net: use libresolv rules for ndots range and validation

BIND libresolv allows values from 0 to 15.

For invalid values and negative numbers, 0 is used.
For numbers greater than 15, 15 is used.

Fixes #15419

Change-Id: I1009bc119c3e87919bcb55a80a35532e9fc3ba52
Reviewed-on: https://go-review.googlesource.com/24901Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/dnsconfig_unix.go

@@ -92,8 +92,10 @@ func dnsReadConfig(filename string) *dnsConfig {
 				switch {
 				case hasPrefix(s, "ndots:"):
 					n, _, _ := dtoi(s[6:])
-					if n < 1 {
-						n = 1
+					if n < 0 {
+						n = 0
+					} else if n > 15 {
+						n = 15
 					}
 					conf.ndots = n
 				case hasPrefix(s, "timeout:"):

src/net/dnsconfig_unix_test.go

@@ -60,6 +60,36 @@ var dnsReadConfigTests = []struct {
 			search:   []string{"domain.local."},
 		},
 	},
+	{
+		name: "testdata/invalid-ndots-resolv.conf",
+		want: &dnsConfig{
+			servers:  defaultNS,
+			ndots:    0,
+			timeout:  5 * time.Second,
+			attempts: 2,
+			search:   []string{"domain.local."},
+		},
+	},
+	{
+		name: "testdata/large-ndots-resolv.conf",
+		want: &dnsConfig{
+			servers:  defaultNS,
+			ndots:    15,
+			timeout:  5 * time.Second,
+			attempts: 2,
+			search:   []string{"domain.local."},
+		},
+	},
+	{
+		name: "testdata/negative-ndots-resolv.conf",
+		want: &dnsConfig{
+			servers:  defaultNS,
+			ndots:    0,
+			timeout:  5 * time.Second,
+			attempts: 2,
+			search:   []string{"domain.local."},
+		},
+	},
 	{
 		name: "testdata/openbsd-resolv.conf",
 		want: &dnsConfig{

src/net/testdata/invalid-ndots-resolv.conf

+options ndots:invalid
\ No newline at end of file

src/net/testdata/large-ndots-resolv.conf

+options ndots:16
\ No newline at end of file

src/net/testdata/negative-ndots-resolv.conf

+options ndots:-1
\ No newline at end of file