net: don't reject domain names with only numbers and hyphens

From https://github.com/golang/go/issues/17659#issuecomment-423113606 ...

> In kubernetes , isDomainName reject Pods "A Record" "pod-ip-address",
> for example: "172-17-0-16", as RFC 3696 section 2 requires
> "top-level domain names not be all-numeric", but this example has
> three hyphen, so I think it should not be reject.

Updates #17659

Change-Id: Ibd8ffb9473d69c45c91525953c09c6749233ca20
Reviewed-on: https://go-review.googlesource.com/136900
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Gudger <igudger@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/dnsclient.go

@@ -75,7 +75,7 @@ func isDomainName(s string) bool {
 	}
 
 	last := byte('.')
-	ok := false // Ok once we've seen a letter.
+	nonNumeric := false // true once we've seen a letter or hyphen
 	partlen := 0
 	for i := 0; i < len(s); i++ {
 		c := s[i]
@@ -83,7 +83,7 @@ func isDomainName(s string) bool {
 		default:
 			return false
 		case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || c == '_':
-			ok = true
+			nonNumeric = true
 			partlen++
 		case '0' <= c && c <= '9':
 			// fine
@@ -94,6 +94,7 @@ func isDomainName(s string) bool {
 				return false
 			}
 			partlen++
+			nonNumeric = true
 		case c == '.':
 			// Byte before dot cannot be dot, dash.
 			if last == '.' || last == '-' {
@@ -110,7 +111,7 @@ func isDomainName(s string) bool {
 		return false
 	}
 
-	return ok
+	return nonNumeric
 }
 
 // absDomainName returns an absolute domain name which ends with a

src/net/dnsname_test.go

@@ -22,6 +22,7 @@ var dnsNameTests = []dnsNameTest{
 	{"foo.com", true},
 	{"1foo.com", true},
 	{"26.0.0.73.com", true},
+	{"10-0-0-1", true},
 	{"fo-o.com", true},
 	{"fo1o.com", true},
 	{"foo1.com", true},