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// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package netutil
import (
"errors"
"net"
"net/netip"
)
// DefaultInterfacePortable looks up the current default interface using a portable lookup method that
// works on most systems with a BSD style socket interface.
//
// Returns the interface name and IP address of the default route interface.
//
// If the default cannot be determined, an error is returned.
// Requires that there is a route on the system servicing UDP IPv4.
func DefaultInterfacePortable() (string, netip.Addr, error) {
// Note: UDP dial just performs a connect(2), and doesn't actually send a packet.
c, err := net.Dial("udp4", "8.8.8.8:53")
if err != nil {
return "", netip.Addr{}, err
}
laddr := c.LocalAddr().(*net.UDPAddr)
c.Close()
ifs, err := net.Interfaces()
if err != nil {
return "", netip.Addr{}, err
}
var (
iface *net.Interface
ipnet *net.IPNet
)
for _, ifc := range ifs {
addrs, err := ifc.Addrs()
if err != nil {
return "", netip.Addr{}, err
}
for _, addr := range addrs {
if ipn, ok := addr.(*net.IPNet); ok {
if ipn.Contains(laddr.IP) {
if ipnet == nil {
ipnet = ipn
iface = &ifc
} else {
newSize, _ := ipn.Mask.Size()
oldSize, _ := ipnet.Mask.Size()
if newSize > oldSize {
ipnet = ipn
iface = &ifc
}
}
}
}
}
}
if iface == nil {
return "", netip.Addr{}, errors.New("no default interface")
}
return iface.Name, laddr.AddrPort().Addr(), nil
}
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