DDWRT DNS Server and Caching

On the Services tab, under Additional DNSMasq Options, you can set these 2 options:

Example 1:

cache-size=0
no-negcache

Example 2:

cache-size=1000
no-negcache

From the DNSMasq man page:

Cache-size 
Set the size of dnsmasq’s cache. The default is 150 names. Setting the cache size to zero disables caching.

no-negcache
Disable negative caching. Negative caching allows dnsmasq to remember “no such domain” answers from upstream nameservers and answer identical queries without forwarding them again.