core: add bindaddr directive, allowing you to specify what address to listen on

config/directives.go

@@ -43,6 +43,7 @@ var directiveOrder = []directive{
 	// Essential directives that initialize vital configuration settings
 	{"root", setup.Root},
 	{"tls", setup.TLS},
+	{"bindaddr", setup.BindAddr},
 
 	// Other directives that don't create HTTP handlers
 	{"startup", setup.Startup},

config/setup/bindaddr.go

@@ -0,0 +1,12 @@
+package setup
+
+import "github.com/mholt/caddy/middleware"
+
+func BindAddr(c *Controller) (middleware.Middleware, error) {
+	for c.Next() {
+		if !c.Args(&c.BindAddress) {
+			return nil, c.ArgErr()
+		}
+	}
+	return nil, nil
+}

server/config.go

@@ -11,6 +11,9 @@ type Config struct {
 	// The hostname or IP on which to serve
 	Host string
 
+	// The address to bind on - defaults to Host if empty
+	BindAddress string
+
 	// The port to listen on
 	Port string
 
@@ -44,6 +47,9 @@ type Config struct {
 
 // Address returns the host:port of c as a string.
 func (c Config) Address() string {
+	if c.BindAddress != "" {
+		return net.JoinHostPort(c.BindAddress, c.Port)
+	}
 	return net.JoinHostPort(c.Host, c.Port)
 }