Tor eats all mbufs on FreeBSD

I'm running tor relay on FreeBSD 11.1, and not long ago the system started to occasionally stop responding to the network with

kernel: [zone: mbuf_cluster] kern.ipc.nmbclusters limit reached

accompanied with

kernel: sonewconn: pcb 0xfffff80003c61570: Listen queue overflow: 193 already in queue awaiting acceptance (211 occurrences)

messages in the logs.

It first happened on Dec 13 and repeated 3 times, the approximate lifetime of the relay is ~1 day.

Seems like a DOS attack which makes tor open a lot of connections and eat all the mbuf space. I don't see any peaks on trafic or pps graphs though, and there are no messages in tor log.

I'm currently trying to gather more information.

Trac:
Username: AMDmi3