doc: add corefile(5) manual page (#1351)

Makefile.doc

@@ -14,6 +14,11 @@ man/coredns.1: coredns.1.md
 	$(RONN) --$(ORG) --manual='CoreDNS' $@.md
 	rm $@.md
 
+man/corefile.5: corefile.5.md
+	sed -e 's/^\(#.*\)/\U\1/' $< > $@.md
+	$(RONN) --$(ORG) --manual='CoreDNS' $@.md
+	rm $@.md
+
 .PHONY: plugins
 plugins:
 	for README in $(READMES); do \

coredns.1.md

@@ -50,4 +50,4 @@ Apache License 2.0
 
 ## See Also
 
-Corefile(7) @@PLUGINS@@.
+Corefile(5) @@PLUGINS@@.

corefile.5.md

@@ -0,0 +1,115 @@
+## Name
+
+*corefile* - configuration file for CoreDNS
+
+## Description
+
+A *corefile* specifies the (internal) servers CoreDNS should run and what plugins each of these
+should chain. The syntax is as follows:
+
+~~~ txt
+[SCHEME://]ZONE [[SCHEME://]ZONE]...[:PORT] {
+    [PLUGIN]...
+}
+~~~
+
+The **ZONE** defines for which name this server should be called, multiple zones are allowed and
+should be *white space* separated. You can use a "reverse" syntax to specify a reverse zone (i.e.
+ip6.arpa and in-addr.arpa), but using an IP address in the CIDR notation. The optional **SCHEME**
+defaults to `dns://`, but can also be `tls://` (DNS over TLS) or `grpc://` (DNS over gRPC).
+
+Specifying a **ZONE** *and* **PORT** combination multiple time for *different* servers will lead to
+an error on startup.
+
+When a query comes in it is matched again all zones for all servers, the server with the longest
+match on the query name will receive the query.
+
+The optional **PORT** controls on which port the server will bind, this default to 53. If you use
+a port number here, you *can't* override it with `-dns.port` (coredns(1)).
+
+**PLUGIN** defines the plugin(s) we want to load into this server. This is optional as well, but as
+server with no plugins will just return SERVFAIL for all queries. Each plugin can have a number of
+properties than can have arguments, see documentation for each plugin.
+
+Comments begin with an unquoted hash `#` and continue to the end of the line. Comments may be
+started anywhere on a line.
+
+Enviroment variables are supported and either the Unix or Windows form may be used: `{$ENV_VAR_1}`
+or `{%ENV_VAR_2%}`.
+
+You can use the `import` "plugin" to include parts of other files, see <https://coredns.io/explugins/import>.
+
+If CoreDNS can’t find a Corefile to load it loads the following builtin one:
+
+~~~ Corefile
+. {
+    whoami
+}
+~~~
+
+## Examples
+
+The **ZONE** is root zone `.`, the **PLUGIN** is chaos. The chaos plugin takes an argument:
+`CoreDNS-001`. This text is returned on a CH class query: `dig CH txt version.bind @localhost`.
+
+~~~ Corefile
+. {
+   chaos CoreDNS-001
+}
+~~~
+
+When defining a new zone, you either create a new server, or add it to an existing one. Here we
+define one server that handles two zones; that potentially chain different plugins:
+
+~~~ Corefile
+example.org {
+    whoami
+}
+org {
+    whoami
+}
+~~~
+
+Is identical to:
+
+~~~ Corefile
+example.org org {
+    whoami
+}
+~~~
+
+Reverse zones can be specified as domain names:
+
+~~~ Corefile
+0.0.10.in-addr.arpa {
+    whoami
+}
+~~~
+
+or by just using the CIDR notation:
+
+~~~ Corefile
+10.0.0.0/24 {
+    whoami
+}
+~~~
+
+This also works on a non octet boundary:
+
+~~~ Corefile
+10.0.0.0/27 {
+    whoami
+}
+~~~
+
+## Authors
+
+CoreDNS Authors.
+
+## Copyright
+
+Apache License 2.0
+
+## See Also
+
+The manual page for CoreDNS: coredns(1) and more documentation on <https://coredns.io>.