Installation guide

Chapter 14. Apache HTTP Server 197
other words, after a reverse lookup is performed, a forward lookup is performed on the result. At least
one of the IP addresses in the forward lookup must match the address from the first reverse lookup.
Generally, you should leave HostnameLookups set to off, because the DNS requests add a load to
your server and may slow it down. If your server is busy, the effects of HostnameLookups will be
noticeable.
HostnameLookups are also an issue for the Internet as a whole. All of the individual connections
made to look up each hostname add up. Therefore, for your own Web server’s benefit, as well as for
the good of the Internet as a whole, you should leave HostnameLookups set to off.
If you like to see the hostnames in your log files, consider running one of the many log analyzer tools
that perform the DNS lookups more efficiently and in bulk when you come to rotate your log files.
14.5.38. ErrorLog
ErrorLog names the file where server errors are logged. As this directive indicates, the error log file
for your Web server is /var/log/httpd/error_log.
The error log is a good place to look if your Web server generates any errors or fails, and you are not
sure what happened.
14.5.39. LogLevel
LogLevel sets how verbose the error messages in the error logs will be. LogLevel can be set (from
least verbose to most verbose) to emerg, alert, crit, error, warn, notice, info or debug. The
default LogLevel is warn.
14.5.40. LogFormat
The LogFormat directives in your httpd.conf file set up a format for the messages in your access
log. The actual LogFormat used will depend on the settings given in the CustomLog directive (see
Section 14.5.41).
14.5.41. CustomLog
CustomLog identifies the log file and the log file format. In your Web server’s default
configuration, CustomLog defines the log file in which accesses to your Web server are recorded:
/var/log/httpd/access_log. You will need to know the location of this file if you want to
generate any access-based server performance statistics for your Web server.
CustomLog also sets the log file format to combined. The common logfile format looks like this:
remotehost rfc931 authuser [date] "request" status bytes referer user-agent
remotehost
The remote hostname. If the hostname is not available from DNS, or if HostnameLookups is
set to Off, then remotehost will be the IP address of the remote host.
rfc931
Not used. You will see a - in the log file in its place.