Stuck in weird issue - need help pls

[blason]

Hi Team,

I am stuck in this weird issue. I have nginx as my reverse proxy set in
front of Apache web server Some how my proxy_pass is not working as expected
and getting 404 not found error while retrieving page. Can someone pls
help?

Reve Proxy IP - 10.122.0.4
Apache 10.122.0.3

On my Rev Proxy /etc/hosts file
10.122.0.3      ipbl.xxxx.xxx

Here is my nginx stanza

server {
        listen 80;
        server_name  threat.list.xxx.xxx;
#       return 301 https://$server_name$request_uri;
        add_header X-Frame-Options "SAMEORIGIN";
        modsecurity on;
        modsecurity_rules_file /etc/nginx/modsec/main.conf;
        error_page 404 403 /custom_404.html;
location = /custom_404.html {
        root /usr/share/nginx/html;
        internal;
        }
        access_log /var/log/nginx/threatlist/access.log;
        error_log /var/log/nginx/threatlist/error.log;
location / {
        if ($request_method !~ "GET") {
        return 403;
        break;
      }
        include /etc/nginx/threatlistacl/ipacls;
        deny all;
        client_max_body_size    10m;
        client_body_buffer_size 128k;
        proxy_send_timeout   90;
        proxy_read_timeout   90;
        proxy_buffer_size    128k;
        proxy_buffers     4 256k;
        proxy_busy_buffers_size 256k;
        proxy_temp_file_write_size 256k;
        proxy_connect_timeout 30s;
        proxy_pass http://ipbl.xxxx.xxxx;
        proxy_set_header   Host   $host;
        proxy_set_header   X-Real-IP  $remote_addr;
        proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;
        }
}

Now if I access ipbl.xxx.xxx/ipbl.txt page it gets accessed successfully

Request URL: http://threat.list.xxx.xxx/ipbl.txt
Request Method: GET
Status Code: 404 Not Found
Remote Address: xxx.xx.xx.xx:80
Referrer Policy: strict-origin-when-cross-origin
Connection: keep-alive
Content-Type: text/html; charset=iso-8859-1
Date: Sat, 13 Mar 2021 04:50:53 GMT
Server: nginx
Transfer-Encoding: chunked
Accept:
text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate
Accept-Language: en-GB,en;q=0.9
Connection: keep-alive
DNT: 1
Host: threat.list.xxx.xxx
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36
(KHTML, like Gecko) Chrome/89.0.4389.82 Safari/537.36

And my access.log

xx.xx.xx.xx - - [13/Mar/2021:10:31:17 +0530] "GET /ipbl.txt HTTP/1.1" 404
183 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36
(KHTML, like Gecko) Chrome/89.0.4389.82 Safari/537.36"

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,290958,290958#msg-290958