Nginx prepends post request body with response data

[faizangulzar]

I have set up my Laravel API with Docker(version: 26.1.1), Nginx(image: nginx:stable-alpine), and PHP(image: php:7.4-fpm-alpine) on Debian vps.

For a few days I'm struggling with a strange problem: The result I receive for my API request does not only include the valid response, but also the parameters that I send along with the post request;

Request:
{ "email": "[email protected]", "password": "xxxxxxx" }

Response:
{ "email": "[email protected]", "password": "xxxxxxx" }{ "token": "xxxxxxxxxxxxxxxxxxxxxxxxxx", "type": "xxxxxxxx" }

This response does not come with a content-type: application/json but as text/html. It seems like the post request body is prepended to the response data of my API. This results in a JSON parse error on my client.
After restarting the nginx server, the problem disappeared temporarily and then started again. I checked in both Nginx and laravel logs, there would be no error.

[oxpa]

Log upstream related metrics (at least address, time, status and length ) and compare those with what you have in nginx response.
If the length matches - it's the upstream that behaves for you.
If upstream response length is lower than the length of the response - post your configuration, please.

But overall, this question has nothing to do with docker image of nginx, really.

[faizangulzar]

As per your instructions, I have logged upstream response length and added information below. Another thing I want to let you know that when I try this whole setup without docker with same versions of nginx and php on another debian vps, it works fine without any issue.

upstream related metrics
[10/Dec/2024:10:24:35 +0000] "POST /url/url/url HTTP/1.1" 200 upstream_response_length=416 upstream_response_time=0.214 upstream_status=200 "-" "PostmanRuntime/7.43.0"

nginx response size in postman

Response Size = 647 B
  Headers = 395 B
  Body = 252 B
  Uncompressed = 393 B

nginx/default.conf

log_format upstream_log '[$time_local] "$request" $status '
                        'upstream_response_length=$upstream_response_length '
                        'upstream_response_time=$upstream_response_time '
                        'upstream_status=$upstream_status '
                        '"$http_referer" "$http_user_agent"';

server {
    listen 80;
    index index.php index.html;
    server_name xx.xxx.com;
    error_log /var/log/nginx/error.log;
    access_log /var/log/nginx/access.log;
    root /var/www/html/public;

    ...

    location ~ \.php$ {
        access_log /var/log/nginx/upstream_access.log upstream_log;
        fastcgi_intercept_errors on;
        try_files $uri =404;
        fastcgi_split_path_info ^(.+\.php)(.+)$;
        fastcgi_pass 127.0.0.1:9000;
        fastcgi_index index.php;
        include fastcgi_params;
        include fastcgi.conf;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param PATH_INFO $fastcgi_path_info;
        fastcgi_param  REQUEST_METHOD $request_method;
        fastcgi_param HTTP_MOD_REWRITE on;
    }

    ...

}

docker-compose.yml

version: "3"

services:
  nginx:
    image: nginx:stable-alpine
    container_name: nginx
    volumes:
      - ../:/var/www/html
      - ./default.conf:/etc/nginx/conf.d/default.conf
    depends_on:
      - php
    network_mode: host

  php:
    build:
      context: .
      dockerfile: Dockerfile
    container_name: php
    environment:
      DB_HOST: xxxxxxxxx
      DB_PORT: xxxxxxxxx
      DB_DATABASE: xxxxxxxxx
      DB_USERNAME: xxxxxxxxx
      DB_PASSWORD: xxxxxxxxx
      REDIS_HOST: xxxxxxxx
    volumes:
      - ../:/var/www/html
    command: |
      sh -c "chown -R www-data:www-data /var/www/html && php-fpm"
    network_mode: host

  redis:
    build:
      context: ./redis
      dockerfile: Dockerfile
    container_name: redis
	command: sh -c 'chown -R redis:redis /data && redis-server'
    volumes:
      - ../../outside_project_folder/data/redis:/data
    network_mode: host

[oxpa]

@faizangulzar first of all, I don't see any matching numbers (body length reported by postman should match something in nginx logs, I suppose). Secondly, I don't know what was the request and response, so it's difficult for me to tell where the problem is.

What i meant is that upstream_response_length is response body length. And you can debug the problem further youself.
Say, if I write a configuration that does "return 200 '123\n';" it returns 4 symbols. And upstream_response_length is 4 for me.
Next, I test it with curl: curl $URL | wc -c this gives me the same "4" as the output.
And this result doesn't depend on compression on anything. And I can see upstream_response_length matches whatever I got from nginx.

Please, try similar approach: you can start a shell in the container with docker exec -it nginx /bin/sh (or similar), install curl with apk add curl.
You can then use postman to get proper curl command as in this SO question https://stackoverflow.com/questions/49432735/converting-a-postman-request-to-curl
And then try the curl | wc -c pointing it at the local nginx. The number from this command should match the number in logs.

If upstream_response_length doesn't match the curl | wc -c then I don't have any good ideas.

[oxpa]

It's also worth mentioning that it's not that easy to make nginx prepend request body to a reply. It is possible but requires some fiddling with configuration. So it's probably not an nginx problem, really