why nginx is not compressing reply from proxied server?

[Peter Volkov]

Hi, I have a HTTP/1.0 web server that streams chunked content to clients.
It does not gzip content so I would like to use nginx as reverse proxy to
compress output from this server. Yet, no matter what I do nginx is not
compressing results. For nginx I'm using following config:

server {
    gzip on;
    gzip_comp_level 5;
    gzip_http_version 1.0;
    gzip_buffers 4 32k;
    gzip_min_length 0;
    gzip_types application/octet-stream;
    gzip_proxied any;
    gzip_vary on;

    proxy_buffering off;

    location / {
        proxy_pass http://10.239.254.17:9102;
    }
}

Then I'm using curl to request content:

curl -v IP:9202 -H 'Accept-Encoding: deflate, gzip' -o output
=====================================================
> GET / HTTP/1.1
> Host: IP:9202
> User-Agent: curl/7.76.1
> Accept: */*
> Accept-Encoding: deflate, gzip
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Server: nginx/1.17.5
< Date: Tue, 11 May 2021 15:05:56 GMT
< Content-Type: application/octet-stream
< Transfer-Encoding: chunked
< Connection: keep-alive
< Keep-Alive: timeout=20
< Accept-Ranges: none
< Cache-Control: no-cache
<
{ [672 bytes data]
100 3343k    0 3343k    0     0  4866k      0 --:--:-- --:--:-- --:--:--
4859k^C
=====================================================

In result output is not compressed. What may cause this behaviour?

This is how request/respond to downstream server looks like:
 curl -v http://10.239.254.17:9102
*   Trying 10.239.254.17:9102...
* TCP_NODELAY set
* Connected to 10.239.254.17 (10.239.254.17) port 9102 (#0)
> GET / HTTP/1.1
> Host: 10.239.254.17:9102
> User-Agent: curl/7.66.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
* HTTP 1.0, assume close after body
< HTTP/1.0 200 OK
< Server: UDP to HTTP tool with smart buffering
< Accept-Ranges: none
< Content-type: application/octet-stream
< Cache-Control: no-cache


Thanks in advance for any help,
--
Peter.
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