How long do browsers cache HTTP 301s?
I am debugging a problem with a HTTP 301 Permanent Redirect. After a quick test, it seems that Safari clears its cache of 301s when it is restarted, but Firefox does not.
When do IE, Chrome, Firefox and Safari clear their cache of 301s?
UPDATE: For example, if I want to redirect example1.com to example2.com, but I accidentally set it to redirect to example3.com, that is a problem. I can correct the mistake, but anyone who has visited example1.com in the meantime will have cached the incorrect redirect to example3.com, and so they will not be able to reach either example1.com or example2.com until their cache is cleared. Upon investigation, I find that there were no Cache-Control and Expires headers set. The headers for the incorrect 301 response would have been like this:
HTTP/1.1 301 Moved Permanently
Date: Wed, 27 Feb 2013 12:05:53 GMT
Server: Apache/2.2.21 (Unix) DAV/2 PHP/5.3.8
X-Powered-By: PHP/5.3.8
Location: http://example3.com/
Content-Type: text/html
My own tests show that:
- IE7, IE8, Android 2.3.4 do not cache at all.
- Firefox 18.0.2, Safari 5.1.7 (on Windows 7), and Opera 12.14 all cache, and clear the cache on browser restart.
- IE10 and Chrome 25 cache, but do not clear on browser restart, so when will they clear?
Answer
In the absense of cache control directives that specify otherwise, a 301 redirect defaults to being cached without any expiry date.
That is, it will remain cached for as long as the browser's cache can accommodate it. It will be removed from the cache if you manually clear the cache, or if the cache entries are purged to make room for new ones.
You can verify this at least in Firefox by going to about:cache and finding it under disk cache. It works this way in other browsers including Chrome and the Chromium based Edge, though they don't have an about:cache for inspecting the cache.
In all browsers it is still possible to override this default behavior using caching directives, as described below:
If you don't want the redirect to be cached
This indefinite caching is only the default caching by these browsers in the absence of headers that specify otherwise. The logic is that you are specifying a "permanent" redirect and not giving them any other caching instructions, so they'll treat it as if you wanted it indefinitely cached.
The browsers still honor the Cache-Control and Expires headers like with any other response, if they are specified.
You can add headers such as Cache-Control: max-age=3600 or Expires: Thu, 01 Dec 2014 16:00:00 GMT to your 301 redirects. You could even add Cache-Control: no-cache so it won't be cached permanently by the browser or Cache-Control: no-store so it can't even be stored in temporary storage by the browser.
Though, if you don't want your redirect to be permanent, it may be a better option to use a 302 or 307 redirect. Issuing a 301 redirect but marking it as non-cacheable is going against the spirit of what a 301 redirect is for, even though it is technically valid. YMMV, and you may find edge cases where it makes sense for a "permanent" redirect to have a time limit. Note that 302 and 307 redirects aren't cached by default by browsers.
If you previously issued a 301 redirect but want to un-do that
If people still have the cached 301 redirect in their browser they will continue to be taken to the target page regardless of whether the source page still has the redirect in place. Your options for fixing this include:
A simple solution is to issue another redirect back again.
If the browser is directed back to a same URL a second time during a redirect, it should fetch it from the origin again instead of redirecting again from cache, in an attempt to avoid a redirect loop. Comments on this answer indicate this now works in all major browsers - but there may be some minor browsers where it doesn't.
If you don't have control over the site where the previous redirect target went to, then you are out of luck. Try and beg the site owner to redirect back to you.
Prevention is better than cure - avoid a 301 redirect if you are not sure you want to permanently de-commission the old URL.