util/dnsname: use vizerror for all errors

The errors emitted by util/dnsname are all written at least moderately
friendly and none of them emit sensitive information. They should be
safe to display to end users.

Updates tailscale/corp#9025

Change-Id: Ic58705075bacf42f56378127532c5f28ff6bfc89
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

util/dnsname/dnsname.go

@@ -5,9 +5,9 @@
 package dnsname
 
 import (
-	"errors"
-	"fmt"
 	"strings"
+
+	"tailscale.com/util/vizerror"
 )
 
 const (
@@ -36,7 +36,7 @@ func ToFQDN(s string) (FQDN, error) {
 		totalLen += 1 // account for missing dot
 	}
 	if totalLen > maxNameLength {
-		return "", fmt.Errorf("%q is too long to be a DNS name", s)
+		return "", vizerror.Errorf("%q is too long to be a DNS name", s)
 	}
 
 	st := 0
@@ -54,7 +54,7 @@ func ToFQDN(s string) (FQDN, error) {
 		//
 		// See https://github.com/tailscale/tailscale/issues/2024 for more.
 		if len(label) == 0 || len(label) > maxLabelLength {
-			return "", fmt.Errorf("%q is not a valid DNS label", label)
+			return "", vizerror.Errorf("%q is not a valid DNS label", label)
 		}
 		st = i + 1
 	}
@@ -97,23 +97,23 @@ func (f FQDN) Contains(other FQDN) bool {
 // ValidLabel reports whether label is a valid DNS label.
 func ValidLabel(label string) error {
 	if len(label) == 0 {
-		return errors.New("empty DNS label")
+		return vizerror.New("empty DNS label")
 	}
 	if len(label) > maxLabelLength {
-		return fmt.Errorf("%q is too long, max length is %d bytes", label, maxLabelLength)
+		return vizerror.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)
+		return vizerror.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)
+		return vizerror.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 vizerror.Errorf("%q is not a valid DNS label: contains invalid character %q", label, label[i])
 		}
 	}
 	return nil