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ooni / probe

OONI Probe network measurement tool for detecting internet censorship
https://ooni.org/install
BSD 3-Clause "New" or "Revised" License
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webconnectivity v0.5: disjoint TH and probe discovered addrs ASNs #2517

Closed bassosimone closed 5 months ago

bassosimone commented 11 months ago

This issue is about documenting a possible error that can occur with Web Connectivity v0.5. To illustrate this error, please consider the 20230706183840.201925_PK_webconnectivity_19f5e0d803cbaea7 measurement.

The https://github.com/ooni/probe-cli/pull/1190 pull request contains a test case illustrating how this bug happens.

The gist of the bug is that all the addresses discovered by the TH belong to AS137 (Apple, Inc.) while some of the addresses discovered by the probe belong to AS36183 (Akamai Technologies, Inc.). Additionally, the URL we're testing here always returns eof_error to the probe. Therefore, it is not possible to validate IP addresses using TLS.

Because the ASNs are disjoint, the probe concludes that the lookups by the probe are not correct. Therefore, it says that there is DNS blocking while the reality is that the TH and the probe probably seees disjoint valid ASNs.

I am inclined to say the probe results are valid (and not censorship) because the CNAME is correct for the tested URL but in all honesty I am not even sure I have enough data to conclude that, I only think this is the most likely explanation.

bassosimone commented 5 months ago

There isn't much we can do here, but reckon that the problem exists. The underpinning reason why this happens is that we get different IP addresses because we're in a different geographic location. The potential solution to this issue has been well described and explained by @fortuna here: https://github.com/ooni/probe/issues/1523. So, I do not think we gain much by keeping this issue open and rather we should prefer keeping https://github.com/ooni/probe/issues/1523 open and it seems enough to just reference https://github.com/ooni/probe/issues/1523 in this issue to link it with https://github.com/ooni/probe/issues/1523.