I was having trouble understanding the iptables hashlimit module and couldn’t dig up anything that really helped. The man pages are definitely lacking a clear explanation and /proc/net/ipt_hashlimit/ leaves out some information that would clarify things immensely. After some testing I managed to work it all out, so let’s go through it and see if I can help make sense of it for you too.
I’ll try not to assume too much prior knowledge about the module. We’ll be coming at this with the goal of blocking traffic that exceeds a certain amount of packets per second. From the man page:
hashlimit uses hash buckets to express a rate limiting match (like the limit match) for a group of connections using a single iptables rule. Grouping can be done per-hostgroup (source and/or destination address) and/or per-port. It gives you the ability to express “N packets per time quantum per group” or “N bytes per seconds”
