// Copyright 2017 flannel authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// +build !windows

package ipsec

import (
	"encoding/json"
	"fmt"
	"sync"

	log "github.com/golang/glog"
	"golang.org/x/net/context"

	"github.com/coreos/flannel/backend"
	"github.com/coreos/flannel/pkg/ip"
	"github.com/coreos/flannel/subnet"
)

/*
	Flannel's approach to IPSec uses Strongswan to handle the key exchange (using IKEv2) and the kernel to handle the
	actual encryption.

	Strongswan's "charon" is bundled in the flannel container. Flannel runs it as a child process when the ipsec backend
	is selected and communicates with it using the "VICI" interface. Strongswan ships a utility "swanctl" which also
	uses the VICI interface. This utility is bundled in the flannel container and can help with debugging.

	The file "handle_charon.go" contains the logic for working with the charon. It supports creating a "CharonIKEDaemon"
	which supports loading the PSK into the charon and adding and removing connections.

	The file "handle_xfrm.go" contains functions for adding and removing the ipsec polcies.

	ipsec_network.go ties it all together, loading the PSK for current host on startu and as new hosts are added and
	removed it, adds/removes the PSK and connection details to strongswan and adds/remove the policy to the kernel.
*/

const (
	defaultESPProposal = "aes128gcm16-sha256-prfsha256-ecp256"
	minPasswordLength  = 96
)

func init() {
	backend.Register("ipsec", New)
}

type IPSECBackend struct {
	sm       subnet.Manager
	extIface *backend.ExternalInterface
}

func New(sm subnet.Manager, extIface *backend.ExternalInterface) (
	backend.Backend, error) {
	be := &IPSECBackend{
		sm:       sm,
		extIface: extIface,
	}

	return be, nil
}

func (be *IPSECBackend) RegisterNetwork(
	ctx context.Context, wg sync.WaitGroup, config *subnet.Config) (backend.Network, error) {

	cfg := struct {
		UDPEncap    bool
		ESPProposal string
		PSK         string
	}{
		UDPEncap:    false,
		ESPProposal: defaultESPProposal,
	}

	if len(config.Backend) > 0 {
		if err := json.Unmarshal(config.Backend, &cfg); err != nil {
			return nil, fmt.Errorf("error decoding IPSEC backend config: %v", err)
		}
	}

	if len(cfg.PSK) < minPasswordLength {
		return nil, fmt.Errorf(
			"config error, password should be at least %d characters long",
			minPasswordLength)
	}

	log.Infof("IPSec config: UDPEncap=%v ESPProposal=%s", cfg.UDPEncap, cfg.ESPProposal)

	attrs := subnet.LeaseAttrs{
		PublicIP:    ip.FromIP(be.extIface.ExtAddr),
		BackendType: "ipsec",
	}

	l, err := be.sm.AcquireLease(ctx, &attrs)

	switch err {
	case nil:

	case context.Canceled, context.DeadlineExceeded:
		return nil, err

	default:
		return nil, fmt.Errorf("failed to acquire lease: %v", err)
	}

	ikeDaemon, err := NewCharonIKEDaemon(ctx, wg, cfg.ESPProposal)
	if err != nil {
		return nil, fmt.Errorf("error creating CharonIKEDaemon struct: %v", err)
	}

	return newNetwork(be.sm, be.extIface, cfg.UDPEncap, cfg.PSK, ikeDaemon, l)
}