cmd/k8s-operator: support setting a custom hostname.

Updates #502

Signed-off-by: David Anderson <danderson@tailscale.com>

util/dnsname/dnsname.go

@@ -6,6 +6,7 @@
 package dnsname
 
 import (
+	"errors"
 	"fmt"
 	"strings"
 )
@@ -94,6 +95,31 @@ func (f FQDN) Contains(other FQDN) bool {
 	return strings.HasSuffix(other.WithTrailingDot(), cmp)
 }
 
+// ValidLabel reports whether label is a valid DNS label.
+func ValidLabel(label string) error {
+	if len(label) == 0 {
+		return errors.New("empty DNS label")
+	}
+	if len(label) > maxLabelLength {
+		return fmt.Errorf("%q is too long, max length is %d bytes", label, maxLabelLength)
+	}
+	if !isalphanum(label[0]) {
+		return fmt.Errorf("%q is not a valid DNS label: must start with a letter or number", label)
+	}
+	if !isalphanum(label[len(label)-1]) {
+		return fmt.Errorf("%q is not a valid DNS label: must end with a letter or number", label)
+	}
+	if len(label) < 2 {
+		return nil
+	}
+	for i := 1; i < len(label)-1; i++ {
+		if !isdnschar(label[i]) {
+			return fmt.Errorf("%q is not a valid DNS label: contains invalid character %q", label, label[i])
+		}
+	}
+	return nil
+}
+
 // SanitizeLabel takes a string intended to be a DNS name label
 // and turns it into a valid name label according to RFC 1035.
 func SanitizeLabel(label string) string {