diff options
Diffstat (limited to 'middleware.md')
-rw-r--r-- | middleware.md | 148 |
1 files changed, 0 insertions, 148 deletions
diff --git a/middleware.md b/middleware.md deleted file mode 100644 index f946e63e5..000000000 --- a/middleware.md +++ /dev/null @@ -1,148 +0,0 @@ -# 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.RcodeRefused) -* 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/2017/03/01/how-to-add-middleware-to-coredns/> -* <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. If the middleware supports dynamic health -reporting it should also have *Health* section detailing on its inner workings. - -## 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. -~~~ - -## Qualifying for main repo - -Middleware for CoreDNS can live out-of-tree, `middleware.cfg` defaults to CoreDNS' repo but other -repos work just as well. So when do we consider the inclusion of a new middleware in the main repo? - -* First, the middleware should be useful for other people. "Useful" is a subjective term. We will - probably need to further refine this. -* It should be sufficiently different from other middleware to warrant inclusion. -* Current internet standards need be supported: IPv4 and IPv6, so A and AAAA records should be - handled (if your middleware is in the business of dealing with address records that is). -* It must have tests. -* It must have a README.md for documentation. |