Maintained by: NLnet Labs

"Blocking regime" behavior question

Dhalgren Tor
Wed Apr 13 03:10:52 CEST 2016

A quick experiment shows that if an infra-cache entry with "rto
120000" (two minutes) is present for all a requested domain's name
servers, then SERVFAIL is returned immediately to further queries.
However, the infra-cache entries are per-domain-per-nameserver.  When
a different domain with a NS record pointing to the same nameserver is
queried, Unbound creates and tracks a separate state for the second
domain and does not combine information regarding reachability of
particular nameservers.

So I suppose queries against eighty-plus unresponsive name servers for
hundreds-to-thousands of domains easily explains the 700 entry active
request queue.

One odd quirk is that Unbound sometimes returns SERVFAIL in from ten
to thirty seconds the first time a request for a non-responding
nameserver domain is made.  Subsequent requests then take however long
until the "rto 120000" state is achieved for the infra-cache domain
entry.  Maximum time till SERVFAIL seems to be ten minutes.

'Eventdns' is now tuned to give up after five seconds and permit up to
16k under-five-seconds active requests, so all the above is academic.