I got this error when trying to use NGINX as a proxy, setting the http_host header like this:
location / { proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for; proxy_set_header Host $host; proxy_pass http://192.168.8.2:6081/; proxy_connect_timeout 5; }
Everything worked fine except when clients would send a request without a host field set.
This happened with a few clients whose job it was to test the server and make sure it ran properly, like monit. To fix the issue, I create a specific location for the test target:
location = /test_target.html { root /var/www/public; }