refactor the code related to xtcp (#3449)

pkg/nathole/utils.go

@@ -16,6 +16,7 @@ package nathole
 
 import (
 	"bytes"
+	"fmt"
 	"net"
 	"strconv"
 
@@ -63,3 +64,49 @@ func (s *ChangedAddress) GetFrom(m *stun.Message) error {
 func (s *ChangedAddress) String() string {
 	return net.JoinHostPort(s.IP.String(), strconv.Itoa(s.Port))
 }
+
+func ListAllLocalIPs() ([]net.IP, error) {
+	addrs, err := net.InterfaceAddrs()
+	if err != nil {
+		return nil, err
+	}
+	ips := make([]net.IP, 0, len(addrs))
+	for _, addr := range addrs {
+		ip, _, err := net.ParseCIDR(addr.String())
+		if err != nil {
+			continue
+		}
+		ips = append(ips, ip)
+	}
+	return ips, nil
+}
+
+func ListLocalIPsForNatHole(max int) ([]string, error) {
+	if max <= 0 {
+		return nil, fmt.Errorf("max must be greater than 0")
+	}
+
+	ips, err := ListAllLocalIPs()
+	if err != nil {
+		return nil, err
+	}
+
+	filtered := make([]string, 0, max)
+	for _, ip := range ips {
+		if len(filtered) >= max {
+			break
+		}
+
+		// ignore ipv6 address
+		if ip.To4() == nil {
+			continue
+		}
+		// ignore localhost IP
+		if ip.IsLoopback() || ip.IsLinkLocalUnicast() || ip.IsLinkLocalMulticast() {
+			continue
+		}
+
+		filtered = append(filtered, ip.String())
+	}
+	return filtered, nil
+}