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Debugging Varnish VCL configuration files | Midwestern Mac, LLC
Over the past few years varnish has become the “go to” way for Magento systems to paper over serious backend performance problems – which means varnish debugging is quickly becoming a must have skill for Magento developers working at all levels of scalability.
This entire article’s worth a read, but the ability to log from VCL files dirctly to the system log is what drew my attention
In your VCL (at the top), add in import std; to import the std library. Then, anywhere in the vcl, you can log messages to the system log using something like:
std.syslog(0, req.http.Cookie);