How do I rewrite URLs in a proxy response in NGINX

You can use the following nginx configuration example:

upstream adminhost {
  server adminhostname:8080;
}

server {
  listen 80;

  location ~ ^/admin/(.*)$ {
    proxy_pass http://adminhost/$1$is_args$args;
    proxy_redirect off;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Host $server_name;
  }
}

You may also need the following directive to be set before the first "sub_filter" for backend-servers with data compression:

proxy_set_header Accept-Encoding "";

Otherwise it may not work. For your example it will look like:

location /admin/ {
    proxy_pass http://localhost:8080/;
    proxy_set_header Accept-Encoding "";
    sub_filter "http://your_server/" "http://your_server/admin/";
    sub_filter_once off;
}

We should first read the documentation on proxy_pass carefully and fully.

The URI passed to upstream server is determined based on whether "proxy_pass" directive is used with URI or not. Trailing slash in proxy_pass directive means that URI is present and equal to /. Absense of trailing slash means hat URI is absent.

Proxy_pass with URI:

location /some_dir/ {
    proxy_pass http://some_server/;
}

With the above, there's the following proxy:

http:// your_server/some_dir/ some_subdir/some_file ->
http:// some_server/          some_subdir/some_file

Basically, /some_dir/ gets replaced by / to change the request path from /some_dir/some_subdir/some_file to /some_subdir/some_file.

Proxy_pass without URI:

location /some_dir/ {
    proxy_pass http://some_server;
}

With the second (no trailing slash): the proxy goes like this:

http:// your_server /some_dir/some_subdir/some_file ->
http:// some_server /some_dir/some_subdir/some_file

Basically, the full original request path gets passed on without changes.

---

So, in your case, it seems you should just drop the trailing slash to get what you want.

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Caveat

Note that automatic rewrite only works if you don't use variables in proxy_pass. If you use variables, you should do rewrite yourself:

location /some_dir/ {
  rewrite    /some_dir/(.*) /$1 break;
  proxy_pass $upstream_server;
}

There are other cases where rewrite wouldn't work, that's why reading documentation is a must.

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Edit

Reading your question again, it seems I may have missed that you just want to edit the html output.

For that, you can use the sub_filter directive. Something like ...

location /admin/ {
    proxy_pass http://localhost:8080/;
    sub_filter "http://your_server/" "http://your_server/admin/";
    sub_filter_once off;
}

Basically, the string you want to replace and the replacement string