Distributing DNS load via DHCP

Simon Hobson dhcp1 at thehobsons.co.uk
Fri Dec 16 18:03:33 UTC 2011

Jeff Wieland wrote:
>We have a fairly large pool of address, and we'd like to distribute
>the DNS load on this pool between two DNS servers.

How about just :

   pool {
     option domain-name-servers,;
   pool {
     option domain-name-servers,;

In the long term, once all addresses have been used one then the 
distribution of leases between pools will be more or less random. In 
the short term, you could make whichever pool gets used first* 
smaller until it's full and devices have been forced into the other 
pool - and then enlarge the pool a bit until both are at full size.

* In 3.x and below, unused addresses were used "top down" (ie highest 
address first) due to the way the internal hashing worked. It looks 
like 4.x does it bottom up.

However, I'm not sure this will entirely achieve what you want. For 
example, if one of the DNS servers goes down for any reason, then I 
believe some (all ?) versions of Windows will put the "dead" server 
to the bottom of it's query order - and leave it there indefinitely 
(ie until one of the other servers goes down). At my last job, I know 
some of my colleagues in other divisions were using this trick to 
have clients query an internal server before going external - which 
broke the first time the internal server went down.

And on that note, some clients will query all the servers anyway for 
non-resolving addresses. What my colleagues were doing was to have an 
internal server which resolved internal domains. If that failed to 
give an answer, then the Windows client would try the next server (an 
external one) which resolved external addresses.

If that is still how they work, then your clients would query one 
server, get nxdomain response, and then query the other for the same 
non-resolving address.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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