Aug 26, 2025 07:00:00

ISPs change DNS behavior to avoid sites publishing Germany's secret internet blocklists, possibly to cover up mistakes

Lina, a German software developer, runs a project called '

CUII-Liste, ' which publishes lists of sites that are DNS-blocked by major German Internet Service Providers (ISPs) and exposes unfair domain blocking. However, Lina reports that CUII-Liste has been disrupted by ISPs and is no longer functioning properly.

A German ISP tampered with their DNS - specifically to sabotage my website - lina's blog
https://lina.sh/blog/telefonica-sabotages-me

In March 2021, major German ISPs and copyright holders joined forces to establish a private organization called 'CUII' to combat online copyright infringement. CUII serves as Germany's 'consultation body for internet copyright,' reviewing websites suspected of distributing pirated content and recommending domains for DNS blocking. Germany's four largest ISPs -- Telekom, Vodafone, 1&1, and Telefonica -- are all members of CUII and block websites in accordance with CUII's instructions.

Lina criticized CUII, saying, 'The judges and review methods are not transparent, and a group of ISPs and major copyright holders are rewriting our free internet for corporate interests.' As a countermeasure, Lina created CUII-Liste, which reveals the blocklists CUII refuses to make public.

According to Lina, sites blocked by CUII can be easily identified because they return a CNAME to notice.cuii.info. In fact, Lina discovered that some of the sites blocked by CUII had been unjustly blocked for more than two years, and German online media

reported this .

Shortly after this news was published, in February 2025, CUII-Liste stopped working as usual. Previously, if you searched for a specific domain on the site, it would return a response that it was blocked. However, Telecom, 1&1, and Vodafone stopped responding and now all return 'that domain does not exist.' This makes it impossible to determine whether the site is blocked or simply a non-existent domain.

Meanwhile, Telefonica, one of Germany's four major ISPs, initially did not follow the change and returned the response 'notice.cuii.info' indicating that the site was blocked. However, according to Lina, after detecting that the 'blau-sicherheit.info' domain of the 'Blau' brand owned by Telefonica was blocked by CUII, Telefonica changed the DNS response method. As a result, Lina's program malfunctioned, interpreting it as 'CUII suddenly unblocking hundreds of domains.' Lina speculates that 'Telefonica may have blocked their own domain and then made changes to pretend that the domain did not exist, in an attempt to investigate the behavior of my site.'

Lina fixed the site's behavior, and it's now working properly again, but it's now harder to spot that it was blocked by CUII. 'Is it a coincidence that CUII made it harder to track us so soon after it was featured in the German media? Or is it a move to cover up future mistakes, knowing that their mistake was exposed? Whatever the intention, the result is undoubtedly a move toward less transparency and more scrutiny, which benefits CUII and not the public,' Lina said.