Nginx: reverse proxy passing client IP to the server

On an existing nginx reverse proxy, I need to pass the client IP to a server (from nginx to the frontend served in NG-Engine, and from this to the backend), I tried a lot of sites, but the server is just receiving 127.0.0.1. Extra info (added after Michael Hampton question), our Virtual Machines infrastructure:

Client browser (another VM or remote desktop)
(VM1) nginx reverse proxy (it works, HTTPS layer I added at least)
(VM1) NgEngine serving the homemade frontend
(VM1) RESTful API served by a Java/Spring homemade backend (running in IntelliJ idea)
(VM2..x) other servers feeding the backend with data (out of the question)

I checked doc, tried here (including removing the X-Real-IP header, as they suggest in this other question). I tried also to add:

set_real_ip_from 127.0.0.1;
real_ip_header X-Forwarded-For

My nginx has real_ip module (output of nginx -V below), and this is my nginx.conf... What I am doing wrong?

user www-data;
worker_processes auto;
pid /run/nginx.pid;

events {
  worker_connections 768;
  # multi_accept on;
}

http {
  sendfile on;
  tcp_nopush on;
  tcp_nodelay on;
  keepalive_timeout 65;
  types_hash_max_size 2048;
  # server_tokens off;

  server_names_hash_bucket_size 64;
  # server_name_in_redirect off;

  include /etc/nginx/mime.types;
  default_type application/octet-stream;

  ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # Dropping SSLv3, ref: POODLE
  ssl_prefer_server_ciphers on;

  gzip on;
  gzip_disable "msie6";

  server {
    listen 443 ssl;
    server_name          test-server;
    ssl_certificate      /etc/nginx/ssl/nginx.crt;
    ssl_certificate_key  /etc/nginx/ssl/nginx.key;

    add_header              Strict-Transport-Security  "max-age=63072000; includeSubdomains; preload" always;
    add_header              X-Frame-Options            SAMEORIGIN;
    add_header              X-Content-Type-Options     nosniff;

    # allow nginx to start regardless of upstream endpoint state by using intermediary variable
    set                     $UPSTREAM_SERVICE          10.10.10.15:8080;

    location  / {
      proxy_pass               http://localhost:6789
      proxy_http_version       1.1;

      proxy_buffering          off;
      proxy_buffer_size        128k;
      proxy_busy_buffers_size  256k;
      proxy_buffers            4                   256k;
      proxy_set_header         Host                $host;
      proxy_set_header         X-Real-IP           $remote_addr;
      proxy_set_header         X-Forwarded-For     $proxy_add_x_forwarded_for;
      proxy_set_header         X-Forwarded-Proto   $scheme;
      proxy_set_header         Upgrade             $http_upgrade;
      proxy_set_header         Connection          $http_connection;

    }
  }
}

Output of 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-mcUg8N/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_flv_module --with-http_geoip_module=dynamic --with-http_gunzip_module --with-http_gzip_static_module --with-http_image_filter_module=dynamic --with-http_mp4_module --with-http_perl_module=dynamic --with-http_random_index_module --with-http_secure_link_module --with-http_sub_module --with-http_xslt_module=dynamic --with-mail=dynamic --with-mail_ssl_module --with-stream=dynamic --with-stream_ssl_module --with-stream_ssl_preread_module --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/http-headers-more-filter --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/http-auth-pam --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/http-cache-purge --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/http-dav-ext --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/http-ndk --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/http-echo --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/http-fancyindex --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/nchan --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/http-lua --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/rtmp --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/http-uploadprogress --add-dy
namic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/http-upstream-fair --add-dynamic-module=/build/nginx-mcUg8N/nginx-1.14.0/debian/modules/http-subs-filter

Thank you very much and best regards

On an existing nginx reverse proxy, I need to pass the client IP to a server (both running in the same OS), I tried a lot of sites, but the server is just receiving 127.0.0.1.

As a reverse proxy, your back-end server will always receive connection from your nginx process, which, depends on your *_pass directive, will contain your nginx's IP address (in your case, 127.0.0.1).

As such, you may not fetch client IP by checking client address, you'll only get your front-end server IP address.

In reverse proxy and load balancing, uh, business, we have several ways to extract client IP from reverse proxy on the backend:

Non-standard de-facto X-Forwarded-For HTTP request header, contains all the proxies passed and requesting client IP address(es).
Forwarded HTTP request header, is a new standardized way to replace X-Forwarded-For, also contains IP addresses for all proxies passed and requesting client.
X-Real-IP HTTP request header, or any custom HTTP request header, to arbitrarily contain client IP address.

Check your back-end application if they support any of these headers.

My nginx has real_ip module

ngx_http_realip_module isn't used for this case. It is used if nginx is behind a proxy so that it could validate and fetch real client IP address and store it in a specified variable.

Nginx: reverse proxy passing client IP to the server

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