// 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 contains helpers for looking up system network interfaces.
package interfaces

import (
	"net"
	"strings"
)

// Tailscale returns the current machine's Tailscale interface, if any.
// If none is found, all zero values are returned.
// A non-nil error is only returned on a problem listing the system interfaces.
func Tailscale() (net.IP, *net.Interface, error) {
	ifs, err := net.Interfaces()
	if err != nil {
		return nil, nil, err
	}
	for _, iface := range ifs {
		if !maybeTailscaleInterfaceName(iface.Name) {
			continue
		}
		addrs, err := iface.Addrs()
		if err != nil {
			continue
		}
		for _, a := range addrs {
			if ipnet, ok := a.(*net.IPNet); ok && IsTailscaleIP(ipnet.IP) {
				return ipnet.IP, &iface, nil
			}
		}
	}
	return nil, nil, nil
}

// HaveIPv6GlobalAddress reports whether the machine appears to have a
// global scope unicast IPv6 address.
//
// It only returns an error if there's a problem querying the system
// interfaces.
func HaveIPv6GlobalAddress() (bool, error) {
	ifs, err := net.Interfaces()
	if err != nil {
		return false, err
	}
	for i := range ifs {
		iface := &ifs[i]
		if !isUp(iface) || isLoopback(iface) {
			continue
		}
		addrs, err := iface.Addrs()
		if err != nil {
			continue
		}
		for _, a := range addrs {
			ipnet, ok := a.(*net.IPNet)
			if !ok {
				continue
			}
			if ipnet.IP.To4() != nil || !ipnet.IP.IsGlobalUnicast() {
				continue
			}
			return true, nil
		}
	}
	return false, nil
}

// maybeTailscaleInterfaceName reports whether s is an interface
// name that might be used by Tailscale.
func maybeTailscaleInterfaceName(s string) bool {
	return strings.HasPrefix(s, "wg") ||
		strings.HasPrefix(s, "ts") ||
		strings.HasPrefix(s, "tailscale") ||
		strings.HasPrefix(s, "utun")
}

// IsTailscaleIP reports whether ip is an IP in a range used by
// Tailscale virtual network interfaces.
func IsTailscaleIP(ip net.IP) bool {
	return cgNAT.Contains(ip)
}

func isUp(nif *net.Interface) bool       { return nif.Flags&net.FlagUp != 0 }
func isLoopback(nif *net.Interface) bool { return nif.Flags&net.FlagLoopback != 0 }

// LocalAddresses returns the machine's IP addresses, separated by
// whether they're loopback addresses.
func LocalAddresses() (regular, loopback []string, err error) {
	// TODO(crawshaw): don't serve interface addresses that we are routing
	ifaces, err := net.Interfaces()
	if err != nil {
		return nil, nil, err
	}
	for i := range ifaces {
		iface := &ifaces[i]
		if !isUp(iface) {
			// Down interfaces don't count
			continue
		}
		ifcIsLoopback := isLoopback(iface)

		addrs, err := iface.Addrs()
		if err != nil {
			return nil, nil, err
		}
		for _, a := range addrs {
			switch v := a.(type) {
			case *net.IPNet:
				// TODO(crawshaw): IPv6 support.
				// Easy to do here, but we need good endpoint ordering logic.
				ip := v.IP.To4()
				if ip == nil {
					continue
				}
				// TODO(apenwarr): don't special case cgNAT.
				// In the general wireguard case, it might
				// very well be something we can route to
				// directly, because both nodes are
				// behind the same CGNAT router.
				if cgNAT.Contains(ip) {
					continue
				}
				if linkLocalIPv4.Contains(ip) {
					continue
				}
				if ip.IsLoopback() || ifcIsLoopback {
					loopback = append(loopback, ip.String())
				} else {
					regular = append(regular, ip.String())
				}
			}
		}
	}
	return regular, loopback, nil
}

var cgNAT = func() *net.IPNet {
	_, ipNet, err := net.ParseCIDR("100.64.0.0/10")
	if err != nil {
		panic(err)
	}
	return ipNet
}()

var linkLocalIPv4 = func() *net.IPNet {
	_, ipNet, err := net.ParseCIDR("169.254.0.0/16")
	if err != nil {
		panic(err)
	}
	return ipNet
}()