Allow docker API location to be specified or blanked

prog/plugin/main.go

@@ -28,14 +28,22 @@ var Log = common.Log
 func main() {
 	var (
 		justVersion      bool
+		dockerAPI        string
 		address          string
 		meshAddress      string
 		logLevel         string
 		noMulticastRoute bool
+
+		defaultDockerHost = "unix:///var/run/docker.sock"
 	)
 
+	if val := os.Getenv("DOCKER_HOST"); val != "" {
+		defaultDockerHost = val
+	}
+
 	flag.BoolVar(&justVersion, "version", false, "print version and exit")
 	flag.StringVar(&logLevel, "log-level", "info", "logging level (debug, info, warning, error)")
+	flag.StringVar(&dockerAPI, "docker-api", defaultDockerHost, "Docker API endpoint")
 	flag.StringVar(&address, "socket", "/run/docker/plugins/weave.sock", "socket on which to listen")
 	flag.StringVar(&meshAddress, "meshsocket", "/run/docker/plugins/weavemesh.sock", "socket on which to listen in mesh mode")
 	flag.BoolVar(&noMulticastRoute, "no-multicast-route", false, "deprecated (this is now the default)")
@@ -51,17 +59,25 @@ func main() {
 
 	weave := weaveapi.NewClient(os.Getenv("WEAVE_HTTP_ADDR"), Log)
 
-	// API 1.21 is the first version that supports docker network commands
-	dockerClient, err := docker.NewVersionedClientFromEnv("1.21")
-	if err != nil {
-		Log.Fatalf("unable to connect to docker: %s", err)
+	var dockerClient *docker.Client
+	var err error
+	if dockerAPI != "" {
+		// API 1.21 is the first version that supports docker network commands
+		dockerClient, err = docker.NewVersionedClient(dockerAPI, "1.21")
+		if err != nil {
+			Log.Fatalf("unable to connect to docker: %s", err)
+		}
 	}
 
 	Log.Println("Weave plugin", version, "Command line options:", os.Args[1:])
 	if noMulticastRoute {
 		Log.Warning("--no-multicast-route option has been removed; multicast is off by default")
 	}
-	Log.Info(dockerClient.Info())
+	if dockerClient == nil {
+		Log.Info("Running without Docker API connection")
+	} else {
+		Log.Info(dockerClient.Info())
+	}
 
 	err = run(dockerClient, weave, address, meshAddress)
 	if err != nil {