Revert "Use a single syscall for route table for health check IP"

This reverts commit ae35229180833f22dfbc0795d642fa5feaf6d577.

cr: https://code.amazon.com/reviews/CR-160879895

agent/platform/platform.go

@@ -17,6 +17,7 @@ package platform
 import (
 	"fmt"
 	"net"
+	"sort"
 	"unicode/utf8"
 
 	"github.com/aws/amazon-ssm-agent/agent/log"
@@ -85,16 +86,25 @@ func Hostname(log log.T) (name string, err error) {
 
 // IP of the network interface
 func IP() (selected string, err error) {
-	if addrs, err := net.InterfaceAddrs(); err == nil {
+	var interfaces []net.Interface
+	if interfaces, err = net.Interfaces(); err == nil {
+		interfaces = filterInterface(interfaces)
+		sort.Sort(byIndex(interfaces))
 		candidates := make([]net.IP, 0)
-		for _, addr := range addrs {
-			switch v := addr.(type) {
-			case *net.IPAddr:
-				candidates = append(candidates, v.IP.To4())
-				candidates = append(candidates, v.IP.To16())
-			case *net.IPNet:
-				candidates = append(candidates, v.IP.To4())
-				candidates = append(candidates, v.IP.To16())
+		for _, i := range interfaces {
+			var addrs []net.Addr
+			if addrs, err = i.Addrs(); err != nil {
+				continue
+			}
+			for _, addr := range addrs {
+				switch v := addr.(type) {
+				case *net.IPAddr:
+					candidates = append(candidates, v.IP.To4())
+					candidates = append(candidates, v.IP.To16())
+				case *net.IPNet:
+					candidates = append(candidates, v.IP.To4())
+					candidates = append(candidates, v.IP.To16())
+				}
 			}
 		}
 		selectedIp, err := selectIp(candidates)
@@ -147,6 +157,23 @@ func isIpv4(ip net.IP) bool {
 	return ip.To4() != nil
 }
 
+// filterInterface removes interface that's not up or is a loopback/p2p
+func filterInterface(interfaces []net.Interface) (i []net.Interface) {
+	for _, v := range interfaces {
+		if (v.Flags&net.FlagUp != 0) && (v.Flags&net.FlagLoopback == 0) && (v.Flags&net.FlagPointToPoint == 0) {
+			i = append(i, v)
+		}
+	}
+	return
+}
+
+// byIndex implements sorting for net.Interface.
+type byIndex []net.Interface
+
+func (b byIndex) Len() int           { return len(b) }
+func (b byIndex) Less(i, j int) bool { return b[i].Index < b[j].Index }
+func (b byIndex) Swap(i, j int)      { b[i], b[j] = b[j], b[i] }
+
 func IsPlatformNanoServer(log log.T) (bool, error) {
 	return isPlatformNanoServer(log)
 }