A very common DDoS attack uses UDP services to amplify your request to a bigger response, but then spoof your src ip to the target.

Having followed many reports of denial of service activity of Lemmy, I don’t think this is the common mode. Attacks I’d heard of involve:

• Using regular lemmy APIs backed by heavy database queries. I haven’t heard discussion of query rates, but Lemmy instances are typically single-machine deployments on modest 4-core to 32-core hardware. Dozens to thousands of queries per second to the heaviest API endpoints are sufficient to saturate them.
• Uploading garbage images to fill storage.

Essentially the low-hanging fruit is low enough that distributed attacks, amplification, and attacks on bandwidth or the networking stack itself are just unnecessary. A WAF is still a good if indeed OPs instance is getting attacked, but I’d be surprised if wafs has built-in rules for lemmy yet. I somewhat suspect one would have to do the DB query analysis to identify slow queries and then write custom waf rules to rate limit the corresponding API calls. But it’s worth noting that OP has provided no evidence of an attack. It’s at least equally likely that they dos’ed themselves by running too many services on a crappy VPS and running out of ram. The place to start is probably basic capacity analysis.

Some recent sources:

• sh.itjust.works/post/2732891
• lemmy.world/post/2923697