A root-server at the Internet’s core lost touch with its peers. We still don’t know why.

For more than four days, a server at the very core of the Internet’s domain name system was out of sync with its 12 root server peers due to an unexplained glitch that could have caused stability and security problems worldwide. This server, maintained by Internet carrier Cogent Communications, is one of the 13 root servers that provision the Internet’s root zone, which sits at the top of the hierarchical distributed database known as the domain name system, or DNS.

For reasons that remain unclear outside of Cogent—which declined to comment for this post—all 12 instances of the c-root it’s responsible for maintaining suddenly stopped updating on Saturday. Stéphane Bortzmeyer, a French engineer who was among the first to flag the problem in a Tuesday post, noted then that the c-root was three days behind the rest of the root servers.