71 lines
1.3 KiB
Go
71 lines
1.3 KiB
Go
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package main
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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 defaultRouteIP() (net.IP, error) {
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idx, err := defaultRouteInterfaceIndex()
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if err != nil {
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return nil, 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 nil, err
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}
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addrs, err := iface.Addrs()
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if err != nil {
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return nil, err
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}
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for _, addr := range addrs {
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ip := addr.(*net.IPNet).IP
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if ip.To4() != nil {
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return ip, nil
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}
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}
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return nil, errors.New("no ipv4 addr")
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}
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func defaultRouteInterfaceIndex() (int, error) {
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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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for _, message := range msgs {
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routeMessage := message.(*route.RouteMessage)
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if routeMessage.Flags&(syscall.RTF_UP|syscall.RTF_GATEWAY|syscall.RTF_STATIC) == 0 {
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continue
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}
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addresses := routeMessage.Addrs
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destination, ok := addresses[0].(*route.Inet4Addr)
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if !ok {
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continue
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}
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if destination.IP != [4]byte{0, 0, 0, 0} {
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continue
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}
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switch addresses[1].(type) {
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case *route.Inet4Addr:
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return routeMessage.Index, nil
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default:
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continue
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}
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}
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return 0, fmt.Errorf("ambiguous gateway interfaces found")
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}
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