recursive-clients, what value ?

Richard Maynard ephur at corp.earthlink.net
Tue Jun 15 00:18:11 UTC 2004


> I read that increasing "recursive-clients" could help. I'm 
> going to set 
> it to 2000 but is there a way to know what would be a good value ?

As Kevin already pointed out, this is a question that can vary widely
depending on your environment. Firstly, I would advise against doubling the
limit, if you eventually want to get there that's okay, but you may want to
do it in smaller incremental steps. The recursive-clients limitations are
not only memory related, but you also have to factor in extra machine load,
not just in managing the additional queries, but also all the extra
connections it will have to keep track of. Doubling the amount of queries
will also increase network traffic, if you are on a network that is even
lightly congested then increases in queries could cause a greater number of
queries to fail than would previously. 

The other part of what you put in your logs, is that localhost is the one
triggering these failures. If it's a caching only name server that serves
services on that machine only you could also investigate other items such as
increasing the cache time, or caching invalid responses, which I believe by
default is off. Caching negative responses can GREATLY reduce the
recursive-clients problem. If DNS for a high traffic domain is failing, and
you're recursive clients are all hitting their timeouts waiting for a
response when one won't be coming back then you'll end up again, seeing
errors like the ones you saw. 

> Is there a way to monitor in real time the number of 
> simultaneous clients ?

You could probably get this from turning up debugging/query logging and
parsing the data, but outside of that I'm not sure of any immediate way to
get the number of simultaneous clients. I would suggest keeping track of
your stats daily and looking at your long term trends, and be sure to look
at your cache hit rate, it gives a good idea of what other things you can
start tuning. While, most decent hardware running on a good network will
have no problems with an extra thousand recursive clients, it may not be the
ultimate answer, by increasing the recursive clients you could end up
masking a different issue.

-- Richard Maynard



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