How to debug VCL in varnish?

Using a vcl config file, import the additional included "standard library", which includes a bunch of utility functions:

import std;

# To 'varnishlog'
std.log("varnish log info:" + req.host);

# To syslog
std.syslog( LOG_USER|LOG_ALERT, "There is serious trouble");

v6.x - https://varnish-cache.org/docs/6.0/reference/vmod_generated.html#void-log-string-s

v5.x - https://varnish-cache.org/docs/5.0/reference/vmod_std.generated.html?#func-log

v4.x - https://varnish-cache.org/docs/4.0/reference/vmod_std.generated.html?#func-log

v3.x - https://varnish-cache.org/docs/3.0/reference/vmod_std.html#log

See also man varnishlog

You can see URL with requested URLs varnishlog utility (it able to write log files)

varnishlog -i RxURL

Or output some info to syslog with vmod std and syslog function for Varnish 3.x https://www.varnish-cache.org/docs/trunk/reference/vmod_std.html#syslog Varnish 5.1 https://varnish-cache.org/docs/5.1/reference/vmod_std.generated.html#func-syslog

Example:

import std;

sub vcl_recv {
  ...
  std.syslog(180, "RECV: " + req.http.host + req.url);
  ...
}

Or with C-snippet on Varnish 2.x https://www.varnish-cache.org/trac/wiki/VCLExampleSyslog