// 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.

// +build !android

package monitor

import (
	"fmt"
	"net"
	"time"

	"github.com/jsimonetti/rtnetlink"
	"github.com/mdlayher/netlink"
	"golang.org/x/sys/unix"
	"inet.af/netaddr"
	"tailscale.com/net/tsaddr"
	"tailscale.com/types/logger"
)

// unspecifiedMessage is a minimal message implementation that should not
// be ignored. In general, OS-specific implementations should use better
// types and avoid this if they can.
type unspecifiedMessage struct{}

func (unspecifiedMessage) ignore() bool { return false }

// nlConn wraps a *netlink.Conn and returns a monitor.Message
// instead of a netlink.Message. Currently, messages are discarded,
// but down the line, when messages trigger different logic depending
// on the type of event, this provides the capability of handling
// each architecture-specific message in a generic fashion.
type nlConn struct {
	logf     logger.Logf
	conn     *netlink.Conn
	buffered []netlink.Message
}

func newOSMon(logf logger.Logf) (osMon, error) {
	conn, err := netlink.Dial(unix.NETLINK_ROUTE, &netlink.Config{
		// Routes get us most of the events of interest, but we need
		// address as well to cover things like DHCP deciding to give
		// us a new address upon renewal - routing wouldn't change,
		// but all reachability would.
		Groups: unix.RTMGRP_IPV4_IFADDR | unix.RTMGRP_IPV6_IFADDR | unix.RTMGRP_IPV4_ROUTE | unix.RTMGRP_IPV6_ROUTE,
	})
	if err != nil {
		return nil, fmt.Errorf("dialing netlink socket: %v", err)
	}
	return &nlConn{logf: logf, conn: conn}, nil
}

func (c *nlConn) Close() error { return c.conn.Close() }

func (c *nlConn) Receive() (message, error) {
	if len(c.buffered) == 0 {
		var err error
		c.buffered, err = c.conn.Receive()
		if err != nil {
			return nil, err
		}
		if len(c.buffered) == 0 {
			// Unexpected. Not seen in wild, but sleep defensively.
			time.Sleep(time.Second)
			return ignoreMessage{}, nil
		}
	}
	msg := c.buffered[0]
	c.buffered = c.buffered[1:]

	// See https://github.com/torvalds/linux/blob/master/include/uapi/linux/rtnetlink.h
	// And https://man7.org/linux/man-pages/man7/rtnetlink.7.html
	switch msg.Header.Type {
	case unix.RTM_NEWADDR, unix.RTM_DELADDR:
		var rmsg rtnetlink.AddressMessage
		if err := rmsg.UnmarshalBinary(msg.Data); err != nil {
			c.logf("failed to parse type %v: %v", msg.Header.Type, err)
			return unspecifiedMessage{}, nil
		}
		return &newAddrMessage{
			Label:  rmsg.Attributes.Label,
			Addr:   netaddrIP(rmsg.Attributes.Local),
			Delete: msg.Header.Type == unix.RTM_DELADDR,
		}, nil
	case unix.RTM_NEWROUTE, unix.RTM_DELROUTE:
		typeStr := "RTM_NEWROUTE"
		if msg.Header.Type == unix.RTM_DELROUTE {
			typeStr = "RTM_DELROUTE"
		}
		var rmsg rtnetlink.RouteMessage
		if err := rmsg.UnmarshalBinary(msg.Data); err != nil {
			c.logf("%s: failed to parse: %v", typeStr, err)
			return unspecifiedMessage{}, nil
		}
		src := netaddrIPPrefix(rmsg.Attributes.Src, rmsg.SrcLength)
		dst := netaddrIPPrefix(rmsg.Attributes.Dst, rmsg.DstLength)
		gw := netaddrIP(rmsg.Attributes.Gateway)

		if msg.Header.Type == unix.RTM_NEWROUTE && rmsg.Table == tsTable && rmsg.DstLength == 32 {
			// Don't log. Spammy and normal to see a bunch of these on start-up,
			// which we make ourselves.
		} else {
			c.logf("%s: src=%v, dst=%v, gw=%v, outif=%v, table=%v", typeStr,
				condNetAddrPrefix(src), condNetAddrPrefix(dst), condNetAddrIP(gw),
				rmsg.Attributes.OutIface, rmsg.Attributes.Table)
		}
		if msg.Header.Type == unix.RTM_DELROUTE {
			// Just logging it for now.
			// (Debugging https://github.com/tailscale/tailscale/issues/643)
			return unspecifiedMessage{}, nil
		}
		return &newRouteMessage{
			Table:   rmsg.Table,
			Src:     src,
			Dst:     dst,
			Gateway: gw,
		}, nil
	default:
		c.logf("unhandled netlink msg type %+v, %q", msg.Header, msg.Data)
		return unspecifiedMessage{}, nil
	}
}

func netaddrIP(std net.IP) netaddr.IP {
	ip, _ := netaddr.FromStdIP(std)
	return ip
}

func netaddrIPPrefix(std net.IP, bits uint8) netaddr.IPPrefix {
	ip, _ := netaddr.FromStdIP(std)
	return netaddr.IPPrefix{IP: ip, Bits: bits}
}

func condNetAddrPrefix(ipp netaddr.IPPrefix) string {
	if ipp.IP.IsZero() {
		return ""
	}
	return ipp.String()
}

func condNetAddrIP(ip netaddr.IP) string {
	if ip.IsZero() {
		return ""
	}
	return ip.String()
}

// newRouteMessage is a message for a new route being added.
type newRouteMessage struct {
	Src, Dst netaddr.IPPrefix
	Gateway  netaddr.IP
	Table    uint8
}

const tsTable = 52

func (m *newRouteMessage) ignore() bool {
	return m.Table == tsTable || tsaddr.IsTailscaleIP(m.Dst.IP)
}

// newAddrMessage is a message for a new address being added.
type newAddrMessage struct {
	Delete bool
	Addr   netaddr.IP
	Label  string // netlink Label attribute (e.g. "tailscale0")
}

func (m *newAddrMessage) ignore() bool {
	return tsaddr.IsTailscaleIP(m.Addr)
}

type ignoreMessage struct{}

func (ignoreMessage) ignore() bool { return true }