package main

import (
	"fmt"
	"syscall"
	"net"
	"unsafe"
	"sort"
)

type rtInfo struct {
	Dst net.IPNet
	Gateway, PrefSrc net.IP
	OutputIface uint32
	Priority uint32
}

type routeSlice []*rtInfo

type router struct {
	ifaces []net.Interface
	addrs []net.IP
	v4 routeSlice
}

func getRouteInfo() (*router, error) {
	rtr := &router{}

	tab, err := syscall.NetlinkRIB(syscall.RTM_GETROUTE, syscall.AF_INET)
	if err != nil {
		return nil, err	
	}

	msgs, err := syscall.ParseNetlinkMessage(tab)
	if err != nil {
		return nil, err	
	}

	for _, m := range msgs {
		switch m.Header.Type {
			case syscall.NLMSG_DONE:
				break	
			case syscall.RTM_NEWROUTE:
				rtmsg := (*syscall.RtMsg)(unsafe.Pointer(&m.Data[0]))
				attrs, err := syscall.ParseNetlinkRouteAttr(&m)		
				if err != nil {
					return nil, err
				}
				routeInfo := rtInfo{}
				rtr.v4 = append(rtr.v4, &routeInfo)
				for _, attr:= range attrs {
					switch attr.Attr.Type {
					case syscall.RTA_DST:
						routeInfo.Dst.IP = net.IP(attr.Value)
						routeInfo.Dst.Mask = net.CIDRMask(int(rtmsg.Dst_len), len(attr.Value)*8)
					case syscall.RTA_GATEWAY:
						routeInfo.Gateway = net.IPv4(attr.Value[0], attr.Value[1], attr.Value[2], attr.Value[3])
					case syscall.RTA_OIF:
						routeInfo.OutputIface = *(*uint32)(unsafe.Pointer(&attr.Value[0]))
					case syscall.RTA_PRIORITY:
						routeInfo.Priority = *(*uint32)(unsafe.Pointer(&attr.Value[0]))
					case syscall.RTA_PREFSRC:
						routeInfo.PrefSrc = net.IPv4(attr.Value[0], attr.Value[1], attr.Value[2], attr.Value[3])
					}
				}
		}			
	}
	
	sort.Slice(rtr.v4, func(i, j int) bool {
		return rtr.v4[i].Priority < rtr.v4[j].Priority
	})

	ifaces, err := net.Interfaces()
	if err != nil {
		return nil, err
	}

	for i, iface := range ifaces {
		
		if i != iface.Index - 1 {
			break
		}

		if iface.Flags & net.FlagUp == 0{
			continue
		}
		rtr.ifaces = append(rtr.ifaces, iface)
		ifaceAddrs, err := iface.Addrs()
		if err != nil {
			return nil, err
		}
		var addrs net.IP
		for _, addr := range ifaceAddrs {
			if inet, ok := addr.(*net.IPNet); ok {
				if v4 := inet.IP.To4(); v4 != nil {
					if addrs == nil {
						addrs = v4
					}
				}
			}
		}
		rtr.addrs = append(rtr.addrs, addrs)
	}
	return rtr, nil
}


func main() {
	newRoute, err := getRouteInfo()
	if err != nil {
		fmt.Println(err)
		return 
	}

	fmt.Println("**************************************")
	fmt.Printf("%-15v %-15v %-15v\n", "interfaceName", "gateway", "ip")
	for _, rt := range newRoute.v4 { 
		if rt.Gateway != nil {
			fmt.Printf("%-15v %-15v %-15v\n", newRoute.ifaces[rt.OutputIface - 1].Name, rt.Gateway.String(), newRoute.addrs[rt.OutputIface - 1])
		}
	}
	fmt.Println("**************************************")

	newRoute.getRoute(net.ParseIP("192.168.2.100"))
	newRoute.getRoute(net.ParseIP("10.10.100.100"))
	newRoute.getRoute(net.ParseIP("172.1.2.100"))
}

func (r *router)getRoute(dst net.IP) {
	for _, rt := range r.v4 { 
		if rt.Dst.IP != nil && !rt.Dst.Contains(dst) {
			continue
		}
		fmt.Printf("%-15v : ", dst.String())
		if rt.PrefSrc == nil {
			fmt.Println(r.ifaces[rt.OutputIface - 1].Name, rt.Gateway.String(), r.addrs[rt.OutputIface - 1].String())
		} else {
			fmt.Println(r.ifaces[rt.OutputIface - 1].Name, rt.Gateway.String(), rt.PrefSrc.String())
		}
		return
	}
}