// 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 }