Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/bind/README.md

@@ -0,0 +1,22 @@
+# bind
+
+*bind* overrides the host to which the server should bind.
+
+Normally, the listener binds to the wildcard host. However, you may force the listener to bind to
+another IP instead. This directive accepts only an address, not a port.
+
+## Syntax
+
+~~~ txt
+bind ADDRESS
+~~~
+
+**ADDRESS** is the IP address to bind to.
+
+## Examples
+
+To make your socket accessible only to that machine, bind to IP 127.0.0.1 (localhost):
+
+~~~ txt
+bind 127.0.0.1
+~~~

plugin/bind/bind.go

@@ -0,0 +1,11 @@
+// Package bind allows binding to a specific interface instead of bind to all of them.
+package bind
+
+import "github.com/mholt/caddy"
+
+func init() {
+	caddy.RegisterPlugin("bind", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setupBind,
+	})
+}

plugin/bind/bind_test.go

@@ -0,0 +1,30 @@
+package bind
+
+import (
+	"testing"
+
+	"github.com/coredns/coredns/core/dnsserver"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetupBind(t *testing.T) {
+	c := caddy.NewTestController("dns", `bind 1.2.3.4`)
+	err := setupBind(c)
+	if err != nil {
+		t.Fatalf("Expected no errors, but got: %v", err)
+	}
+
+	cfg := dnsserver.GetConfig(c)
+	if got, want := cfg.ListenHost, "1.2.3.4"; got != want {
+		t.Errorf("Expected the config's ListenHost to be %s, was %s", want, got)
+	}
+}
+
+func TestBindAddress(t *testing.T) {
+	c := caddy.NewTestController("dns", `bind 1.2.3.bla`)
+	err := setupBind(c)
+	if err == nil {
+		t.Fatalf("Expected errors, but got none")
+	}
+}

plugin/bind/setup.go

@@ -0,0 +1,24 @@
+package bind
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func setupBind(c *caddy.Controller) error {
+	config := dnsserver.GetConfig(c)
+	for c.Next() {
+		if !c.Args(&config.ListenHost) {
+			return plugin.Error("bind", c.ArgErr())
+		}
+	}
+	if net.ParseIP(config.ListenHost) == nil {
+		return plugin.Error("bind", fmt.Errorf("not a valid IP address: %s", config.ListenHost))
+	}
+	return nil
+}