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// Copyright (c) 2020 Tailscale Inc & 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 interfaces
import (
"errors"
"fmt"
"log"
"net"
"syscall"
"golang.org/x/net/route"
"golang.org/x/sys/unix"
"inet.af/netaddr"
)
func DefaultRouteInterface() (string, error) {
idx, err := DefaultRouteInterfaceIndex()
if err != nil {
return "", err
}
iface, err := net.InterfaceByIndex(idx)
if err != nil {
return "", err
}
return iface.Name, nil
}
// fetchRoutingTable calls route.FetchRIB, fetching NET_RT_DUMP2.
func fetchRoutingTable() (rib []byte, err error) {
return route.FetchRIB(syscall.AF_UNSPEC, syscall.NET_RT_DUMP2, 0)
}
func DefaultRouteInterfaceIndex() (int, error) {
// $ netstat -nr
// Routing tables
// Internet:
// Destination Gateway Flags Netif Expire
// default 10.0.0.1 UGSc en0 <-- want this one
// default 10.0.0.1 UGScI en1
// From man netstat:
// U RTF_UP Route usable
// G RTF_GATEWAY Destination requires forwarding by intermediary
// S RTF_STATIC Manually added
// c RTF_PRCLONING Protocol-specified generate new routes on use
// I RTF_IFSCOPE Route is associated with an interface scope
rib, err := fetchRoutingTable()
if err != nil {
return 0, fmt.Errorf("route.FetchRIB: %w", err)
}
msgs, err := route.ParseRIB(syscall.NET_RT_IFLIST2, rib)
if err != nil {
return 0, fmt.Errorf("route.ParseRIB: %w", err)
}
indexSeen := map[int]int{} // index => count
for _, m := range msgs {
rm, ok := m.(*route.RouteMessage)
if !ok {
continue
}
const RTF_GATEWAY = 0x2
const RTF_IFSCOPE = 0x1000000
if rm.Flags&RTF_GATEWAY == 0 {
continue
}
if rm.Flags&RTF_IFSCOPE != 0 {
continue
}
indexSeen[rm.Index]++
}
if len(indexSeen) == 0 {
return 0, errors.New("no gateway index found")
}
if len(indexSeen) == 1 {
for idx := range indexSeen {
return idx, nil
}
}
return 0, fmt.Errorf("ambiguous gateway interfaces found: %v", indexSeen)
}
func init() {
likelyHomeRouterIP = likelyHomeRouterIPDarwinFetchRIB
}
func likelyHomeRouterIPDarwinFetchRIB() (ret netaddr.IP, ok bool) {
rib, err := fetchRoutingTable()
if err != nil {
log.Printf("routerIP/FetchRIB: %v", err)
return ret, false
}
msgs, err := route.ParseRIB(syscall.NET_RT_IFLIST2, rib)
if err != nil {
log.Printf("routerIP/ParseRIB: %v", err)
return ret, false
}
for _, m := range msgs {
rm, ok := m.(*route.RouteMessage)
if !ok {
continue
}
const RTF_GATEWAY = 0x2
const RTF_IFSCOPE = 0x1000000
if rm.Flags&RTF_GATEWAY == 0 {
continue
}
if rm.Flags&RTF_IFSCOPE != 0 {
continue
}
if len(rm.Addrs) > unix.RTAX_GATEWAY {
dst4, ok := rm.Addrs[unix.RTAX_DST].(*route.Inet4Addr)
if !ok || dst4.IP != ([4]byte{0, 0, 0, 0}) {
// Expect 0.0.0.0 as DST field.
continue
}
gw, ok := rm.Addrs[unix.RTAX_GATEWAY].(*route.Inet4Addr)
if !ok {
continue
}
return netaddr.IPv4(gw.IP[0], gw.IP[1], gw.IP[2], gw.IP[3]), true
}
}
return ret, false
}
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