How to debug nginx configuration

Check if your nginx was built with --with-debug via nginx -V

Sample output of nginx -V

# nginx -V              
nginx version: nginx/1.14.0 (Ubuntu)
built with OpenSSL 1.1.0g  2 Nov 2017
TLS SNI support enabled
configure arguments: --with-cc-opt='-g -O2 -fdebug-prefix-map=/build/nginx-FIJPpj/nginx-1.14.0=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -Wdate-time -D_FORTIFY_SOURCE=2' --with-ld-opt='-Wl,-Bsymbolic-functions -Wl,-z,relro -Wl,-z,now -fPIC' --prefix=/usr/share/nginx --conf-path=/etc/nginx/nginx.conf --http-log-path=/var/log/nginx/access.log --error-log-path=/var/log/nginx/error.log --lock-path=/var/lock/nginx.lock --pid-path=/run/nginx.pid --modules-path=/usr/lib/nginx/modules --http-client-body-temp-path=/var/lib/nginx/body --http-fastcgi-temp-path=/var/lib/nginx/fastcgi --http-proxy-temp-path=/var/lib/nginx/proxy --http-scgi-temp-path=/var/lib/nginx/scgi --http-uwsgi-temp-path=/var/lib/nginx/uwsgi --with-debug --with-pcre-jit --with-http_ssl_module --with-http_stub_status_module --with-http_realip_module --with-http_auth_request_module --with-http_v2_module --with-http_dav_module --with-http_slice_module --with-threads --with-http_addition_module --with-http_geoip_module=dynamic --with-http_gunzip_module --with-http_gzip_static_module --with-http_image_filter_module=dynamic --with-http_sub_module --with-http_xslt_module=dynamic --with-stream=dynamic --with-stream_ssl_module --with-mail=dynamic --with-mail_ssl_module

If --with-debug exists, add this line to your configuration (inside server block)

error_log /dev/stdout debug;

Otherwise, add this line

error_log /dev/stdout notice;

Then restart nginx server

To print your own header. In http block

log_format my_custom_format "My Debug Format: $host"

You can check nginx default log format

log_format compression '$remote_addr - $remote_user [$time_local] '
                       '"$request" $status $bytes_sent '
                       '"$http_referer" "$http_user_agent" "$gzip_ratio"';

Or see nginx variable list. Especially, the $http_name is a dynamic variable, is used to represent any header from a request.

For example: $http_origin's value is the Origin (or origin) header value

Next, in the location or server block, add

access_log /dev/stdout my_custom_format;

Have fun