net: Don't read beyond end of slice when parsing resolv.conf options.

Fixes #8252.

LGTM=adg
R=ruiu, josharian, adg
CC=golang-codereviews
https://golang.org/cl/102470046

src/pkg/net/dnsconfig_unix.go

@@ -75,19 +75,19 @@ func dnsReadConfig(filename string) (*dnsConfig, error) {
 			for i := 1; i < len(f); i++ {
 				s := f[i]
 				switch {
-				case len(s) >= 6 && s[0:6] == "ndots:":
+				case hasPrefix(s, "ndots:"):
 					n, _, _ := dtoi(s, 6)
 					if n < 1 {
 						n = 1
 					}
 					conf.ndots = n
-				case len(s) >= 8 && s[0:8] == "timeout:":
+				case hasPrefix(s, "timeout:"):
 					n, _, _ := dtoi(s, 8)
 					if n < 1 {
 						n = 1
 					}
 					conf.timeout = n
-				case len(s) >= 8 && s[0:9] == "attempts:":
+				case hasPrefix(s, "attempts:"):
 					n, _, _ := dtoi(s, 9)
 					if n < 1 {
 						n = 1
@@ -103,3 +103,7 @@ func dnsReadConfig(filename string) (*dnsConfig, error) {
 
 	return conf, nil
 }
+
+func hasPrefix(s, prefix string) bool {
+	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
+}

src/pkg/net/testdata/resolv.conf

@@ -3,3 +3,4 @@
 domain Home
 nameserver 192.168.1.1
 options ndots:5 timeout:10 attempts:3 rotate
+options attempts 3