[cobalt-users] Fw: simple DNS question

["Gerald Waugh" <gerald@xxxxxxxxx>]

Forwarding this to the list, gives a fairly good explanation of
named services.
 
> > would my DNS server, if it cant resolve some name, ask the root
> > directly? or where in the hierarchy would it start. You have
> > forwarders configured in your name servers, would it be step by step
> > worked up the hierarchy, or would it directly go to the root.
> 
> Depends on what is already in your name server's cache.  If it already
> knows about some level of the hierarchy, it will start there.  If not, it
> will start at a root NS.
> 
> Let's assume for example that a client has asked your name server to
> resolve www.mydomain.com.  Let's also assume your NS's cache is absolutely
> empty (unlikely except right after startup).  Also assume a pretty normal
> plain-jane named.conf.
> 
> o Your NS looks in its cache and sees that it doesn't know what NS's
>   are authoritative for the mydomain.com domain, and it doesn't even
>   know what NS's are authoritative for the .com TLD.  However it does
>   know (from its hints zone file) where the root NS's are.
> o Your NS queries a root NS (let's say a.root-servers.net).
> o a.root-servers.net refers your NS to the NS's authoritative for
>   the .com TLD.
> o Your NS queries one of those TLD servers (let's say a.gtld-servers.net).
> o a.gtld-servers.net refers your NS to the NS's authoritative for
>   mydomain.com.
> o Your NS queries one of those NS's (let's say ns1.mydomain.com).
> o ns1.mydomain.com returns the IP address of www.mydomain.com.
> o Your NS returns that IP address to the client.
> 
> Now, on a normally-configured NS that has been running for more than a few
> minutes the cache won't be empty.  Most likely our NS will have already
> had to resolve several .com domains, so the locations of the .com TLD
> servers will be in cache.  If so, your NS will not have to query a root
> server for that information. It can go directly to one of the .com TLD
> servers, so the sequence is shortened to this:
> 
> o Your NS looks in it's cache and sees that it doesn't know what NS's
>   are authoritative for mydomain.com, but it does know what NS's are
>   authoritative for the .com TLD.
> o Your NS queries one of those TLD servers (let's say a.gtld-servers.net).
> o a.gtld-servers.net refers your NS to the NS's authoritative for
>   mydomain.com.
> o Your NS queries one of those NS's (let's say ns1.mydomain.com).
> o ns1.mydomain.com returns the IP address of www.mydomain.com.
> o Your NS returns that IP address to the client.
> 
> Having gone through that, your NS now knows (it's in the cache) what NS's
> are authoritative for mydomain.com.  Now suppose a client asks your NS to
> resolve smtp.mydomain.com.  The sequence is shortened to this:
> 
> o Your NS looks in its cache and sees that it already knows what NS's
>   are authoritative for the mydomain.com domain.
> o Your NS queries one of those NS's (let's say ns1.mydomain.com).
> o ns1.mydomain.com returns the IP address of smtp.mydomain.com.
> o Your NS returns that IP address to the client.
> 
> Or to shorten it even more, let's assume a client asks for
> www.mydomain.com after your NS has already looked up that address for
> another client.
> 
> o Your NS looks in its cache and sees that it already knows the IP
>   address for www.mydomain.com.
> o Your NS returns that IP address to the client.
> 
> So, where in the hierarchy your NS starts looking depends on how much of
> the hierarch it has already queried and cached.
> 
> -- 
> Chip Old (Francis E. Old)               E-Mail:  fold@xxxxxxxx
> Manager, BCPL Network Services          Voice:   410-887-6180
> Manager, BCPL.NET Internet Services     FAX:     410-887-2091
> 320 York Road
> Towson, Maryland 21204 U.S.A.
> 
> 
>