Reducing the time that DNS records are cached

Expiration of DNS records before TTL expiration

By default, NetCache honors the TTL that the server specifies and does not expire the DNS record in the NetCache DNS cache until after the server-specified TTL expires. The assumption is that the mapping between the IP address and host name in the appliance DNS cache is correct until that TTL expires. However, in some cases, the mapping might not be correct for the duration of the TTL. The Per-Domain TTL Expiration option enables you to set NetCache to expire the DNS records in its cache for the domains that you specify before the TTL expires.

For example, in environments that use DHCP (Dynamic Host Configuration Protocol) servers and DDNS (Dynamic DNS) servers to dynamically assign IP addresses, a client that was dynamically assigned an IP address might release that IP address before the server-specified TTL expires. When this event occurs, the DHCP server and the DDNS server are automatically updated, and the DHCP server can assign that released IP address to another client. The record in the appliance DNS cache is not updated until after the TTL expires.

To maintain consistency between the DDNS server and the appliance DNS cache, you must force the expiration of the DNS records in the appliance's DNS cache before the TTL expires. The Per-Domain TTL Expiration option forces expiration of DNS records for a specified domain after 15 seconds.

Examples of entries

Assume that the Widget Company's wireless laptops are managed by DHCP. The laptops are assigned addresses in the range 10.23.64.0/24, and these IP addresses are in the wireless.widget.com subdomain. You would enter domain names or in-addr.arpa format domain names rather than IP addresses in CIDR format.

The Per-Domain TTL Expiration option settings are as follows:

.wireless.widget.com. .64.23.10.in-addr.arpa.

For the domains listed in this option, NetCache will expire the DNS records in its cache after 15 seconds.