Make sure the culster name is valid before creating it.

MAke sure the cluster name is a RFC 1123 compliant host name, since the
cluster name is used as base for expanding into a auto generated
host names for k3s nodes.

cli/commands.go

@@ -24,6 +24,37 @@ import (
 )
 
 const defaultRegistry = "docker.io"
+const clusterNameMaxSize int = 35
+
+// Make sure a cluster name is also a valid host name according to RFC 1123.
+// We further restrict the length of the cluster name to shorter than 'clusterNameMaxSize'
+// so that we can construct the host names based on the cluster name, and still stay
+// within the 64 characters limit.
+func checkClusterName(name string) error {
+       if len(name) > clusterNameMaxSize {
+	       return fmt.Errorf("cluster name is too long")
+       }
+
+       if name[0] == '-' || name[len(name) - 1] == '-' {
+	       return fmt.Errorf("cluster name can not start or end with - (dash)")
+       }
+
+
+       for _ , c := range name {
+		switch {
+			case '0' <= c && c <= '9':
+			case 'a' <= c && c <= 'z':
+			case 'A' <= c && c <= 'Z':
+			case c == '-':
+				break;
+		default:
+	       		return fmt.Errorf("cluster name contains charaters other than 'Aa-Zz', '0-9' or '-'")
+
+       		}
+	}
+
+       return nil
+}
 
 // CheckTools checks if the docker API server is responding
 func CheckTools(c *cli.Context) error {
@@ -45,6 +76,10 @@ func CheckTools(c *cli.Context) error {
 // CreateCluster creates a new single-node cluster container and initializes the cluster directory
 func CreateCluster(c *cli.Context) error {
 
+	if err := checkClusterName(c.String("name")); err != nil {
+		return err;
+	}
+
 	// define image
 	image := c.String("image")
 	if c.IsSet("version") {