[Resolved] For https://forum.nginx.org/read.php?2,286645

README.md

Issue

According to some references below, I assume nginx would downgrade strong etags to weak ones when it modifies response content (e.g. gzip compression). But nginx removes strong etags on gzip compression instead of a downgrade.

• http://nginx.org/en/CHANGES (See "Changes with nginx 1.7.3 08 Jul 2014")

  • *) Feature: weak entity tags are now preserved on response modifications, and strong ones are changed to weak.

• https://github.com/nginx/nginx/commit/def16742a1ec22ece8279185eb2b798eb5ffa031

  • Entity tags: downgrade strong etags to weak ones as needed.

Step to reproduce

git clone https://gist.github.com/86f2d5b464f5e88821133c43a9cf4956.git
cd 86f2d5b464f5e88821133c43a9cf4956
docker-compose up -d

curl http://localhost:80/strong_etag -i -H "Accept-Encoding: gzip"

docker-compose up -d runs 2 nginx servers.

1. proxy
   • It just does gzip compression of response. See proxy-nginx.conf.
2. upstream
   • It just returns a JSON response with an etag. /weak_etag returns a weak etag, /strong_etag returns a strong one. See upstream-nginx.conf.

Expected behavior

• Etag header exists.
• nginx downgrades the strong Etag to weak Etag as described at https://github.com/nginx/nginx/commit/def16742a1ec22ece8279185eb2b798eb5ffa031.

HTTP/1.1 200 OK
Server: nginx/1.17.6
Date: Wed, 01 Jan 2020 11:34:44 GMT
Content-Type: application/json
Transfer-Encoding: chunked
Connection: keep-alive
Etag: d41d8cd98f00b204e9800998ecf8427e
Content-Encoding: gzip

�V�M-.NLOU�RP�H����Q(��,V����\�҂⒢��\E�Z5[,%

Actual behavior

• Etag is removed.

HTTP/1.1 200 OK
Server: nginx/1.17.6
Date: Wed, 01 Jan 2020 11:34:44 GMT
Content-Type: application/json
Transfer-Encoding: chunked
Connection: keep-alive
Content-Encoding: gzip

�V�M-.NLOU�RP�H����Q(��,V����\�҂⒢��\E�Z5[,%

Other information

When upstream returns a weak etag

• The weak etag passes through the proxy nginx.

curl http://localhost:80/weak_etag -i -H "Accept-Encoding: gzip"
HTTP/1.1 200 OK
Server: nginx/1.17.6
Date: Wed, 01 Jan 2020 11:54:12 GMT
Content-Type: application/json
Transfer-Encoding: chunked
Connection: keep-alive
Etag: W/d41d8cd98f00b204e9800998ecf8427e
Content-Encoding: gzip

�V�M-.NLOU�RP�H����Q(��,V����\�҂⒢��\E�Z5[,%

When gzip is off

• The strong etag passes through the proxy nginx.

$ curl http://localhost:80/strong_etag -i
HTTP/1.1 200 OK
Server: nginx/1.17.6
Date: Wed, 01 Jan 2020 11:56:29 GMT
Content-Type: application/json
Content-Length: 44
Connection: keep-alive
Etag: d41d8cd98f00b204e9800998ecf8427e

{"message": "Hello, this is from upstream!"}%

docker-compose.yml

version: '3'
services:
  proxy:
    image: nginx:1.17.6-alpine
    ports:
      - "80:80"
    volumes:
      - ./proxy-nginx.conf:/etc/nginx/nginx.conf

  upstream:
    image: nginx:1.17.6-alpine
    volumes:
      - ./upstream-nginx.conf:/etc/nginx/nginx.conf

proxy-nginx.conf

events {
}

http {
  server {
    gzip on;
    gzip_types application/json;

    location / {
      proxy_pass http://upstream:80;
    }
  }
}

upstream-nginx.conf

events {
}

http {
  server {
    location /weak_etag {
      add_header Etag W/d41d8cd98f00b204e9800998ecf8427e;
      default_type application/json;
      return 200 '{"message": "Hello, this is from upstream!"}';
    }

    location /strong_etag {
      add_header Etag d41d8cd98f00b204e9800998ecf8427e;
      default_type application/json;
      return 200 '{"message": "Hello, this is from upstream!"}';
    }
  }
}