MOTD profanity ban wave — "unknown host"
Last night I got an unexpected "unknown host" error trying to connect to my friend group's server. I spent literal hours pouring over DNS records, routing tables, port forwards, TCP dumps, all that.
Turns out, Mojang maintains a list of "blocked servers" (as hashes so it's harder to search). If your domain or IP address shows up in the list, minecraft will refuse to connect. It won't even give you the courtesy of a proper error message, you just get "unknown host" message and no log messages.
After concluding we were on this list, we got sent down another rabbithole checking server logs for any unauthorized joins that may have caused problems when we accidentally left the white-list off. Nothing.
Eventually, we found and were exploring mc-blacklist, which keeps a record of changes to the blacklist. Well, there was a massive ban wave a month ago and additional ban/unban waves every few days since. Looking through the list of known servers, nearly every recently banned server had some swear word in their MOTD, and nearly every unbanned server had an MOTD with words like "shoot" or messages like "MOTD Changed per Mojang Enforcement Request".
We had a mild swear word in our MOTD. We changed it, and were unbanned several hours later. We had no official way of knowing this is what we had to do, it was actively hidden from us; we couldn't even find official mentions of the block list, let alone an appeal form.
Just throwing this on here in case it saves someone 4 hours of debugging a strange "unknown host" error.