How to setup Varnish logging?

If you want to log HTTP requests in NCSA Common Log Format you need to use varnishncsa. On CentOS/RedHat, the Varnish RPM package includes a varnishncsa init script that you can use to start logging. By default it logs to logfile="/var/log/varnish/varnishncsa.log".

Additionally if you wish to serve several different hosts through a single Varnish install, you'll want to include the host name in the log as well. This can be accomplished with the following setting in /etc/sysconfig/varnishncsa

DAEMON_OPTS="$DAEMON_OPTS -F '%{Host}i %h %l %u %t \"%r\" %s %b \"%{Referer}i\" \"%{User-agent}i\"'"

Please note that the method described in the link by Anshu only logs the requests that are passed through by Varnish to the backend servers. Cache hit requests will never be recorded (before Varnish 5.0 - see below). Therefore the HTTPD logs gathered this way cannot be used for statistical analysis.

Update: As @VikrantPogula mentioned, as of Varnish 5.0 all client requests are logged - including cache hits. This is the default behavior, and can be switched on explicitly using the -c switch.

Metrics in Varnish are a common area that people struggle with.

It's relatively easy to get some one off request metrics but overall tracking over time generally involves using varnishncsa. Here is a guide on Varnish installation that includes metric setup