coredns/middleware.md

# Middleware

## 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 assumes
something has been written to the client (by the middleware).

## Hooking It Up

See a couple of blog posts on how to write and add middleware to CoreDNS:

* <https://blog.coredns.io/#> TO BE PUBLISHED.
* <https://blog.coredns.io/2016/12/19/writing-middleware-for-coredns/>, slightly older, but useful.

## Metrics

When exporting metrics the *Namespace* should be `middleware.Namespace` (="coredns"), and the
*Subsystem* should be the name of the middleware. The README.md for the middleware should then
also contain a *Metrics* section detailing the metrics.

## Documentation

Each middleware should have a README.md explaining what the middleware does and how it is
configured. The file should have the following layout:

* Title: use the middleware's name
* Subsection titled: "Syntax"
* Subsection titled: "Examples"

More sections are of course possible.

### Style

We use the Unix manual page style:

* The name of middleware in the running text should be italic: *middleware*.
* all CAPITAL: user supplied argument, in the running text references this use strong text: `**`:
  **EXAMPLE**.
* Optional text: in block quotes: `[optional]`.
* Use three dots to indicate multiple options are allowed: `arg...`.
* Item used literal: `literal`.

### Example Domain Names

Please be sure to use `example.org` or `example.net` in any examples you provide. These are the
standard domain names created for this purpose.

## Fallthrough

In a perfect world the following would be true for middleware: "Either you are responsible for
a zone or not". If the answer is "not", the middleware should call the next middleware in the chain.
If "yes" it should handle *all* names that fall in this zone and the names below - i.e. it should
handle the entire domain.

~~~ txt
. {
    file example.org db.example
}
~~~
In this example the *file* middleware is handling all names below (and including) `example.org`. If
a query comes in that is not a subdomain (or equal to) `example.org` the next middleware is called.

Now, the world isn't perfect, and there are good reasons to "fallthrough" to the next middlware,
meaning a middleware is only responsible for a subset of names within the zone. The first of these
to appear was the *reverse* middleware that synthesis PTR and A/AAAA responses (useful with IPv6).

The nature of the *reverse* middleware is such that it only deals with A,AAAA and PTR and then only
for a subset of the names. Ideally you would want to layer *reverse* **in front off** another
middleware such as *file* or *auto* (or even *proxy*). This means *reverse* handles some special
reverse cases and **all other** request are handled by the backing middleware. This is exactly what
"fallthrough" does. To keep things explicit we've opted that middlewares implement such behavior
should implement a `fallthrough` keyword.

### Example Fallthrough Usage

The following Corefile example, sets up the *reverse* middleware, but disables fallthrough. It
also defines a zonefile for use with the *file* middleware for other names in the `compute.internal`.

~~~ txt
arpa compute.internal {
    reverse 10.32.0.0/16 {
        hostname ip-{ip}.{zone[2]}
        #fallthrough
    }
    file db.compute.internal compute.internal
}
~~~

This works for returning a response to a PTR request:

~~~ sh
% dig +nocmd @localhost +noall +ans -x 10.32.0.1
1.0.32.10.in-addr.arpa.	3600	IN	PTR	ip-10-32-0-1.compute.internal.
~~~

And for the forward:

~~~ sh
% dig +nocmd @localhost +noall +ans A ip-10-32-0-1.compute.internal
ip-10-32-0-1.compute.internal. 3600 IN	A	10.32.0.1
~~~

But a query for `mx compute.internal` will return SERVFAIL. Now when we remove the '#' from
fallthrough and reload (on Unix: `kill -SIGUSR1 $(pidof coredns)`) CoreDNS, we *should* get an
answer for the MX query:

~~~ sh
% dig +nocmd @localhost +noall +ans MX compute.internal
compute.internal.	3600	IN	MX	10 mx.compute.internal.
~~~