net/internal/nettest/interface.go

// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package nettest

import "net"

// IsMulticastCapable reports whether ifi is an IP multicast-capable
// network interface. Network must be "ip", "ip4" or "ip6".
func IsMulticastCapable(network string, ifi *net.Interface) (net.IP, bool) {
	switch network {
	case "ip", "ip4", "ip6":
	default:
		return nil, false
	}
	if ifi == nil || ifi.Flags&net.FlagUp == 0 || ifi.Flags&net.FlagMulticast == 0 {
		return nil, false
	}
	return hasRoutableIP(network, ifi)
}

// RoutedInterface returns a network interface that can route IP
// traffic and satisfies flags. It returns nil when an appropriate
// network interface is not found. Network must be "ip", "ip4" or
// "ip6".
func RoutedInterface(network string, flags net.Flags) *net.Interface {
	switch network {
	case "ip", "ip4", "ip6":
	default:
		return nil
	}
	ift, err := net.Interfaces()
	if err != nil {
		return nil
	}
	for _, ifi := range ift {
		if ifi.Flags&flags != flags {
			continue
		}
		if _, ok := hasRoutableIP(network, &ifi); !ok {
			continue
		}
		return &ifi
	}
	return nil
}

func hasRoutableIP(network string, ifi *net.Interface) (net.IP, bool) {
	ifat, err := ifi.Addrs()
	if err != nil {
		return nil, false
	}
	for _, ifa := range ifat {
		switch ifa := ifa.(type) {
		case *net.IPAddr:
			if ip := routableIP(network, ifa.IP); ip != nil {
				return ip, true
			}
		case *net.IPNet:
			if ip := routableIP(network, ifa.IP); ip != nil {
				return ip, true
			}
		}
	}
	return nil, false
}

func routableIP(network string, ip net.IP) net.IP {
	if !ip.IsLoopback() && !ip.IsLinkLocalUnicast() && !ip.IsGlobalUnicast() {
		return nil
	}
	switch network {
	case "ip4":
		if ip := ip.To4(); ip != nil {
			return ip
		}
	case "ip6":
		if ip.IsLoopback() { // addressing scope of the loopback address depends on each implementation
			return nil
		}
		if ip := ip.To16(); ip != nil && ip.To4() == nil {
			return ip
		}
	default:
		if ip := ip.To4(); ip != nil {
			return ip
		}
		if ip := ip.To16(); ip != nil {
			return ip
		}
	}
	return nil
}