Error if using proxy_pass with variable and limit_except

[Haxe18@…]

Hi nginx guys,

i use a nginx in front of a varnish server.
I purge my varnish via purge method.

Nginx uses the following VHost config:

server {
    listen       *:80 default_server;

    location / {
        limit_except GET POST {
            allow 127.0.0.1/32;
            deny all;
        }

        set $upstream http://127.0.0.1:8080;

        if ($http_user_agent = 'mobile') {
            set $upstream http://127.0.0.1:8080;
        }

        proxy_pass              $upstream;
        proxy_set_header        Host $host;
        proxy_set_header        X-Forwarded-For $remote_addr;
    }
}

Suggested: From not localhost i only can request GET/HEAD/POST, localhost can do everything.

From remote it works as expected:

root@test:~# curl -X PURGE -I EXTIP
HTTP/1.1 403 Forbidden
Server: nginx
Date: Mon, 18 Sep 2017 10:39:23 GMT
Content-Type: text/html
Content-Length: 162
Connection: keep-alive
Vary: Accept-Encoding

But from localhost:

root@test:~# curl -X PURGE -I http://127.0.0.1
HTTP/1.1 500 Internal Server Error
Server: nginx
Date: Mon, 18 Sep 2017 10:39:06 GMT
Content-Type: text/html
Content-Length: 186
Connection: close

Nginx error log tells me:

==> /var/log/nginx/error.log <==
2017/09/18 12:39:06 [error] 2483#2483: *2 invalid URL prefix in "", client: 127.0.0.1, server: , request: "PURGE / HTTP/1.1", host: "127.0.0.1"

Without using Variables in VHost:

server {
    listen       *:80 default_server;

    location / {
        limit_except GET POST {
            allow 127.0.0.1/32;
            deny all;
        }

        proxy_pass              http://127.0.0.1:8080;
        proxy_set_header        Host $host;
        proxy_set_header        X-Forwarded-For $remote_addr;
    }
}

Works as expected:

root@test:~# curl -X PURGE -I http://127.0.0.1
HTTP/1.1 200 OK
Server: nginx
Date: Mon, 18 Sep 2017 10:45:35 GMT
Content-Type: text/html; charset=UTF-8
Transfer-Encoding: chunked
Connection: keep-alive
Vary: Accept-Encoding

Other tests with a variable proxy_pass e.g. using the get method instead of purge also fails with same error.

Please take a look why nginx fails when combining limit_except with proxypass and variables.
Thanks

[Maxim Dounin]

The problem that rewrite module directives (set, if) are not inherited into the limit_except block, and not executed there. As such, the $upstream variable is not set, and proxy_pass complains. This behaviour is basically identical to a nested location block.

An obvious workaround would be to use map instead, for example:

map $http_user_agent $upstream {
    default http://127.0.0.1:8080;
    mobile http://127.0.0.1:8080;
}

server {
    listen       *:80 default_server;

    location / {
        limit_except GET POST {
            allow 127.0.0.1/32;
            deny all;
        }

        proxy_pass              $upstream;
        proxy_set_header        Host $host;
        proxy_set_header        X-Forwarded-For $remote_addr;
    }
}

It doesn't looks possible to fix this without disallowing all the configuration directives inside an if in location first. See also #633, #86.