple experiments, mostly the result of a bit of vibe coding. -- Marco

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Asking for credits for a university project
by Serban Orza 16 Jul '25

16 Jul '25

Hello, Atlas community! I am Serban Orza, a second-year Computer Science student at Delft University of Technology. Together with another 4 students, we are working on an open-source project that measures the accuracy and reliability of NTP servers. We already implemented the basics, but we would like to integrate RIPE Atlas to have a global perspective and to avoid the bias of a single measurement point. We would appreciate it a lot if somebody could provide us with some RIPE API credits to perform measurements and analyze the reachability of NTP servers. My account at RIPE NCC is on this email: orza(a)student.tudelft.nl Thank you! Serban

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sw-probe reducing syslog messages
by Hans Mayer 14 Jul '25

14 Jul '25

Dear All, I am running the package ripe-atlas 1.5-3 on Debian 12 bookworm with probe id 1010735. Is there a way to reduce the syslog messages for really necessary events like errors ? Kind regards Hans --

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software probe disconnected: REGINIT exit with error
by Marek Küthe 09 Jul '25

09 Jul '25

Hello, For a few hours now, my two software probes (#1008711 and #1008669) are no longer connected. I have attached the log. I build the software probe myself [1] and use a somewhat hardened systemd unit [2]. Does anyone know what I could do? Greetings Marek Küthe [1] https://codeberg.org/mark22k/dn42-router/src/commit/ae9b7cc8c36a7cb0dd9ce8d… [2] https://codeberg.org/mark22k/dn42-router/src/commit/ae9b7cc8c36a7cb0dd9ce8d… -- Marek Küthe m.k(a)mk16.de er/ihm he/him

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Location quantisation?
by Ray Bellis 08 Jul '25

08 Jul '25

In an effort to find and report probes with incorrect geolocation, I'm rather curious to find out what's special about a lake just west of Wichita (Kansas) that results in it having 11 probes in the middle of it? I suspect some sort of very coarse location quantisation, yet the probes there are not just distributed all over the US, but I also see a probe from Madrid and another from near Ljubljana. cheers, Ray

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Probe areas
by Yohan Prod'homme 07 Jul '25

07 Jul '25

Hello, When you start a measurement, you can specify an area to chose probes from. These areas are like "West", "North-Central", "South-East"... What do they corresponds to? If I use the "West" area, this is the west of what? Of the europe? Because "west of the world" barely makes any sense to me, and I can't find any documentation about these areas. I feel like I'm missing something obvious, so I'd be grateful if someone can enlighten me :) Thanks, Yohan

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Probe down ?
by Colin Johnston 30 Jun '25

30 Jun '25

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