I setup a Friendica instance earlier in the week. Turns out the DNS queries had increased somewhat dramatically
https://lemmy.fwgx.uk/post/1159

I setup a Friendica instance earlier in the week. Turns out the DNS queries had increased somewhat dramatically - FWGX Lemmy
I set up friendica as my first foray on to the fediverse. It worked well, but as
it turns out doesn’t work that well with Lemmy, which was my main usecase. Well
whilst trying to fix DNS issues setting up a Lemmy instance instead, I noticed
my DNS logs were rather full. My Unbound DNS was getting 40k requests every 10
mins from *.activitypub-troll.cf. I don’t know who or what that is, but blocking
it didn’t reduce the activity. At first I thought it was something to do with
Lemmy as I’d forgotten I still had Friendica running. Thankfully stopping the
Friendica service reduced the DNS request back to normal. So if you’ve set
something up recently, you might want to check if there have been any
consequences in your service logs
I am new to the fediverse, and I don't use Friendica, so I could be entirely wrong about this. However, from what is described, perhaps Friendica has some sort of feature in which would trigger your instance to go out to fetch some data from another instance. Someone exploited this feature, spammed your instance with content from assortment of subdomains on the *.activitypub-troll.cf domain, and most if not all of them are probably non-existent. As result of that, your server is re-checking every 10 minutes to see if they've came back online. This would also explain why shutting down the Friendica service resolved the problem for you.
That's about what I concluded too.
I do wonder if the fediverse grows to tens / hundreds of millions of users, just how scalable the networking will be - and how susceptible to DDOS. I haven't a detailed understanding of the communication protocols, I've just noticed a reasonable amount of traffic in the log files of my single user Lemmy instance.