programming / protocol question about client-side caching

Michael B Allen ioplex at
Mon Jun 16 17:02:05 UTC 2008

On 6/16/08, Chris Buxton <cbuxton at> wrote:
> On Jun 15, 2008, at 11:39 PM, Michael B Allen wrote:
> > Hi,
> >
> > I'm not sure where to ask generic protocol questions (is c.p.d dead?)
> > so please redirect me if necessary.
> >
> > I have an HTTP server module that does a lot of DNS SRV and A lookups
> > for kerberos, ldap, etc. I have my own DNS client library to which I
> > am currently adding a cache. However, I don't recall ever seeing a DNS
> > client cache query results so I have to wonder if there's a reason.
> >
> > Is there any reason why a DNS client should *not* cache query results?
> >
> > Also, my cache can "round-robin" entries for a given name to emulate
> > the behavior of a DNS server. But it's not smart enough to know if the
> > server from which it retrieved the entries uses round-robin behavior
> > or not. Is there any reason why a DNS client cache should *not*
> > round-robin entries?
> >
> > Mike
> >
>  Most stub resolvers now do include a cache.
>  - Windows has its DNS Client Service.
>  - Mac OS X has used lookupd, and now uses DirectoryService.
>  - Some Unix-like OS's use nscd, the Name Service Caching Daemon.
>  - Others use BIND 9's lwresd.
>  Just be sure that your cache mechanism honors TTL's, other than possibly
> applying a reasonable minimum and maximum.
>  There is no reason you should not use either round-robin or randomized
> order, or any other ordering you like. This is the default behavior of BIND
> and probably of several other implementations. There is no way for the
> resolver to tell if the responding server is applying RRSet reordering.



Michael B Allen
PHP Active Directory SPNEGO SSO

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