82 lines
2.2 KiB
Go
82 lines
2.2 KiB
Go
// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build darwin,!redo,!ios
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// (Exclude redo, because we don't want this code in the App Store
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// version's sandbox, where it won't work, and also don't want it on
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// iOS. This is just for utun-using non-sandboxed cmd/tailscaled on macOS.
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package interfaces
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import (
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"errors"
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"fmt"
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"net"
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"syscall"
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"golang.org/x/net/route"
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)
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func DefaultRouteInterface() (string, error) {
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idx, err := DefaultRouteInterfaceIndex()
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if err != nil {
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return "", err
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}
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iface, err := net.InterfaceByIndex(idx)
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if err != nil {
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return "", err
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}
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return iface.Name, nil
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}
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func DefaultRouteInterfaceIndex() (int, error) {
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// $ netstat -nr
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// Routing tables
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// Internet:
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// Destination Gateway Flags Netif Expire
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// default 10.0.0.1 UGSc en0 <-- want this one
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// default 10.0.0.1 UGScI en1
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// From man netstat:
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// U RTF_UP Route usable
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// G RTF_GATEWAY Destination requires forwarding by intermediary
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// S RTF_STATIC Manually added
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// c RTF_PRCLONING Protocol-specified generate new routes on use
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// I RTF_IFSCOPE Route is associated with an interface scope
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rib, err := route.FetchRIB(syscall.AF_UNSPEC, syscall.NET_RT_DUMP2, 0)
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if err != nil {
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return 0, fmt.Errorf("route.FetchRIB: %w", err)
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}
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msgs, err := route.ParseRIB(syscall.NET_RT_IFLIST2, rib)
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if err != nil {
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return 0, fmt.Errorf("route.ParseRIB: %w", err)
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}
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indexSeen := map[int]int{} // index => count
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for _, m := range msgs {
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rm, ok := m.(*route.RouteMessage)
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if !ok {
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continue
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}
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const RTF_GATEWAY = 0x2
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const RTF_IFSCOPE = 0x1000000
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if rm.Flags&RTF_GATEWAY == 0 {
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continue
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}
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if rm.Flags&RTF_IFSCOPE != 0 {
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continue
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}
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indexSeen[rm.Index]++
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}
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if len(indexSeen) == 0 {
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return 0, errors.New("no gateway index found")
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}
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if len(indexSeen) == 1 {
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for idx := range indexSeen {
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return idx, nil
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}
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}
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return 0, fmt.Errorf("ambiguous gateway interfaces found: %v", indexSeen)
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}
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