Intercepting backend 301/302 redirects (proxy_pass) and rewriting to another location block possible?

Question

We have a couple of backends sitting behind our nginx front ends.

Is it possible to intercept 301 / 302 redirects sent by these backends and have nginx handle them?

We were thinging something alone the lines of:

error_page 302 = @target;

But I doubt 301/302 redirects can be handled the same as 404's etc etc... I mean, error_page probably doesnt apply to 200, etc error codes?

So to summarize:

Our backends send back 301/302s once in a while. We would like to have nginx intercept these, and rewrite them to another location block, where we could do any number of other things with them.

Possible?

Thanks!

Answer 1

You could use proxy_redirect directive:

http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_redirect

Nginx will still return 301/302 to the client but proxy_redirect will modify Location header and the client should make a new request to the URL given in the Location header.

Something like this should make the subsequent request back to nginx:

proxy_redirect http://upstream:port/ http://$http_host/;

Answer 2

I succeeded in solving a more generic case when a redirect location can be any external URL.

server {
    ...

    location / {
        proxy_pass http://backend;
        # You may need to uncomment the following line if your redirects are relative, e.g. /foo/bar
        #proxy_redirect / /;
        proxy_intercept_errors on;
        error_page 301 302 307 = @handle_redirects;
    }

    location @handle_redirects {
        set $saved_redirect_location '$upstream_http_location';
        proxy_pass $saved_redirect_location;
    }
}

Alternative approach, which is closer to what you describe, is covered in ServerFault answer to this question: https://serverfault.com/questions/641070/nginx-302-redirect-resolve-internally

Answer 3

If you need to follow multiple redirects, modify Vlad's solution as follows:

1) Add

recursive_error_pages on;

to location /.

2) Add

  proxy_intercept_errors on;
  error_page 301 302 307 = @handle_redirect;

to the location @handle_redirects section.

Answer 4

More on proxy_redirect, for relative locations

Case

location /api/ {
  proxy_pass http://${API_HOST}:${API_PORT}/;
}

• the backend redirects to a relative location, which miss the /api/ prefix
• the browser follows the redirection and hits a wall of incomprehension

Solution

location /api/ {
  proxy_pass http://${API_HOST}:${API_PORT}/;
  proxy_redirect ~^/(.*) http://$http_host/api/$1;
}