# Writing middleware

From the Caddy docs:

> Oh yes, those pesky return values on ServeHTTP(). You read the documentation so you already know
> what they mean. But what does that imply for the behavior of your middleware?
>
> Basically, return a status code only if you did NOT write to the response body. If you DO write to
> the response body, return a status code of 0. Return an error value if your middleware encountered
> an error that you want logged. It is common to return an error status and an error value together,
> so that the error handler up the chain can write the correct error page.
>
> The returned status code is not logged directly; rather, it tells middleware higher up the chain
> what status code to use if/when the response body is written. Again, return a 0 status if you've
> already written a body!

In the DNS status codes are called rcodes and it's slightly harder to return the correct
answer in case of failure.

So CoreDNS treats

* SERVFAIL (dns.RcodeServerFailure)
* REFUSED (dns.RecodeRefused)
* FORMERR (dns.RcodeFormatError)
* NOTIMP (dns.RcodeNotImplemented)

as special and will then assume nothing has written to the client. In all other cases it is assume
something has been written to the client.