How do I rewrite URLs in a proxy response in NGINX

Question

I'm used to using Apache with mod_proxy_html, and am trying to achieve something similar with NGINX. The specific use case is that I have an admin UI running in Tomcat on port 8080 on a server at the root context:

http://localhost:8080/

I need to surface this on port 80, but I have other contexts on the NGINX server running on this host, so want to try and access this at:

http://localhost:80/admin/

I was hoping that the following super simple server block would do it, but it doesn't quite:

server {
    listen  80;
    server_name screenly.local.akana.com;

    location /admin/ {
        proxy_pass http://localhost:8080/;
    }
}

The problem is that the returned content (html) contains URLs to scripts and style info that is all accessed at the root context, so I need to get these URLs rewritten to start with /admin/ instead of /.

How do I do this in NGINX?

Answer 1

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.

---

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.

---

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

Answer 2

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;
}

Answer 3

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;
  }
}

Answer 4

If there are no hyperlinks which need to be rewritten with sub_filter, you might just use the proxy_redirect directive:

location /admin/ {
    proxy_pass http://localhost:8080/;
    proxy_redirect / /admin/
}

It changes the Location-Header of the response according to the given 'match-rewrite' rule.