HTML 500 error page for .htaccess errors - apache

Question

I know what HTML 500 errors are and I know how to research solving them,
I run a server with apache 2 and admin control is through cPanel and WHM.
What I am looking for is some guidance on how to feed back a useful and informative HTML 500 error page to the end user, after it has occurred on my website.
I have read: PHP: 500 Error to error page
and https://ux.stackexchange.com/questions/15955/how-to-create-a-useful-500-internal-server-error-page
but these don't seem to be what I'm looking for, for example http://www.smashingmagazine.com/2011/05/27/getting-started-with-defensive-web-design/ (referenced from one of the above links) tells me how to design the art work for the 500 error page but no indication as to how to deploy this page when the error occurs.
My most common point of the rare times I do have a few moments of HTML 500 errors, is with errors in the .htaccess file, for the website, this website may only be down for a few minutes but with ~50k visitors a month, this down time, even if momentary, will be seen by dozens of people and really should be presented well, nicely, informative and not the current standard which is:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
I have a server error page for 500 errors already set up and created and loadable in my server admin (cPanel), but the error page example (above) claims not to be able to find it, I also have an errorDocument handle in .htaccess which directs to this same page.
So my question is:
What processes do I need to set up to deploy a coherent and informative error 500 page when I get these errors?
specifically when I have .htaccess errors
I have a feeling that due to the nature of the error (server) it means that error pages can not be deployed from the same server as the error occurs, so the server can not work asynchronously?
Does the above mean that HTML 500 errors are only really deployable from multi-server situations or CDNs?

Answer 1

I have a feeling that due to the nature of the error (server) it means that error pages can not be deployed from the same server as the error occurs, so the server can not work asynchronously?
Does the above mean that HTML 500 errors are only really deployable from multi-server situations or CDNs?
I'd say they are only reliably deployable from a multi-server configuration.
The math is really simple:
An error page is itself a request to the server.
If the server is unable to complete requests due to misconfiguration or faulty code, there is no way to guarantee that the same will not happen to an error page.
But ultimately, it depends on your config.
If, for example, you make a mistake in a .htaccess file in some subfolder, causing infinite recursion, the error page will load just fine, provided it is not served from that directory.
If, however, you cause infinite redirection on all URLs, then the request to the error document will be affected by this too, which is precisely what happens when the server tells you:
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
This is like reporting a bug reporter bug - depending on its severity, this might or might not be possible.
If there is another server with a separate configuration, it will not be affected by the failures of server 1.
It doesn't need to be a physical server, simply launching a second web server will do as long as
one acts as a proxy or load balancer for the other
that proxy is configured to redirect responses from the other server with status code 500
that proxy is itself not misconfigured
For apache, using mod_proxy_balancer, such a configuration could look like
<VirtualHost *:80>
ServerName 127.0.0.1
DocumentRoot /var/www/errorpages/
</VirtualHost>
<VirtualHost *:80>
ServerName www.example.com
ServerAlias *.example.com
ServerAlias example.com
ProxyRequests Off
ProxyPreserveHost On
ProxyPass / balancer://whatever/
ProxyPassReverse / balancer://whatever/
<Proxy balancer://whatever/>
ProxySet failonstatus=500
BalancerMember http://127.0.0.1:42 retry=1
BalancerMember http://127.0.0.1:80 status=+H
</Proxy>
</VirtualHost>
This server would need to be started on port 80, and would pass all incoming requests to port 42 (hence the other server would need to listen to port 42, advisably only on 127.0.0.1). If any of the responses returned by the server running on port 42 were to have a status code of 500, the server on port 80 would resend the request to port 80 on 127.0.0.1, thus passing the request to itself, but with a different Host header, so that it wouldn't hit the load balancer again.