Different synthetic tests probe different layers of the Internet infrastructure we reviewed earlier. Each kind of test provides information on the health of a component or a service, and all are useful for determining where issues lie.
As you’ve seen, DNS is an important source of latency for mashups and a common culprit when sites aren’t accessible. DNS services are also a common point of attack for hackers, who try to “poison” domain name listings, which can misdirect visitors to other locations. In other words, you need to watch:
The response time for DNS lookups
DNS resolution of every site involved in building a page, not just your own
Whether the DNS lookup returns the correct IP addresses
If you’re using a CDN to speed up the delivery of your web pages to the far reaches of the Internet, the CDN may be operating your DNS on your behalf. This isn’t an excuse not to test DNS resolution; in fact, you may want to watch more closely to be sure that IP addresses don’t change without your approval. However, CDNs use resilient DNS services and Global Server Load Balancing (GSLB), which significantly improve the performance and availability of DNS resolution.
In most cases, you won’t test DNS by itself. It will be a part of a synthetic test, and will be shown as the first element of latency in a test’s results. Verifying that the content of a page is what you expect it to be will also let you know if your page has been hacked or if users are ...