Conditional nginx auth_request

Question

I want to have my nginx proxy perform a subrequest for authentication only if the client is not already authenticated. The conditional part is where I am stuck. How can I craft a configuration so that the client is only authenticated once per session?

I am able to successfully perform an auth_request to Apache and pull back the headers I want to pass on to the back-end, but this is occurring on every request and is expensive.

In the example here, my goal is to only perform the auth_request if the "Authorization" header is missing or empty or alternately a cookie containing the token

# DEFAULT BACKEND
    location / {

        proxy_pass_request_body off;

        if ($http_authorization ~* '')
        {
            rewrite ^(.*)$ /__login;
        }

        if ($user !~* "([aa-zZ]+)@example.com")
        {

        }

        if ($http_cookie !~* "(auth_cookie=([aa-zZ]+)@example.com)")
        {
            add_header Set-Cookie "auth_cookie=$user;domain=.example.com;Max-Age=3000";

        }

        proxy_pass_header x-webauth-user;
        proxy_pass_header Set-Cookie;
        proxy_pass http://example.com:6762/;

   }

location /__login { internal;

    auth_request /auth;
    auth_request_set $user $upstream_http_x_webauth_user;
    set $xuser $user;

    add_header Auth-User $user;
    proxy_set_header User-Name $user;
    proxy_set_header Authorization $http_authorization;

    #proxy_pass_header x-webauth-user;
    #proxy_pass_header Set-Cookie;

    proxy_pass http://example:6762/;

    access_log /etc/nginx/login_debug.log;
   }


location = /auth{
    internal;
    proxy_pass http://example.com:81/;

    proxy_pass_request_body off;
    proxy_set_header Content-Length "";
    proxy_set_header Host $host;
    proxy_set_header X-Forwarded-for $proxy_add_x_forwarded_for;
    #proxy_pass_header  Set-Cookie;
    #proxy_pass_header  x-webauth-user;
}

The Auth-User header gets lost on all requests after the first and the cookie never seems to get set, beyond that the page doesn't actually seem to render in a browser. I am obviously doing something very wrong, could some please help me figure this out.

Answer 1

Please check out the NJS (https://nginx.org/en/docs/njs/) module. It's really simple and for sure can do what you want. Here is the example solution:

file: /etc/nginx/conf.d/default.conf:

server {
    listen 80;
    server_name "SOME_SERVER";
    # make an authentication subrequest for every request
    auth_request /auth;

    # create a new variable AuthToken and set its value to the res.SOMEVALUE from the later subrequest... 
    auth_request_set $AuthToken $sent_http_token;
    
    # add new AuthToken to the request
    proxy_set_header Authorization $AuthToken;

    location / {
        proxy_pass http://SOME_ENDPOINT;
    }

    location = /auth {
        internal;

        proxy_pass_request_body off;
        proxy_set_header Content-Length "";
        proxy_set_header X-Original-URI $request_uri;

        js_content auth.main;
    }

    location /get-new-token-location {
       internal;

       proxy_pass http://SOMEURL;
    }
}

and the example of nginx.conf file to show how to enable the NJS module:

...
pid /var/run/nginx.pid;

load_module /usr/lib/nginx/modules/ngx_http_js_module.so;

events {
    use epoll;
    worker_connections 10000;
}


http {
    # import njs scripts
    js_import auth from /path/to/the/auth.js;
    
    include /etc/nginx/conf.d/default.conf;
}

and finally, the main function from auth.js file:

export default {main}

function main(r) {
    var token = "";
    // search token in Authorization header
    if (this.requestHeaderExists(r, 'Authorization')) {
        var m = r.headersIn.Authorization.match(/Bearer\s+(.+)/);

        if (m !== null && typeof m[1] !== 'undefined') {
            token = m[1];
        }
    }

    // search token in cookie
    if (token.length == 0) {
       ... code here ...
    }

    // token was found, you can somehow validate it if you want
    if (token.length > 0) {
       .., make sure token is valid...
    }
    else { // there is no token, so ask for the new one
       r.subrequest('/get-new-token-location, { method: 'GET' }, function(reply) {  
         var res = JSON.parse(reply.responseBody);
         // add token to the response headers of this sub-request
         r.headersOut['token'] = res.SOMEVALUE;
       }
    }
   r.return(200);
   return;
}

Please treat it as an example. Ok, maybe it looks complicated, but it is really powerful and for sure you can find more examples in the world wide web.

Answer 2

The Nginx wiki warns that if inside location may give unexpected results, but that rewrite ... last; is safe. Here is an example:

location / {
    if ($cookie_UserName = "") {
        rewrite ^ /__login$uri last;
    }
    proxy_pass http://backend-app;
}
location /__login {  internal;
    rewrite ^/__login(?<realurl>/.*)$ $realurl break;
    auth_request /auth;
    auth_request_set $user $upstream_http_x_webauth_user;
    proxy_set_header Cookie UserName=$user;
    proxy_pass http://backend-app;
    add_header Set-Cookie "UserName=$user;Max-Age=300";
}
location = /auth {  internal;
    proxy_pass_request_body off;
    proxy_set_header Content-Length "";
    proxy_pass http://auth-server/validate;
}

There are two cases: Cookie:UserName exists or not. If it exists the first proxy_pass is executed. Otherwise /__login is used. Note that $uri is passed, so that it can be sent to backend-app.

For more advanced conditionals, you may use map instead of if.

Beware, though, that not authenticating every request runs the risk of accepting requests with a "faked" cookie/header.