Validate resolv.conf for presence of nameserver entries

Co-authored-by: Brad Davidson <brad@oatmail.org>
Signed-off-by: linxin <linxin@geedgenetworks.com>
(cherry picked from commit f24ba9d3a9)
Signed-off-by: Brad Davidson <brad.davidson@rancher.com>

pkg/agent/config/config.go

@@ -316,19 +316,22 @@ func isValidResolvConf(resolvConfFile string) bool {
 
 	nameserver := regexp.MustCompile(`^nameserver\s+([^\s]*)`)
 	scanner := bufio.NewScanner(file)
+	foundNameserver := false
 	for scanner.Scan() {
 		ipMatch := nameserver.FindStringSubmatch(scanner.Text())
 		if len(ipMatch) == 2 {
 			ip := net.ParseIP(ipMatch[1])
 			if ip == nil || !ip.IsGlobalUnicast() {
 				return false
+			} else {
+				foundNameserver = true
 			}
 		}
 	}
 	if err := scanner.Err(); err != nil {
 		return false
 	}
-	return true
+	return foundNameserver
 }
 
 func locateOrGenerateResolvConf(envInfo *cmds.Agent) string {