Cannot read "data" from js_body_filter

[joaosobreira]

I'm trying to hash the content of a file (sent in chunks) in a response from the frontend.

Log of "data" in js_body_filter show unreadable data that I can't decode as shown below.

default.conf

server_tokens off;

#frontend.
server {
    listen 443 ssl;
    server_name 132.226.195.22;
    ssl_certificate      /home/zerossl/certificateTRUE.crt; 
    ssl_certificate_key  /home/zerossl/private.key;
    ssl_protocols TLSv1.2 TLSv1.3;
    
    add_header X-Frame-Options "SAMEORIGIN";
    add_header Strict-Transport-Security "max-age=31536000; includeSubdomains; preload";
    add_header Content-Security-Policy "default-src 'self' http: https: data: blob: 'unsafe-inline'" always;

    location / {
        js_body_filter main.audit;
        proxy_pass https://138.3.245.53:3000/;
    }
}

nginx.conf

user  nginx;
worker_processes  auto;

error_log  /var/log/nginx/error.log debug;
pid        /var/run/nginx.pid;

load_module modules/ngx_http_js_module.so;
load_module modules/ngx_stream_js_module.so;


events {
    worker_connections  1024;
}


http {
    js_path "/etc/nginx/njs/";
    js_import main from hash.js;

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

    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;

    sendfile        on;
    keepalive_timeout  65;

    include /etc/nginx/conf.d/*.conf;
}

hash.js

function audit(r, data, flags) {
    var cr = require('crypto');
    var hash = cr.createHash('md5');
    let response = '';
    let responseHash = '';
    let finalHash = '';
    const refHash = '8978cf8669edb983c3bad1973fce0155';

    if (r.uri == "/static/js/main.chunk.js") {
        r.log(data)
        if (flags.last) {
            let finalHash = hash.update(responseHash).digest('hex');
            r.log(response)
            if (finalHash == refHash) {
                r.log(response)
                r.return(502);
            }
        }
    }

    r.sendBuffer(data, flags);
}

export default { audit }

[xeioex]

Hi @joaosobreira,

Most probably you are seeing a compressed response from your backend.
I suggest to play around with Accept-encoding header and proxy_set_directive.