Varnish 403 - Headers Too Large

Varnish has a set maximum length value for headers accepted, this can be easily overloaded by Magento cache tags or CSP allow headers.

Varnish will output an error message in varnishlog complaining that the headers are too long/large.

Resolution

You can increase the max header length for Varnish, by editing the launch params to specify http_resp_hdr_len to a larger value.

Edit Launch Params (systemd)

systemctl edit varnish.service --full

Edit the ExecStart and add -p http_resp_hdr_len=64000

Then reload the systemd daemon and restart varnish.

systemctl daemon-reload
systemctl restart varnish