Nginx Cache and WP canonical URL create infinite loop (bug ?)

[Simone Fumagalli]

Hello everybody.

Just want to report this to all people who have an NGINX that cache request in front a Wordpress installation

Scenario

 - Nginx is configured as reverse proxy in front of an Apache that run Wordpress. 
 - proxy_cache_key is not defined so is $scheme$proxy_host$request_uri;

Let's say a client require the page http://www.mysite.com/my-page// (yes double slash at the end) that is not cached

 1) Nginx pass the request to Apache/Wordpress
 2) Apache/Wordpress process the request and reply with a 301 and sends the client to http://www.mysite.com/my-page/ because is the canonical URL
 3) Nginx save in the cache the response with these data 
        KEY: http://www.mysite.com/my-page/
        HTTP/1.0 301 Moved Permanently
	[..]
        Location: http://www.mysite.com/my-page/
	[..]
    As you can see the KEY is equal to Location
 4) The user/browser send a request for http://www.mysite.com/my-page/   (just one slash)
 5) Nginx find the key in the cache and so redirect the browser to http://www.mysite.com/my-page/ causing an infinite loop
 
Conclusions

 - All the users that require the page http://www.mysite.com/my-page/ while is cached are send in the loop
 - This happen of course also for requests with a number of slashes at the end > 2
 - I can't call this bug but is just an "unexpected" behaviour, for sure a malicious user can overload your site or make pages un-available with these kind of requests.
 - There are probably other type of URL that create this situation. Have found any until now, can someone point one out ?
 - Is there a way to avoid this ?

Thanks

--
Simone