admin: Disable host checking if wildcard interface is specified

To clarify, listening on wildcard interfaces is NOT the default and
should only be done under certain circumstances and when you know
what you're doing. Emits a warning in the log.

Fixes https://github.com/caddyserver/caddy-docker/issues/71

2 files changed, 31 insertions, 13 deletions

diff --git a/admin.go b/admin.go
index 7217eb7..6831686 100644
--- a/admin.go
+++ b/admin.go
@@ -60,10 +60,11 @@ type AdminConfig struct {
 	// default.
 	EnforceOrigin bool `json:"enforce_origin,omitempty"`
 
-	// The list of allowed origins for API requests. Only used if
-	// `enforce_origin` is true. If not set, the listener address
-	// will be the default value. If set but empty, no origins will
-	// be allowed.
+	// The list of allowed origins/hosts for API requests. Only needed
+	// if accessing the admin endpoint from a host different from the
+	// socket's network interface or if `enforce_origin` is true. If not
+	// set, the listener address will be the default value. If set but
+	// empty, no origins will be allowed.
 	Origins []string `json:"origins,omitempty"`
 
 	// Options related to configuration management.
@@ -99,6 +100,7 @@ func (admin AdminConfig) listenAddr() (NetworkAddress, error) {
 func (admin AdminConfig) newAdminHandler(addr NetworkAddress) adminHandler {
 	muxWrap := adminHandler{
 		enforceOrigin:  admin.EnforceOrigin,
+		enforceHost:    !addr.isWildcardInterface(),
 		allowedOrigins: admin.allowedOrigins(addr),
 		mux:            http.NewServeMux(),
 	}
@@ -234,12 +236,15 @@ func replaceAdmin(cfg *Config) error {
 
 	go adminServer.Serve(ln)
 
-	Log().Named("admin").Info(
-		"admin endpoint started",
+	Log().Named("admin").Info("admin endpoint started",
 		zap.String("address", addr.String()),
 		zap.Bool("enforce_origin", adminConfig.EnforceOrigin),
-		zap.Strings("origins", handler.allowedOrigins),
-	)
+		zap.Strings("origins", handler.allowedOrigins))
+
+	if !handler.enforceHost {
+		Log().Named("admin").Warn("admin endpoint on open interface; host checking disabled",
+			zap.String("address", addr.String()))
+	}
 
 	return nil
 }
@@ -271,6 +276,7 @@ type AdminRoute struct {
 
 type adminHandler struct {
 	enforceOrigin  bool
+	enforceHost    bool
 	allowedOrigins []string
 	mux            *http.ServeMux
 }
@@ -292,11 +298,13 @@ func (h adminHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 // be called more than once per request, for example if a request
 // is rewritten (i.e. internal redirect).
 func (h adminHandler) serveHTTP(w http.ResponseWriter, r *http.Request) {
-	// DNS rebinding mitigation
-	err := h.checkHost(r)
-	if err != nil {
-		h.handleError(w, r, err)
-		return
+	if h.enforceHost {
+		// DNS rebinding mitigation
+		err := h.checkHost(r)
+		if err != nil {
+			h.handleError(w, r, err)
+			return
+		}
 	}
 
 	if h.enforceOrigin {
diff --git a/listeners.go b/listeners.go
index bfbe6dd..39bd811 100644
--- a/listeners.go
+++ b/listeners.go
@@ -302,6 +302,16 @@ func (na NetworkAddress) isLoopback() bool {
 	return false
 }
 
+func (na NetworkAddress) isWildcardInterface() bool {
+	if na.Host == "" {
+		return true
+	}
+	if ip := net.ParseIP(na.Host); ip != nil {
+		return ip.IsUnspecified()
+	}
+	return false
+}
+
 func (na NetworkAddress) port() string {
 	if na.StartPort == na.EndPort {
 		return strconv.FormatUint(uint64(na.StartPort), 10)