Nginx não servirá arquivos PHP de um servidor remoto

Quando eu chamo o servidor NGINX, ele deve servir arquivos php armazenados em um servidor PHP remoto. Atualmente recebo 404 Not Found - nginx/1.10.3 (Ubuntu). Se eu mudar root /srv/www/sitepara root /var/www/htmlentão, ele serve com sucesso index.htmldo NGINX, então a solicitação nunca atinge o servidor PHP.

• SERVIDOR PHP: 192.168.99.31 ( PHP 7.1.15)
• SERVIDOR NGINX: 192.168.99.32 ( nginx version: nginx/1.10.3 (Ubuntu))

O servidor PHP é acessível a partir do servidor NGINX pela porta 9000, como visto abaixo, então não acho que haja problemas relacionados à conexão aqui.

vagrant@nginx:~$ nc -zv 192.168.99.31 9000
Connection to 192.168.99.31 9000 port [tcp/*] succeeded!

PHP

vagrant@php:~$ cat /etc/php/7.1/fpm/pool.d/www.conf

[www]
user                   = www-data
group                  = www-data

listen                 = 9000
listen.allowed_clients = 192.168.99.32
listen.owner           = www-data
listen.group           = www-data

pm = dynamic
pm.max_children        = 5
pm.start_servers       = 2
pm.min_spare_servers   = 1
pm.max_spare_servers   = 3

-

vagrant@php:~$ dpkg --list | grep php

ii  php-common                       1:60+ubuntu16.04.1+deb.sury.org+1
ii  php7.1                           7.1.15-1+ubuntu16.04.1+deb.sury.org+2
ii  php7.1-cgi                       7.1.15-1+ubuntu16.04.1+deb.sury.org+2
ii  php7.1-cli                       7.1.15-1+ubuntu16.04.1+deb.sury.org+2
ii  php7.1-common                    7.1.15-1+ubuntu16.04.1+deb.sury.org+2
ii  php7.1-fpm                       7.1.15-1+ubuntu16.04.1+deb.sury.org+2
ii  php7.1-json                      7.1.15-1+ubuntu16.04.1+deb.sury.org+2
ii  php7.1-mbstring                  7.1.15-1+ubuntu16.04.1+deb.sury.org+2
ii  php7.1-mcrypt                    7.1.15-1+ubuntu16.04.1+deb.sury.org+2
ii  php7.1-mysql                     7.1.15-1+ubuntu16.04.1+deb.sury.org+2
ii  php7.1-opcache                   7.1.15-1+ubuntu16.04.1+deb.sury.org+2
ii  php7.1-readline                  7.1.15-1+ubuntu16.04.1+deb.sury.org+2
ii  php7.1-zip                       7.1.15-1+ubuntu16.04.1+deb.sury.org+2

-

vagrant@php:~$ cat /srv/www/site/index.php 
<?php
echo 'Hello from PHP host'.PHP_EOL;

NGINX

vagrant@nginx:~$ cat /etc/nginx/sites-available/default

server {
        listen 80 default_server;
        listen [::]:80 default_server;

        root /srv/www/site;
        index index.php index.html;
        server_name _;

        location / {
                try_files $uri $uri/ =404;
        }

        location ~ \.php$ {
                include snippets/fastcgi-php.conf;
                fastcgi_pass 192.168.99.31:9000;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
                include fastcgi_params;
        }

        location ~ /\.ht {
                deny all;
        }

        error_log /var/log/nginx/site_error.log;
        access_log /var/log/nginx/site_access.log;
}

-

vagrant@nginx:~$ cat /etc/nginx/snippets/fastcgi-php.conf

fastcgi_split_path_info ^(.+\.php)(/.+)$;
try_files $fastcgi_script_name =404;
set $path_info $fastcgi_path_info;
fastcgi_param PATH_INFO $path_info;
fastcgi_index index.php;
include fastcgi.conf;

-

vagrant@nginx:~$ cat /etc/nginx/fastcgi.conf

fastcgi_param  SCRIPT_FILENAME    $document_root$fastcgi_script_name;
fastcgi_param  QUERY_STRING       $query_string;
fastcgi_param  REQUEST_METHOD     $request_method;
fastcgi_param  CONTENT_TYPE       $content_type;
fastcgi_param  CONTENT_LENGTH     $content_length;

fastcgi_param  SCRIPT_NAME        $fastcgi_script_name;
fastcgi_param  REQUEST_URI        $request_uri;
fastcgi_param  DOCUMENT_URI       $document_uri;
fastcgi_param  DOCUMENT_ROOT      $document_root;
fastcgi_param  SERVER_PROTOCOL    $server_protocol;
fastcgi_param  REQUEST_SCHEME     $scheme;
fastcgi_param  HTTPS              $https if_not_empty;

fastcgi_param  GATEWAY_INTERFACE  CGI/1.1;
fastcgi_param  SERVER_SOFTWARE    nginx/$nginx_version;

fastcgi_param  REMOTE_ADDR        $remote_addr;
fastcgi_param  REMOTE_PORT        $remote_port;
fastcgi_param  SERVER_ADDR        $server_addr;
fastcgi_param  SERVER_PORT        $server_port;
fastcgi_param  SERVER_NAME        $server_name;

fastcgi_param  REDIRECT_STATUS    200;