TTL for 'www' Subdomain
Checks the TTL (Time to Live) for the www subdomain's 'A' record.
The 'A' record for a domain name specifies IP address for the website. The TTL (Time to Live) number allows recursive name servers to cache this information for this amount of time. If the recursive name server has the information cached, then the lookup happens quickly and the user can load the website quickly. If the recursive name server has to look up the information, then there is a delay while this happens and the website will load a tiny bit slower. Therefore, for performance reasons, it is recommended to have the TTL be as long as practical for your particular server environment so that the recursive name servers can cache this information longer.
In emergency situations it is sometimes necessary to switch the IP address to point to a backup server, so you should select the TTL wisely. If you want to provide high-availability and you have a system in place to automatically fail over and update the DNS records, then a short TTL may be a good choice, but not too short because this could impact performance and overload your name servers. If you update your DNS records manually or do not have the need for high-availability, then a longer TTL may be wise because it will improve performance for the user.
This test will report an error if the TTL for the 'A' record of the 'www' subdomain is less than 5 minutes and it will provide a warning if it is less than 30 minutes. There is also a danger in having the TTL be too long because this could prevent you from responding to an emergency. This test will report an error if the TTL is longer than 2 days. We recommend setting the TTL to 1-2 hours and then if you are planning a change, you can update the TTL a few hours ahead of time to temporarily lower it.
For very large and popular websites, the recursive name servers will almost always have the data cached, even with short TTLs, so this test becomes less important for large websites and there may be legitimate scenarios where short TTLs are important for large high-availability websites.