How to debug VCL in varnish?

Question

How can I print a log in VCL?

Can I print log info on screen?

Can I do like this?

sub vcl_recv {
  ....
  log.info(req.http.host); // can i write a log here?
  ....
}

Answer 1

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

Answer 2

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 troble");

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

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

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

See also man varnishlog