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Chapter 20. Server Performance > Turn Off Reverse DNS Lookups

20.1. Turn Off Reverse DNS Lookups

Reverse DNS maps IP numbers to machine names. The web server uses reverse DNS for two things: writing a machine name rather than simply an IP address in the web servers logs, and giving CGI programs easy access to the client machine name. The CGI program can access the client machine name via the REMOTE_HOST environment variable if reverse DNS is on.

The problems with reverse DNS are that it can take a long time and is generally implemented as a blocking system call. This means that the current process or thread can do no further work until the call either returns with an answer or times out. While the time required generally isn't a problem for a single HTTP request, it rapidly becomes the server bottleneck if there are more than a few users making requests.


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