Thread Error Using Many Hostname in Aliases
If many hostnames are present in aliases, the following error may appear in the system log:
filterdns: Unable to create monitoring thread for host myhost.example.com! It will not be monitored
This error indicated that the filterdns daemon hit the limit of the number of concurrent threads it could launch in order to resolve and monitor the hostnames properly.
The cure for this is to raise the number of allowed threads per process. This can be done by navigating to System > Advanced on the System Tunables tab. There, create an entry for kern.threads.max_threads_per_proc and set it to 4096.
After raising that value, filterdns will need to be restarted. This can be done by any method that triggers a filter reload, such as navigating to Status > Filter Reload and clicking Reload Filter.