-0, --dns-forward-max=<queries>

Set the maximum number of concurrent DNS queries. The default value is 150, which should be fine for most setups. The only known situation where this needs to be increased is when using web-server log file resolvers, which can generate large numbers of concurrent queries. This parameter actually controls the number of concurrent queries per server group, where a server group is the set of server(s) associated with a single domain. So if a domain has its own server via --server=/example.com/1.2.3.4 and 1.2.3.4 is not responding, but queries for *.example.com cannot go elsewhere, then other queries will not be affected. On configurations with many such server groups and tight resources, this value may need to be reduced.