admin: Always enforce Host header checks

With a simple heuristic for loopback addresses, we can enable this by
default without adding unnecessary inconvenience.

admin.go

@@ -21,6 +21,7 @@ import (
 	"expvar"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"net/http/pprof"
 	"net/url"
@@ -78,27 +79,27 @@ type ConfigSettings struct {
 
 // listenAddr extracts a singular listen address from ac.Listen,
 // returning the network and the address of the listener.
-func (admin AdminConfig) listenAddr() (string, string, error) {
+func (admin AdminConfig) listenAddr() (NetworkAddress, error) {
 	input := admin.Listen
 	if input == "" {
 		input = DefaultAdminListen
 	}
 	listenAddr, err := ParseNetworkAddress(input)
 	if err != nil {
-		return "", "", fmt.Errorf("parsing admin listener address: %v", err)
+		return NetworkAddress{}, fmt.Errorf("parsing admin listener address: %v", err)
 	}
 	if listenAddr.PortRangeSize() != 1 {
-		return "", "", fmt.Errorf("admin endpoint must have exactly one address; cannot listen on %v", listenAddr)
+		return NetworkAddress{}, fmt.Errorf("admin endpoint must have exactly one address; cannot listen on %v", listenAddr)
 	}
-	return listenAddr.Network, listenAddr.JoinHostPort(0), nil
+	return listenAddr, nil
 }
 
 // newAdminHandler reads admin's config and returns an http.Handler suitable
 // for use in an admin endpoint server, which will be listening on listenAddr.
-func (admin AdminConfig) newAdminHandler(listenAddr string) adminHandler {
+func (admin AdminConfig) newAdminHandler(addr NetworkAddress) adminHandler {
 	muxWrap := adminHandler{
 		enforceOrigin:  admin.EnforceOrigin,
-		allowedOrigins: admin.allowedOrigins(listenAddr),
+		allowedOrigins: admin.allowedOrigins(addr),
 		mux:            http.NewServeMux(),
 	}
 
@@ -140,14 +141,30 @@ func (admin AdminConfig) newAdminHandler(listenAddr string) adminHandler {
 // If admin.Origins is nil (null), the provided listen address
 // will be used as the default origin. If admin.Origins is
 // empty, no origins will be allowed, effectively bricking the
-// endpoint, but whatever.
-func (admin AdminConfig) allowedOrigins(listen string) []string {
+// endpoint for non-unix-socket endpoints, but whatever.
+func (admin AdminConfig) allowedOrigins(addr NetworkAddress) []string {
 	uniqueOrigins := make(map[string]struct{})
 	for _, o := range admin.Origins {
 		uniqueOrigins[o] = struct{}{}
 	}
 	if admin.Origins == nil {
-		uniqueOrigins[listen] = struct{}{}
+		if addr.isLoopback() {
+			if addr.IsUnixNetwork() {
+				// RFC 2616, Section 14.26:
+				// "A client MUST include a Host header field in all HTTP/1.1 request
+				// messages. If the requested URI does not include an Internet host
+				// name for the service being requested, then the Host header field MUST
+				// be given with an empty value."
+				uniqueOrigins[""] = struct{}{}
+			} else {
+				uniqueOrigins[net.JoinHostPort("localhost", addr.port())] = struct{}{}
+				uniqueOrigins[net.JoinHostPort("::1", addr.port())] = struct{}{}
+				uniqueOrigins[net.JoinHostPort("127.0.0.1", addr.port())] = struct{}{}
+			}
+		}
+		if !addr.IsUnixNetwork() {
+			uniqueOrigins[addr.JoinHostPort(0)] = struct{}{}
+		}
 	}
 	allowed := make([]string, 0, len(uniqueOrigins))
 	for origin := range uniqueOrigins {
@@ -195,14 +212,14 @@ func replaceAdmin(cfg *Config) error {
 	}
 
 	// extract a singular listener address
-	netw, addr, err := adminConfig.listenAddr()
+	addr, err := adminConfig.listenAddr()
 	if err != nil {
 		return err
 	}
 
 	handler := adminConfig.newAdminHandler(addr)
 
-	ln, err := Listen(netw, addr)
+	ln, err := Listen(addr.Network, addr.JoinHostPort(0))
 	if err != nil {
 		return err
 	}
@@ -219,7 +236,7 @@ func replaceAdmin(cfg *Config) error {
 
 	Log().Named("admin").Info(
 		"admin endpoint started",
-		zap.String("address", addr),
+		zap.String("address", addr.String()),
 		zap.Bool("enforce_origin", adminConfig.EnforceOrigin),
 		zap.Strings("origins", handler.allowedOrigins),
 	)
@@ -263,6 +280,7 @@ type adminHandler struct {
 func (h adminHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	Log().Named("admin.api").Info("received request",
 		zap.String("method", r.Method),
+		zap.String("host", r.Host),
 		zap.String("uri", r.RequestURI),
 		zap.String("remote_addr", r.RemoteAddr),
 		zap.Reflect("headers", r.Header),
@@ -274,14 +292,14 @@ 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) {
-	if h.enforceOrigin {
-		// DNS rebinding mitigation
-		err := h.checkHost(r)
-		if err != nil {
-			h.handleError(w, r, err)
-			return
-		}
+	// DNS rebinding mitigation
+	err := h.checkHost(r)
+	if err != nil {
+		h.handleError(w, r, err)
+		return
+	}
 
+	if h.enforceOrigin {
 		// cross-site mitigation
 		origin, err := h.checkOrigin(r)
 		if err != nil {