Issue link: https://read.uberflip.com/i/1146286
64 | Chapter 9: Rule 9: Reduce DNS Lookups Simple enough? Hold on a minute—most browsers have their own caches, separate from the operating system's cache. As long as the browser keeps a DNS record in its own cache, it doesn't bother the operating system with a request for the record. Only after the browser's cache discards the record does it ask the operating system for the address—and then the operating system either satisfies the request out of its cache or sends a request to a remote server, which is where potential slowdowns occur. To make things yet more complicated, designers realize that IP addresses change and that caches consume memory. Therefore, the DNS records have to be periodically flushed from the cache, and several different configuration settings determine how often they are discarded. Factors Affecting DNS Caching First, the server has a say in how long records should be cached. The DNS record returned from a lookup contains a time-to-live (TTL) value. This tells the client how long the record can be cached. Although operating system caches respect the TTL, browsers often ignore it and set their own time limits. Furthermore, the Keep-Alive feature of the HTTP protocol, discussed in Chapter B, can override both the TTL and the browser's time limit. In other words, as long as the browser and the web server are happily communicating and keeping their TCP connection open, there's no reason for a DNS lookup. Browsers put a limit on the number of DNS records cached, regardless of the time the records have been in the cache. If the user visits many different sites with different Figure 9-1. DNS caching from the browser's perspective User's computer Browser DNS client service DNS DNS User's ISP DNS resolver DNS Wider domain name system