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