[dns-operations] Order of dropped queries when the maximum number of recursive clients has been reached

[Paul Vixie]

> I have encountered the following error in my logs:
> 
> 26-Apr-2006 20:39:19.101 client <ip removed>#58633: \
>       no more recursive clients: quota reached
> 
> I know why we're getting these messages, but I do not know the order in 
> which queries are dropped if the server continues to answer queries.  
> Are the oldest dropped first?

noting that implementation-specific questions are often better asked in
implementation-specific forums, for example, bind-users at isc.org in this
case, i believe i know the answer.  the oldest query in the queue -- the
one bind9 has been working on the longest -- is the one that gets dropped.
alternatives such as "the one whose current outstanding upstream query
has been outstanding the longest" and "choose at random" were considered
and rejected.

my reason for answering this here is that, apart from what BIND does, there
is an interesting question, what's the best thing for a recursive server
to do?  (if there's a case to be made, perhaps BIND could be encouraged
to change its behaviour, along with other implementations?)