[tor-relays] DirPort DOS activity against Fallback Directories

Recently I noticed excessive DirPort requests to my relay, where DirPort bandwidth reached 15% of ORPort bandwidth. Normal DirPort load is around 2%.

https://lists.torproject.org/pipermail/tor-relays/2018-May/015253.html

Just looked over a sample of FallBackDir relays in Relay Search and it appears this excess-load abuse is directed at them in particular. Some fall-back directories show more than a month of excess request traffic, presumably on the DirPort. Logs here indicate six weeks of abuse escalating in increments. Possibly this foreshadows a major increase in an effort to impair FallBackDir relay functionality.

Either an iptables connection-rate limit or disabling DirPort resolves the problem.