addrs, err := net.InterfaceAddrs()
	if err != nil {
		return false, false, err
	}
	ipv4 := false
	for _, address := range addrs {
		ipnet, ok := address.(*net.IPNet)
		if !ok || !ipnet.IP.IsLoopback() {
			continue
		}
		if netutils.IsIPv6(ipnet.IP) {
			ipv6 = true
			continue
		}
		ipv4 = true
	}
	ipv6only = ipv6 && !ipv4
	return ipv6, ipv6only, nil

			//   IPv6 addresses.
			//
			// - When IP.To16() == nil and IP.To4() != nil,
			//   the raw connection consists of an IPv4
			//   socket using only IPv4 addresses.
			//
			// - Otherwise, the raw connection is a dual
			//   stack socket, an IPv6 socket using IPv6
			//   addresses including IPv4-mapped or
			//   IPv4-embedded IPv6 addresses.
			if addr.IP.To16() != nil && addr.IP.To4() == nil {

	}
	defer l.Close()
	if a := l.Addr(); isNotIPv4(a) {
		t.Errorf("address does not contain IPv4: %v", a)
	}
}

func TestUDP4ListenZero(t *testing.T) {
	c, err := ListenPacket("udp4", "0.0.0.0:0")
	if err != nil {
		t.Fatal(err)
	}
	defer c.Close()
	if a := c.LocalAddr(); isNotIPv4(a) {
		t.Errorf("address does not contain IPv4: %v", a)
	}

	"runtime"
	"sync"
	_ "unsafe" // for linkname
)

// BUG(rsc,mikio): On DragonFly BSD and OpenBSD, listening on the
// "tcp" and "udp" networks does not listen for both IPv4 and IPv6
// connections. This is due to the fact that IPv4 traffic will not be
// routed to an IPv6 socket - two separate sockets are required if
// both address families are to be supported.
// See inet6(4) for details.

			//   IPv6 addresses.
			//
			// - When IP.To16() == nil and IP.To4() != nil,
			//   the raw connection consists of an IPv4
			//   socket using only IPv4 addresses.
			//
			// - Otherwise, the raw connection is a dual
			//   stack socket, an IPv6 socket using IPv6
			//   addresses including IPv4-mapped or
			//   IPv4-embedded IPv6 addresses.
			if addr.IP.To16() != nil && addr.IP.To4() == nil {

		}
	}

	// Test functionality of IPv4 communication using AF_INET6
	// sockets.
	if !supportsIPv4map() && supportsIPv4() && (network == "tcp" || network == "udp" || network == "ip") && wildcard {
		// At this point, we prefer IPv4 when ip is nil.
		// See favoriteAddrFamily for further information.
		if ip.To16() != nil && ip.To4() == nil && cip.To4() != nil { // a pair of IPv6 server and IPv4 client
			return false
		}

package net

/*
#include <netdb.h>
*/
import "C"

// NOTE(rsc): In theory there are approximately balanced
// arguments for and against including AI_ADDRCONFIG
// in the flags (it includes IPv4 results only on IPv4 systems,
// and similarly for IPv6), but in practice setting it causes
// getaddrinfo to return the wrong canonical name on Linux.
// So definitely leave it out.

	}
	return IPAddrs, nil
}

func TestGetLocalIP(t *testing.T) {
	tests := []struct {
		name     string
		lipas    func() ([]net.Addr, error)
		isDS     bool
		expected bool
	}{
		{
			name: "ipv4 only local ip addresses",
			lipas: func() ([]net.Addr, error) {
				return tesrLocalIPAddrs([]netip.Addr{
					netip.MustParseAddr("127.0.0.1"),
					netip.MustParseAddr("1.2.3.5"),
				})
			},
			isDS:     false,

	if isIpv4 {
		addr, err = unix.GetsockoptIPv6MTUInfo(
			int(fd),
			unix.IPPROTO_IP,
			constants.SoOriginalDst)
		if err != nil {
			log.Errorf("Error ipv4 getsockopt: %v", err)
			return
		}
		// See struct sockaddr_in
		daddr = net.IPv4(
			addr.Addr.Addr[0], addr.Addr.Addr[1], addr.Addr.Addr[2], addr.Addr.Addr[3])
	} else {
		addr, err = unix.GetsockoptIPv6MTUInfo(
			int(fd), unix.IPPROTO_IPV6,

	return ift, nil
}

const (
	// See linux/if_arp.h.
	// Note that Linux doesn't support IPv4 over IPv6 tunneling.
	sysARPHardwareIPv4IPv4 = 768 // IPv4 over IPv4 tunneling
	sysARPHardwareIPv6IPv6 = 769 // IPv6 over IPv6 tunneling
	sysARPHardwareIPv6IPv4 = 776 // IPv6 over IPv4 tunneling
	sysARPHardwareGREIPv4  = 778 // any over GRE over IPv4 tunneling
	sysARPHardwareGREIPv6  = 823 // any over GRE over IPv6 tunneling
)