// Copyright 2015 Matthew Holt and The Caddy 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.

package caddy

import (
	"context"
	"crypto/tls"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"net"
	"net/netip"
	"os"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"

	"github.com/quic-go/quic-go"
	"github.com/quic-go/quic-go/http3"
	h3qlog "github.com/quic-go/quic-go/http3/qlog"
	"go.uber.org/zap"
	"golang.org/x/time/rate"

	"github.com/caddyserver/caddy/v2/internal"
)

// NetworkAddress represents one or more network addresses.
// It contains the individual components for a parsed network
// address of the form accepted by ParseNetworkAddress().
type NetworkAddress struct {
	// Should be a network value accepted by Go's net package or
	// by a plugin providing a listener for that network type.
	Network string

	// The "main" part of the network address is the host, which
	// often takes the form of a hostname, DNS name, IP address,
	// or socket path.
	Host string

	// For addresses that contain a port, ranges are given by
	// [StartPort, EndPort]; i.e. for a single port, StartPort
	// and EndPort are the same. For no port, they are 0.
	StartPort uint
	EndPort   uint
}

// ListenAll calls Listen for all addresses represented by this struct, i.e. all ports in the range.
// (If the address doesn't use ports or has 1 port only, then only 1 listener will be created.)
// It returns an error if any listener failed to bind, and closes any listeners opened up to that point.
func (na NetworkAddress) ListenAll(ctx context.Context, config net.ListenConfig) ([]any, error) {
	var listeners []any
	var err error

	// if one of the addresses has a failure, we need to close
	// any that did open a socket to avoid leaking resources
	defer func() {
		if err == nil {
			return
		}
		for _, ln := range listeners {
			if cl, ok := ln.(io.Closer); ok {
				cl.Close()
			}
		}
	}()

	// an address can contain a port range, which represents multiple addresses;
	// some addresses don't use ports at all and have a port range size of 1;
	// whatever the case, iterate each address represented and bind a socket
	for portOffset := uint(0); portOffset < na.PortRangeSize(); portOffset++ {
		select {
		case <-ctx.Done():
			return nil, ctx.Err()
		default:
		}

		// create (or reuse) the listener ourselves
		var ln any
		ln, err = na.Listen(ctx, portOffset, config)
		if err != nil {
			return nil, err
		}
		listeners = append(listeners, ln)
	}

	return listeners, nil
}

// Listen is similar to net.Listen, with a few differences:
//
// Listen announces on the network address using the port calculated by adding
// portOffset to the start port. (For network types that do not use ports, the
// portOffset is ignored.)
//
// First Listen checks if a plugin can provide a listener from this address. Otherwise,
// the provided ListenConfig is used to create the listener. Its Control function,
// if set, may be wrapped by an internally-used Control function. The provided
// context may be used to cancel long operations early. The context is not used
// to close the listener after it has been created.
//
// Caddy's listeners can overlap each other: multiple listeners may be created on
// the same socket at the same time. This is useful because during config changes,
// the new config is started while the old config is still running. How this is
// accomplished varies by platform and network type. For example, on Unix, SO_REUSEPORT
// is set except on Unix sockets, for which the file descriptor is duplicated and
// reused; on Windows, the close logic is virtualized using timeouts. Like normal
// listeners, be sure to Close() them when you are done.
//
// This method returns any type, as the implementations of listeners for various
// network types are not interchangeable. The type of listener returned is switched
// on the network type. Stream-based networks ("tcp", "unix", "unixpacket", etc.)
// return a net.Listener; datagram-based networks ("udp", "unixgram", etc.) return
// a net.PacketConn; and so forth. The actual concrete types are not guaranteed to
// be standard, exported types (wrapping is necessary to provide graceful reloads).
//
// Unix sockets will be unlinked before being created, to ensure we can bind to
// it even if the previous program using it exited uncleanly; it will also be
// unlinked upon a graceful exit (or when a new config does not use that socket).
// Listen synchronizes binds to unix domain sockets to avoid race conditions
// while an existing socket is unlinked.
func (na NetworkAddress) Listen(ctx context.Context, portOffset uint, config net.ListenConfig) (any, error) {
	if na.IsUnixNetwork() {
		unixSocketsMu.Lock()
		defer unixSocketsMu.Unlock()
	}

	// check to see if plugin provides listener
	if ln, err := getListenerFromPlugin(ctx, na.Network, na.Host, na.port(), portOffset, config); ln != nil || err != nil {
		return ln, err
	}

	// create (or reuse) the listener ourselves
	return na.listen(ctx, portOffset, config)
}

func (na NetworkAddress) listen(ctx context.Context, portOffset uint, config net.ListenConfig) (any, error) {
	var (
		ln           any
		err          error
		address      string
		unixFileMode fs.FileMode
	)

	// split unix socket addr early so lnKey
	// is independent of permissions bits
	if na.IsUnixNetwork() {
		address, unixFileMode, err = internal.SplitUnixSocketPermissionsBits(na.Host)
		if err != nil {
			return nil, err
		}
	} else if na.IsFdNetwork() {
		address = na.Host
	} else {
		address = na.JoinHostPort(portOffset)
	}

	if strings.HasPrefix(na.Network, "ip") {
		ln, err = config.ListenPacket(ctx, na.Network, address)
	} else {
		if na.IsUnixNetwork() {
			// if this is a unix socket, see if we already have it open
			ln, err = reuseUnixSocket(na.Network, address)
		}

		if ln == nil && err == nil {
			// otherwise, create a new listener
			lnKey := listenerKey(na.Network, address)
			ln, err = listenReusable(ctx, lnKey, na.Network, address, config)
		}
	}

	if err != nil {
		return nil, err
	}

	if ln == nil {
		return nil, fmt.Errorf("unsupported network type: %s", na.Network)
	}

	if IsUnixNetwork(na.Network) {
		isAbstractUnixSocket := strings.HasPrefix(address, "@")
		if !isAbstractUnixSocket {
			err = os.Chmod(address, unixFileMode)
			if err != nil {
				return nil, fmt.Errorf("unable to set permissions (%s) on %s: %v", unixFileMode, address, err)
			}
		}
	}

	return ln, nil
}

// IsUnixNetwork returns true if na.Network is
// unix, unixgram, or unixpacket.
func (na NetworkAddress) IsUnixNetwork() bool {
	return IsUnixNetwork(na.Network)
}

// IsFdNetwork returns true if na.Network is
// fd or fdgram.
func (na NetworkAddress) IsFdNetwork() bool {
	return IsFdNetwork(na.Network)
}

// JoinHostPort is like net.JoinHostPort, but where the port
// is StartPort + offset.
func (na NetworkAddress) JoinHostPort(offset uint) string {
	if na.IsUnixNetwork() || na.IsFdNetwork() {
		return na.Host
	}
	return net.JoinHostPort(na.Host, strconv.FormatUint(uint64(na.StartPort+offset), 10))
}

// Expand returns one NetworkAddress for each port in the port range.
func (na NetworkAddress) Expand() []NetworkAddress {
	size := na.PortRangeSize()
	addrs := make([]NetworkAddress, size)
	for portOffset := range size {
		addrs[portOffset] = na.At(portOffset)
	}
	return addrs
}

// At returns a NetworkAddress with a port range of just 1
// at the given port offset; i.e. a NetworkAddress that
// represents precisely 1 address only.
func (na NetworkAddress) At(portOffset uint) NetworkAddress {
	na2 := na
	na2.StartPort, na2.EndPort = na.StartPort+portOffset, na.StartPort+portOffset
	return na2
}

// PortRangeSize returns how many ports are in
// pa's port range. Port ranges are inclusive,
// so the size is the difference of start and
// end ports plus one.
func (na NetworkAddress) PortRangeSize() uint {
	if na.EndPort < na.StartPort {
		return 0
	}
	return (na.EndPort - na.StartPort) + 1
}

func (na NetworkAddress) isLoopback() bool {
	if na.IsUnixNetwork() || na.IsFdNetwork() {
		return true
	}
	if na.Host == "localhost" {
		return true
	}
	if ip, err := netip.ParseAddr(na.Host); err == nil {
		return ip.IsLoopback()
	}
	return false
}

func (na NetworkAddress) isWildcardInterface() bool {
	if na.Host == "" {
		return true
	}
	if ip, err := netip.ParseAddr(na.Host); err == nil {
		return ip.IsUnspecified()
	}
	return false
}

func (na NetworkAddress) port() string {
	if na.StartPort == na.EndPort {
		return strconv.FormatUint(uint64(na.StartPort), 10)
	}
	return fmt.Sprintf("%d-%d", na.StartPort, na.EndPort)
}

// String reconstructs the address string for human display.
// The output can be parsed by ParseNetworkAddress(). If the
// address is a unix socket, any non-zero port will be dropped.
func (na NetworkAddress) String() string {
	if na.Network == "tcp" && (na.Host != "" || na.port() != "") {
		na.Network = "" // omit default network value for brevity
	}
	return JoinNetworkAddress(na.Network, na.Host, na.port())
}

// IsUnixNetwork returns true if the netw is a unix network.
func IsUnixNetwork(netw string) bool {
	return strings.HasPrefix(netw, "unix")
}

// IsFdNetwork returns true if the netw is a fd network.
func IsFdNetwork(netw string) bool {
	return strings.HasPrefix(netw, "fd")
}

// ParseNetworkAddress parses addr into its individual
// components. The input string is expected to be of
// the form "network/host:port-range" where any part is
// optional. The default network, if unspecified, is tcp.
// Port ranges are inclusive.
//
// Network addresses are distinct from URLs and do not
// use URL syntax.
func ParseNetworkAddress(addr string) (NetworkAddress, error) {
	return ParseNetworkAddressWithDefaults(addr, "tcp", 0)
}

// ParseNetworkAddressWithDefaults is like ParseNetworkAddress but allows
// the default network and port to be specified.
func ParseNetworkAddressWithDefaults(addr, defaultNetwork string, defaultPort uint) (NetworkAddress, error) {
	var host, port string
	network, host, port, err := SplitNetworkAddress(addr)
	if err != nil {
		return NetworkAddress{}, err
	}
	if network == "" {
		network = defaultNetwork
	}
	if IsUnixNetwork(network) {
		_, _, err := internal.SplitUnixSocketPermissionsBits(host)
		return NetworkAddress{
			Network: network,
			Host:    host,
		}, err
	}
	if IsFdNetwork(network) {
		return NetworkAddress{
			Network: network,
			Host:    host,
		}, nil
	}
	var start, end uint64
	if port == "" {
		start = uint64(defaultPort)
		end = uint64(defaultPort)
	} else {
		before, after, found := strings.Cut(port, "-")
		if !found {
			after = before
		}
		start, err = strconv.ParseUint(before, 10, 16)
		if err != nil {
			return NetworkAddress{}, fmt.Errorf("invalid start port: %v", err)
		}
		end, err = strconv.ParseUint(after, 10, 16)
		if err != nil {
			return NetworkAddress{}, fmt.Errorf("invalid end port: %v", err)
		}
		if end < start {
			return NetworkAddress{}, fmt.Errorf("end port must not be less than start port")
		}
		if (end - start) > maxPortSpan {
			return NetworkAddress{}, fmt.Errorf("port range exceeds %d ports", maxPortSpan)
		}
	}
	return NetworkAddress{
		Network:   network,
		Host:      host,
		StartPort: uint(start),
		EndPort:   uint(end),
	}, nil
}

// SplitNetworkAddress splits a into its network, host, and port components.
// Note that port may be a port range (:X-Y), or omitted for unix sockets.
func SplitNetworkAddress(a string) (network, host, port string, err error) {
	beforeSlash, afterSlash, slashFound := strings.Cut(a, "/")
	if slashFound {
		network = strings.ToLower(strings.TrimSpace(beforeSlash))
		a = afterSlash
		if IsUnixNetwork(network) || IsFdNetwork(network) {
			host = a
			return network, host, port, err
		}
	}

	host, port, err = net.SplitHostPort(a)
	firstErr := err

	if err != nil {
		// in general, if there was an error, it was likely "missing port",
		// so try removing square brackets around an IPv6 host, adding a bogus
		// port to take advantage of standard library's robust parser, then
		// strip the artificial port.
		host, _, err = net.SplitHostPort(net.JoinHostPort(strings.Trim(a, "[]"), "0"))
		port = ""
	}

	if err != nil {
		err = errors.Join(firstErr, err)
	}

	return network, host, port, err
}

// JoinNetworkAddress combines network, host, and port into a single
// address string of the form accepted by ParseNetworkAddress(). For
// unix sockets, the network should be "unix" (or "unixgram" or
// "unixpacket") and the path to the socket should be given as the
// host parameter.
func JoinNetworkAddress(network, host, port string) string {
	var a string
	if network != "" {
		a = network + "/"
	}
	if (host != "" && port == "") || IsUnixNetwork(network) || IsFdNetwork(network) {
		a += host
	} else if port != "" {
		a += net.JoinHostPort(host, port)
	}
	return a
}

// ListenQUIC returns a http3.QUICEarlyListener suitable for use in a Caddy module.
//
// The network will be transformed into a QUIC-compatible type if the same address can be used with
// different networks. Currently this just means that for tcp, udp will be used with the same
// address instead.
//
// NOTE: This API is EXPERIMENTAL and may be changed or removed.
// NOTE: user should close the returned listener twice, once to stop accepting new connections, the second time to free up the packet conn.
func (na NetworkAddress) ListenQUIC(ctx context.Context, portOffset uint, config net.ListenConfig, tlsConf *tls.Config, pcWrappers []PacketConnWrapper, allow0rttconf *bool) (http3.QUICListener, error) {
	lnKey := listenerKey("quic"+na.Network, na.JoinHostPort(portOffset))

	sharedEarlyListener, _, err := listenerPool.LoadOrNew(lnKey, func() (Destructor, error) {
		lnAny, err := na.Listen(ctx, portOffset, config)
		if err != nil {
			return nil, err
		}

		ln := lnAny.(net.PacketConn)

		h3ln := ln
		if len(pcWrappers) == 0 {
			for {
				// retrieve the underlying socket, so quic-go can optimize.
				if unwrapper, ok := h3ln.(interface{ Unwrap() net.PacketConn }); ok {
					h3ln = unwrapper.Unwrap()
				} else {
					break
				}
			}
		} else {
			// wrap packet conn before QUIC
			for _, pcWrapper := range pcWrappers {
				h3ln = pcWrapper.WrapPacketConn(h3ln)
			}
		}

		sqs := newSharedQUICState(tlsConf)
		// http3.ConfigureTLSConfig only uses this field and tls App sets this field as well
		//nolint:gosec
		quicTlsConfig := &tls.Config{
			GetConfigForClient:          sqs.getConfigForClient,
			GetEncryptedClientHelloKeys: sqs.getEncryptedClientHelloKeys,
		}
		// Require clients to verify their source address when we're handling more than 1000 handshakes per second.
		// TODO: make tunable?
		limiter := rate.NewLimiter(1000, 1000)
		tr := &quic.Transport{
			Conn:                h3ln,
			VerifySourceAddress: func(addr net.Addr) bool { return !limiter.Allow() },
		}
		allow0rtt := true
		if allow0rttconf != nil {
			allow0rtt = *allow0rttconf
		}
		earlyLn, err := tr.ListenEarly(
			http3.ConfigureTLSConfig(quicTlsConfig),
			&quic.Config{
				InitialPacketSize: 1200,
				Allow0RTT:         allow0rtt,
				Tracer:            h3qlog.DefaultConnectionTracer,
			},
		)
		if err != nil {
			return nil, err
		}
		// TODO: figure out when to close the listener and the transport
		// using the original net.PacketConn to close them properly
		return &sharedQuicListener{EarlyListener: earlyLn, packetConn: ln, sqs: sqs, key: lnKey}, nil
	})
	if err != nil {
		return nil, err
	}

	sql := sharedEarlyListener.(*sharedQuicListener)
	// add current tls.Config to sqs, so GetConfigForClient will always return the latest tls.Config in case of context cancellation
	ctx, cancel := sql.sqs.addState(tlsConf)

	return &fakeCloseQuicListener{
		sharedQuicListener: sql,
		context:            ctx,
		contextCancel:      cancel,
	}, nil
}

// ListenerUsage returns the current usage count of the given listener address.
func ListenerUsage(network, addr string) int {
	count, _ := listenerPool.References(listenerKey(network, addr))
	return count
}

// contextAndCancelFunc groups context and its cancelFunc
type contextAndCancelFunc struct {
	context.Context
	context.CancelCauseFunc
}

// sharedQUICState manages GetConfigForClient
// see issue: https://github.com/caddyserver/caddy/pull/4849
type sharedQUICState struct {
	rmu           sync.RWMutex
	tlsConfs      map[*tls.Config]contextAndCancelFunc
	activeTlsConf *tls.Config
}

// newSharedQUICState creates a new sharedQUICState
func newSharedQUICState(tlsConfig *tls.Config) *sharedQUICState {
	sqtc := &sharedQUICState{
		tlsConfs:      make(map[*tls.Config]contextAndCancelFunc),
		activeTlsConf: tlsConfig,
	}
	sqtc.addState(tlsConfig)
	return sqtc
}

// getConfigForClient is used as tls.Config's GetConfigForClient field
func (sqs *sharedQUICState) getConfigForClient(ch *tls.ClientHelloInfo) (*tls.Config, error) {
	sqs.rmu.RLock()
	defer sqs.rmu.RUnlock()
	return sqs.activeTlsConf.GetConfigForClient(ch)
}

// getEncryptedClientHelloKeys is used as tls.Config's GetEncryptedClientHelloKeys field.
func (sqs *sharedQUICState) getEncryptedClientHelloKeys(ch *tls.ClientHelloInfo) ([]tls.EncryptedClientHelloKey, error) {
	sqs.rmu.RLock()
	defer sqs.rmu.RUnlock()
	if sqs.activeTlsConf.GetEncryptedClientHelloKeys == nil {
		return nil, nil
	}
	return sqs.activeTlsConf.GetEncryptedClientHelloKeys(ch)
}

// addState adds tls.Config and activeRequests to the map if not present and returns the corresponding context and its cancelFunc
// so that when cancelled, the active tls.Config will change
func (sqs *sharedQUICState) addState(tlsConfig *tls.Config) (context.Context, context.CancelCauseFunc) {
	sqs.rmu.Lock()
	defer sqs.rmu.Unlock()

	if cacc, ok := sqs.tlsConfs[tlsConfig]; ok {
		return cacc.Context, cacc.CancelCauseFunc
	}

	ctx, cancel := context.WithCancelCause(context.Background())
	wrappedCancel := func(cause error) {
		cancel(cause)

		sqs.rmu.Lock()
		defer sqs.rmu.Unlock()

		delete(sqs.tlsConfs, tlsConfig)
		if sqs.activeTlsConf == tlsConfig {
			// select another tls.Config, if there is none,
			// related sharedQuicListener will be destroyed anyway
			for tc := range sqs.tlsConfs {
				sqs.activeTlsConf = tc
				break
			}
		}
	}
	sqs.tlsConfs[tlsConfig] = contextAndCancelFunc{ctx, wrappedCancel}
	// there should be at most 2 tls.Configs
	if len(sqs.tlsConfs) > 2 {
		Log().Warn("quic listener tls configs are more than 2", zap.Int("number of configs", len(sqs.tlsConfs)))
	}
	return ctx, wrappedCancel
}

// sharedQuicListener is like sharedListener, but for quic.EarlyListeners.
type sharedQuicListener struct {
	*quic.EarlyListener
	packetConn net.PacketConn // we have to hold these because quic-go won't close listeners it didn't create
	sqs        *sharedQUICState
	key        string
}

// Destruct closes the underlying QUIC listener and its associated net.PacketConn.
func (sql *sharedQuicListener) Destruct() error {
	// close EarlyListener first to stop any operations being done to the net.PacketConn
	_ = sql.EarlyListener.Close()
	// then close the net.PacketConn
	return sql.packetConn.Close()
}

// fakeClosedErr returns an error value that is not temporary
// nor a timeout, suitable for making the caller think the
// listener is actually closed
func fakeClosedErr(l interface{ Addr() net.Addr }) error {
	return &net.OpError{
		Op:   "accept",
		Net:  l.Addr().Network(),
		Addr: l.Addr(),
		Err:  errFakeClosed,
	}
}

// errFakeClosed is the underlying error value returned by
// fakeCloseListener.Accept() after Close() has been called,
// indicating that it is pretending to be closed so that the
// server using it can terminate, while the underlying
// socket is actually left open.
var errFakeClosed = fmt.Errorf("QUIC listener 'closed' 😉")

type fakeCloseQuicListener struct {
	closed              atomic.Int32
	*sharedQuicListener // embedded, so we also become a quic.EarlyListener
	context             context.Context
	contextCancel       context.CancelCauseFunc
}

// Currently Accept ignores the passed context, however a situation where
// someone would need a hotswappable QUIC-only (not http3, since it uses context.Background here)
// server on which Accept would be called with non-empty contexts
// (mind that the default net listeners' Accept doesn't take a context argument)
// sounds way too rare for us to sacrifice efficiency here.
func (fcql *fakeCloseQuicListener) Accept(_ context.Context) (*quic.Conn, error) {
	conn, err := fcql.sharedQuicListener.Accept(fcql.context)
	if err == nil {
		return conn, nil
	}

	// if the listener is "closed", return a fake closed error instead
	if fcql.closed.Load() == 1 && errors.Is(err, context.Canceled) {
		return nil, fakeClosedErr(fcql)
	}
	return nil, err
}

func (fcql *fakeCloseQuicListener) Close() error {
	if fcql.closed.CompareAndSwap(0, 1) {
		fcql.contextCancel(errFakeClosed)
	} else if fcql.closed.CompareAndSwap(1, 2) {
		_, _ = listenerPool.Delete(fcql.sharedQuicListener.key)
	}
	return nil
}

// RegisterNetwork registers a network type with Caddy so that if a listener is
// created for that network type, getListener will be invoked to get the listener.
// This should be called during init() and will panic if the network type is standard
// or reserved, or if it is already registered. EXPERIMENTAL and subject to change.
func RegisterNetwork(network string, getListener ListenerFunc) {
	network = strings.TrimSpace(strings.ToLower(network))

	if network == "tcp" || network == "tcp4" || network == "tcp6" ||
		network == "udp" || network == "udp4" || network == "udp6" ||
		network == "unix" || network == "unixpacket" || network == "unixgram" ||
		strings.HasPrefix(network, "ip:") || strings.HasPrefix(network, "ip4:") || strings.HasPrefix(network, "ip6:") ||
		network == "fd" || network == "fdgram" {
		panic("network type " + network + " is reserved")
	}

	if _, ok := networkTypes[strings.ToLower(network)]; ok {
		panic("network type " + network + " is already registered")
	}

	networkTypes[network] = getListener
}

var unixSocketsMu sync.Mutex

// getListenerFromPlugin returns a listener on the given network and address
// if a plugin has registered the network name. It may return (nil, nil) if
// no plugin can provide a listener.
func getListenerFromPlugin(ctx context.Context, network, host, port string, portOffset uint, config net.ListenConfig) (any, error) {
	// get listener from plugin if network type is registered
	if getListener, ok := networkTypes[network]; ok {
		Log().Debug("getting listener from plugin", zap.String("network", network))
		return getListener(ctx, network, host, port, portOffset, config)
	}

	return nil, nil
}

func listenerKey(network, addr string) string {
	return network + "/" + addr
}

// ListenerFunc is a function that can return a listener given a network and address.
// The listeners must be capable of overlapping: with Caddy, new configs are loaded
// before old ones are unloaded, so listeners may overlap briefly if the configs
// both need the same listener. EXPERIMENTAL and subject to change.
type ListenerFunc func(ctx context.Context, network, host, portRange string, portOffset uint, cfg net.ListenConfig) (any, error)

var networkTypes = map[string]ListenerFunc{}

// ListenerWrapper is a type that wraps a listener
// so it can modify the input listener's methods.
// Modules that implement this interface are found
// in the caddy.listeners namespace. Usually, to
// wrap a listener, you will define your own struct
// type that embeds the input listener, then
// implement your own methods that you want to wrap,
// calling the underlying listener's methods where
// appropriate.
type ListenerWrapper interface {
	WrapListener(net.Listener) net.Listener
}

// PacketConnWrapper is a type that wraps a packet conn
// so it can modify the input packet conn methods.
// Modules that implement this interface are found
// in the caddy.packetconns namespace. Usually, to
// wrap a packet conn, you will define your own struct
// type that embeds the input packet conn, then
// implement your own methods that you want to wrap,
// calling the underlying packet conn methods where
// appropriate.
type PacketConnWrapper interface {
	WrapPacketConn(net.PacketConn) net.PacketConn
}

// listenerPool stores and allows reuse of active listeners.
var listenerPool = NewUsagePool()

const maxPortSpan = 65535
