When there are no free mbuf clusters available, FreeBSD enters zonelimit state and stops awnsering network requests. You can see if the system has gone into “zoneli” state by using the top command.

To see current mbuf cluster usage

netstat -m

Immediate resolution is to reboot the system. System may not recover from zoneli state by killing affected processes. Sometimes even a reboot will not work from console and machine will need to power cycled.