It's been a while since we last provided some update on this mailing list about our ongoing work fighting several DoS attacks.
We can use the attached graph about detected overload over the last couple of months to show what is going on and what we do/plan to do about it.
The first noteworthy incident on that graph is the sharp rise in overloaded non-exit nodes since the middle of July caused by a drop of onionskins which the relay's available CPU/memory can't handle anymore. There are currently two ideas we are working on to cope with such a flood of onionskins:
1. Developing a Proof of Work (PoW) system to has a rate-limitation knob rejecting the load of onionskins while letting legitimate ones through. We still need to solve some design issues (feel free to help!) but hope to have that feature integrated into Tor soon.
2. Relay operators started to experiment with iptables/nftables rules and having the right ones available might be a good stopgap approach against the onionskin related DoS. We are coordinating that effort, so we have something available to propose to the wider community, which is kept up-to-date and limits the risks of traffic overblocking. Feel free to help as well with that effort.
The other noteworthy incident started around September 13 when exit nodes began to get overloaded (while the other DoS was and is still ongoing). Unfortunately, that exit related DoS is heavily impacting our users' experience as can be seen in our OnionPerf data. While we are still investigating the nature of that DoS attack it turns out that blocking particular IP addresses with ExitPolicy rules seems to help on exit nodes this is tested on. The Artikel10 exit node operators provided even a script recently to help with that (much appreciated, thanks!). This approach is highly experimental at this point and it might help us at least to come up with an actual design idea to counter that particular exit DoS.
 For information about overload in general and what "drop of onionskins" means, see: https://support.torproject.org/relay-operators/#relay-bridge-overloaded. It contains as well a guide on how to enable MetricsPort monitoring yourself so you see the actual metrics of your own relay.
 prop327: Implement PoW over Introduction Circuits (#40634) · Issues · The Tor Project / Core / Tor · GitLab
 Provide a recommended set of iptables/nftables rules to help in case of DoS attacks (#40093) · Issues · The Tor Project / Community / Support · GitLab
 Performance – Tor Metrics
 [tor-relays] Performance issues/DoS from outgoing Exit connections