xK/xS/xS.go

//
// Copyright (c) 2014 - 2022, Přemysl Eric Janouch <p@janouch.name>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
//

// xS is a straight-forward port of xD IRCd from C.
package main

import (
	"bufio"
	"bytes"
	"crypto/sha256"
	"crypto/tls"
	"encoding/hex"
	"errors"
	"flag"
	"fmt"
	"io"
	"io/ioutil"
	"log/syslog"
	"net"
	"os"
	"os/signal"
	"os/user"
	"path/filepath"
	"regexp"
	"strconv"
	"strings"
	"syscall"
	"time"
)

const projectName = "xS"

var projectVersion = "?"

var debugMode = false

// --- Logging -----------------------------------------------------------------

type logPrio int

const (
	prioFatal logPrio = iota
	prioError
	prioWarning
	prioStatus
	prioDebug
)

func (lp logPrio) prefix() string {
	switch lp {
	case prioFatal:
		return "fatal: "
	case prioError:
		return "error: "
	case prioWarning:
		return "warning: "
	case prioStatus:
		return ""
	case prioDebug:
		return "debug: "
	default:
		panic("unhandled log priority")
	}
}

func (lp logPrio) syslogPrio() syslog.Priority {
	switch lp {
	case prioFatal:
		return syslog.LOG_ERR
	case prioError:
		return syslog.LOG_ERR
	case prioWarning:
		return syslog.LOG_WARNING
	case prioStatus:
		return syslog.LOG_INFO
	case prioDebug:
		return syslog.LOG_DEBUG
	default:
		panic("unhandled log priority")
	}
}

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

func logMessageStdio(prio logPrio, format string, args ...interface{}) {
	// TODO: isatty-enabled colors based on prio.
	os.Stderr.WriteString(time.Now().Format("2006-01-02 15:04:05 ") +
		prio.prefix() + fmt.Sprintf(format, args...) + "\n")
}

func logMessageSystemd(prio logPrio, format string, args ...interface{}) {
	if prio == prioFatal {
		// There is no corresponding syslog severity.
		format = "fatal: " + format
	}
	fmt.Fprintf(os.Stderr, "<%d>%s\n",
		prio.syslogPrio(), fmt.Sprintf(format, args...))
}

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

var logMessage = logMessageStdio

func printDebug(format string, args ...interface{}) {
	if debugMode {
		logMessage(prioDebug, format, args...)
	}
}

func printStatus(format string, args ...interface{}) {
	logMessage(prioStatus, format, args...)
}
func printWarning(format string, args ...interface{}) {
	logMessage(prioWarning, format, args...)
}
func printError(format string, args ...interface{}) {
	logMessage(prioError, format, args...)
}

// "fatal" is reserved for failures that would harm further operation.

func printFatal(format string, args ...interface{}) {
	logMessage(prioFatal, format, args...)
}

func exitFatal(format string, args ...interface{}) {
	printFatal(format, args...)
	os.Exit(1)
}

// --- Utilities ---------------------------------------------------------------

// Split a string by a set of UTF-8 delimiters, optionally ignoring empty items.
func splitString(s, delims string, ignoreEmpty bool) (result []string) {
	for {
		end := strings.IndexAny(s, delims)
		if end < 0 {
			break
		}
		if !ignoreEmpty || end != 0 {
			result = append(result, s[:end])
		}
		s = s[end+1:]
	}
	if !ignoreEmpty || s != "" {
		result = append(result, s)
	}
	return
}

// Trivial SSL/TLS autodetection. The first block of data returned by Recvfrom
// must be at least three octets long for this to work reliably, but that should
// not pose a problem in practice. We might try waiting for them.
//
//	SSL2:      1xxx xxxx | xxxx xxxx |    <1>
//	              (message length)  (client hello)
//	SSL3/TLS:    <22>    |    <3>    | xxxx xxxx
//	          (handshake)|  (protocol version)
//
// Note that Go 1.12's crypto/tls offers a slightly more straight-forward
// solution: "If a client sends an initial message that does not look like TLS,
// the server will no longer reply with an alert, and it will expose the
// underlying net.Conn in the new field Conn of RecordHeaderError."
func detectTLS(sysconn syscall.RawConn) (isTLS bool) {
	sysconn.Read(func(fd uintptr) (done bool) {
		var buf [3]byte
		n, _, err := syscall.Recvfrom(int(fd), buf[:], syscall.MSG_PEEK)
		switch {
		case n == 3:
			isTLS = buf[0]&0x80 != 0 && buf[2] == 1
			fallthrough
		case n == 2:
			isTLS = isTLS || buf[0] == 22 && buf[1] == 3
		case n == 1:
			isTLS = buf[0] == 22
		case err == syscall.EAGAIN:
			return false
		}
		return true
	})
	return isTLS
}

// --- File system -------------------------------------------------------------

// Look up the value of an XDG path from environment, or fall back to a default.
func getXDGHomeDir(name, def string) string {
	env := os.Getenv(name)
	if env != "" && env[0] == filepath.Separator {
		return env
	}

	home := ""
	if v, ok := os.LookupEnv("HOME"); ok {
		home = v
	} else if u, _ := user.Current(); u != nil {
		home = u.HomeDir
	}
	return filepath.Join(home, def)
}

func resolveRelativeFilenameGeneric(paths []string, filename string) string {
	for _, path := range paths {
		// As per XDG spec, relative paths are ignored.
		if path == "" || path[0] != filepath.Separator {
			continue
		}

		file := filepath.Join(path, filename)
		if _, err := os.Stat(file); err == nil {
			return file
		}
	}
	return ""
}

// Retrieve all XDG base directories for configuration files.
func getXDGConfigDirs() (result []string) {
	home := getXDGHomeDir("XDG_CONFIG_HOME", ".config")
	if home != "" {
		result = append(result, home)
	}
	dirs := os.Getenv("XDG_CONFIG_DIRS")
	if dirs == "" {
		dirs = "/etc/xdg"
	}
	for _, path := range strings.Split(dirs, ":") {
		if path != "" {
			result = append(result, path)
		}
	}
	return
}

func resolveRelativeConfigFilename(filename string) string {
	return resolveRelativeFilenameGeneric(getXDGConfigDirs(),
		filepath.Join(projectName, filename))
}

func findTildeHome(username string) string {
	if username != "" {
		if u, _ := user.Lookup(username); u != nil {
			return u.HomeDir
		}
	} else if u, _ := user.Current(); u != nil {
		return u.HomeDir
	} else if v, ok := os.LookupEnv("HOME"); ok {
		return v
	}
	printDebug("failed to expand the home directory for %s", username)
	return "~" + username
}

func resolveFilename(filename string, relativeCB func(string) string) string {
	// Absolute path is absolute.
	if filename == "" || filename[0] == filepath.Separator {
		return filename
	}
	if filename[0] != '~' {
		return relativeCB(filename)
	}

	// Paths to home directories ought to be absolute.
	var n int
	for n = 0; n < len(filename); n++ {
		if filename[n] == filepath.Separator {
			break
		}
	}
	return findTildeHome(filename[1:n]) + filename[n:]
}

// --- Simple file I/O ---------------------------------------------------------

// Overwrites filename contents with data; creates directories as needed.
func writeFile(path string, data []byte) error {
	if dir := filepath.Dir(path); dir != "." {
		if err := os.MkdirAll(dir, 0755); err != nil {
			return err
		}
	}
	return ioutil.WriteFile(path, data, 0644)
}

// Wrapper for writeFile that makes sure that the new data has been written
// to disk in its entirety before overriding the old file.
func writeFileSafe(path string, data []byte) error {
	temp := path + ".new"
	if err := writeFile(temp, data); err != nil {
		return err
	}
	return os.Rename(temp, path)
}

// --- Simple configuration ----------------------------------------------------

// This is the bare minimum to make an application configurable.
// Keys are stripped of surrounding whitespace, values are not.

type simpleConfigItem struct {
	key         string // INI key
	def         string // default value
	description string // documentation
}

type simpleConfig map[string]string

func (sc simpleConfig) loadDefaults(table []simpleConfigItem) {
	for _, item := range table {
		sc[item.key] = item.def
	}
}

func (sc simpleConfig) updateFromFile() error {
	basename := projectName + ".conf"
	path := resolveFilename(basename, resolveRelativeConfigFilename)
	if path == "" {
		return &os.PathError{
			Op:   "cannot find",
			Path: basename,
			Err:  os.ErrNotExist,
		}
	}

	f, err := os.Open(path)
	if err != nil {
		return err
	}
	defer f.Close()

	scanner := bufio.NewScanner(f)
	for lineNo := 1; scanner.Scan(); lineNo++ {
		line := strings.TrimLeft(scanner.Text(), " \t")
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}

		equals := strings.IndexByte(line, '=')
		if equals <= 0 {
			return fmt.Errorf("%s:%d: malformed line", path, lineNo)
		}

		sc[strings.TrimRight(line[:equals], " \t")] = line[equals+1:]
	}
	return scanner.Err()
}

func writeConfigurationFile(pathHint string, data []byte) (string, error) {
	path := pathHint
	if path == "" {
		path = filepath.Join(getXDGHomeDir("XDG_CONFIG_HOME", ".config"),
			projectName, projectName+".conf")
	}

	if err := writeFileSafe(path, data); err != nil {
		return "", err
	}
	return path, nil
}

func simpleConfigWriteDefault(pathHint string, prolog string,
	table []simpleConfigItem) (string, error) {
	data := []byte(prolog)
	for _, item := range table {
		data = append(data, fmt.Sprintf("# %s\n%s=%s\n",
			item.description, item.key, item.def)...)
	}
	return writeConfigurationFile(pathHint, data)
}

// Convenience wrapper suitable for most simple applications.
func callSimpleConfigWriteDefault(pathHint string, table []simpleConfigItem) {
	prologLines := []string{
		`# ` + projectName + ` ` + projectVersion + ` configuration file`,
		"#",
		`# Relative paths are searched for in ${XDG_CONFIG_HOME:-~/.config}`,
		`# /` + projectName + ` as well as in $XDG_CONFIG_DIRS/` + projectName,
		``,
		``,
	}

	path, err := simpleConfigWriteDefault(
		pathHint, strings.Join(prologLines, "\n"), table)
	if err != nil {
		exitFatal("%s", err)
	}

	printStatus("configuration written to `%s'", path)
}

// --- Configuration -----------------------------------------------------------

var configTable = []simpleConfigItem{
	{"server_name", "", "Server name"},
	{"server_info", "My server", "Brief server description"},
	{"motd", "", "MOTD filename"},
	{"catalog", "", "Localisation catalog"},

	{"bind", ":6667", "Bind addresses of the IRC server"},
	{"tls_cert", "", "Server TLS certificate (PEM)"},
	{"tls_key", "", "Server TLS private key (PEM)"},
	{"webirc_password", "", "Password for WebIRC"},

	{"operators", "", "IRCop TLS certificate SHA-256 fingerprints"},

	{"max_connections", "0", "Global connection limit"},
	{"ping_interval", "180", "Interval between PINGs (sec)"},
}

// --- Rate limiter ------------------------------------------------------------

type floodDetector struct {
	interval   time.Duration // interval for the limit in seconds
	limit      uint          // maximum number of events allowed
	timestamps []time.Time   // timestamps of last events
	pos        uint          // index of the oldest event
}

func newFloodDetector(interval time.Duration, limit uint) *floodDetector {
	return &floodDetector{
		interval:   interval,
		limit:      limit,
		timestamps: make([]time.Time, limit+1),
		pos:        0,
	}
}

func (fd *floodDetector) check() bool {
	now := time.Now()
	fd.timestamps[fd.pos] = now

	fd.pos++
	if fd.pos > fd.limit {
		fd.pos = 0
	}

	var count uint
	begin := now.Add(-fd.interval)
	for _, ts := range fd.timestamps {
		if ts.After(begin) {
			count++
		}
	}
	return count <= fd.limit
}

// --- IRC token validation ----------------------------------------------------

// Everything as per RFC 2812
const (
	ircMaxNickname      = 9
	ircMaxHostname      = 63
	ircMaxChannelName   = 50
	ircMaxMessageLength = 510
)

const (
	reClassSpecial = "\\[\\]\\\\`_^{|}"
	// "shortname" from RFC 2812 doesn't work how its author thought it would.
	reShortname = "[0-9A-Za-z](-*[0-9A-Za-z])*"
)

var (
	reHostname = regexp.MustCompile(
		`^` + reShortname + `(\.` + reShortname + `)*$`)

	// Extending ASCII to the whole range of Unicode letters.
	reNickname = regexp.MustCompile(
		`^[\pL` + reClassSpecial + `][\pL` + reClassSpecial + `0-9-]*$`)

	// Notably, this won't match invalid UTF-8 characters, although the
	// behaviour seems to be unstated in the documentation.
	reUsername = regexp.MustCompile(`^[^\0\r\n @]+$`)

	reChannelName = regexp.MustCompile(`^[^\0\007\r\n ,:]+$`)
	reKey         = regexp.MustCompile(`^[^\r\n\f\t\v ]{1,23}$`)
	reUserMask    = regexp.MustCompile(`^[^!@]+![^!@]+@[^@!]+$`)
	reFingerprint = regexp.MustCompile(`^[a-fA-F0-9]{64}$`)
)

func ircValidateHostname(hostname string) error {
	if hostname == "" {
		return errors.New("the value is empty")
	}
	if !reHostname.MatchString(hostname) {
		return errors.New("invalid format")
	}
	if len(hostname) > ircMaxHostname {
		return errors.New("the value is too long")
	}
	return nil
}

func ircIsValidNickname(nickname string) bool {
	return len(nickname) <= ircMaxNickname && reNickname.MatchString(nickname)
}

func ircIsValidUsername(username string) bool {
	// XXX: We really should put a limit on this
	// despite the RFC not mentioning one.
	return reUsername.MatchString(username)
}

func ircIsValidChannelName(name string) bool {
	return len(name) <= ircMaxChannelName && reChannelName.MatchString(name)
}

func ircIsValidKey(key string) bool {
	// XXX: Should be 7-bit as well but whatever.
	return reKey.MatchString(key)
}

func ircIsValidUserMask(mask string) bool {
	return reUserMask.MatchString(mask)
}

func ircIsValidFingerprint(fp string) bool {
	return reFingerprint.MatchString(fp)
}

// --- Clients (equals users) --------------------------------------------------

type connCloseWriter interface {
	net.Conn
	CloseWrite() error
}

const ircSupportedUserModes = "aiwros"

const (
	ircUserModeInvisible uint = 1 << iota
	ircUserModeRxWallops
	ircUserModeRestricted
	ircUserModeOperator
	ircUserModeRxServerNotices
)

const (
	ircCapMultiPrefix uint = 1 << iota
	ircCapInviteNotify
	ircCapEchoMessage
	ircCapUserhostInNames
	ircCapServerTime
)

type client struct {
	transport net.Conn        // underlying connection
	tls       *tls.Conn       // TLS, if detected
	conn      connCloseWriter // high-level connection
	recvQ     []byte          // unprocessed input
	sendQ     []byte          // unprocessed output
	reading   bool            // whether a reading goroutine is running
	writing   bool            // whether a writing goroutine is running
	closing   bool            // whether we're closing the connection
	killTimer *time.Timer     // hard kill timeout

	opened            time.Time // when the connection was opened
	nSentMessages     uint      // number of sent messages total
	sentBytes         int       // number of sent bytes total
	nReceivedMessages uint      // number of received messages total
	receivedBytes     int       // number of received bytes total

	hostname string // hostname or IP shown to the network
	port     string // port of the peer as a string
	address  string // full network address

	pingTimer      *time.Timer // we should send a PING
	timeoutTimer   *time.Timer // connection seems to be dead
	registered     bool        // the user has registered
	capNegotiating bool        // negotiating capabilities
	capsEnabled    uint        // enabled capabilities
	capVersion     uint        // CAP protocol version

	tlsCertFingerprint string // client certificate fingerprint

	nickname string // IRC nickname (main identifier)
	username string // IRC username
	realname string // IRC realname (or e-mail)

	mode        uint            // user's mode
	awayMessage string          // away message
	lastActive  time.Time       // last PRIVMSG, to get idle time
	invites     map[string]bool // channel invitations by operators
	antiflood   *floodDetector  // flood detector
}

// --- Channels ----------------------------------------------------------------

const ircSupportedChanModes = "ov" + "beI" + "imnqpst" + "kl"

const (
	ircChanModeInviteOnly uint = 1 << iota
	ircChanModeModerated
	ircChanModeNoOutsideMsgs
	ircChanModeQuiet
	ircChanModePrivate
	ircChanModeSecret
	ircChanModeProtectedTopic

	ircChanModeOperator
	ircChanModeVoice
)

type channel struct {
	name      string    // channel name
	modes     uint      // channel modes
	key       string    // channel key
	userLimit int       // user limit or -1
	created   time.Time // creation time

	topic     string    // channel topic
	topicWho  string    // who set the topic
	topicTime time.Time // when the topic was set

	userModes map[*client]uint // modes for all channel users

	banList       []string // ban list
	exceptionList []string // exceptions from bans
	inviteList    []string // exceptions from +I
}

func (ch *channel) getMode(discloseSecrets bool) string {
	var buf []byte
	if 0 != ch.modes&ircChanModeInviteOnly {
		buf = append(buf, 'i')
	}
	if 0 != ch.modes&ircChanModeModerated {
		buf = append(buf, 'm')
	}
	if 0 != ch.modes&ircChanModeNoOutsideMsgs {
		buf = append(buf, 'n')
	}
	if 0 != ch.modes&ircChanModeQuiet {
		buf = append(buf, 'q')
	}
	if 0 != ch.modes&ircChanModePrivate {
		buf = append(buf, 'p')
	}
	if 0 != ch.modes&ircChanModeSecret {
		buf = append(buf, 's')
	}
	if 0 != ch.modes&ircChanModeProtectedTopic {
		buf = append(buf, 'r')
	}

	if ch.userLimit != -1 {
		buf = append(buf, 'l')
	}
	if ch.key != "" {
		buf = append(buf, 'k')
	}

	// XXX: Is it correct to split it? Try it on an existing implementation.
	if discloseSecrets {
		if ch.userLimit != -1 {
			buf = append(buf, fmt.Sprintf(" %d", ch.userLimit)...)
		}
		if ch.key != "" {
			buf = append(buf, fmt.Sprintf(" %s", ch.key)...)
		}
	}
	return string(buf)
}

// --- IRC server context ------------------------------------------------------

type whowasInfo struct {
	nickname, username, realname, hostname string
}

func newWhowasInfo(c *client) *whowasInfo {
	return &whowasInfo{
		nickname: c.nickname,
		username: c.username,
		realname: c.realname,
		hostname: c.hostname,
	}
}

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

type ircCommand struct {
	requiresRegistration bool
	handler              func(*message, *client)

	nReceived     uint // number of commands received
	bytesReceived int  // number of bytes received total
}

type preparedEvent struct {
	client *client
	host   string // client's hostname or literal IP address
	isTLS  bool   // the client seems to use TLS
}

type readEvent struct {
	client *client
	data   []byte // new data from the client
	err    error  // read error
}

type writeEvent struct {
	client  *client
	written int   // amount of bytes written
	err     error // write error
}

// TODO: Maybe we want to keep it in a struct?
// A better question might be: can we run multiple instances of it?
var (
	// network

	listeners []net.Listener
	clients   = make(map[*client]bool)

	// IRC state

	// XXX: Beware that maps with identifier keys need to be indexed correctly.
	// We might want to enforce accessor functions for users and channels.

	started  time.Time                      // when the server has been started
	users    = make(map[string]*client)     // maps nicknames to clients
	channels = make(map[string]*channel)    // maps channel names to data
	whowas   = make(map[string]*whowasInfo) // WHOWAS registry

	// event loop

	quitting  bool
	quitTimer <-chan time.Time

	sigs     = make(chan os.Signal, 1)
	conns    = make(chan net.Conn)
	prepared = make(chan preparedEvent)
	reads    = make(chan readEvent)
	writes   = make(chan writeEvent)
	timers   = make(chan func())

	// configuration

	config         simpleConfig    // server configuration
	tlsConf        *tls.Config     // TLS connection configuration
	serverName     string          // our server name
	pingInterval   time.Duration   // ping interval
	maxConnections int             // max connections allowed or 0
	motd           []string        // MOTD (none if empty)
	catalog        map[int]string  // message catalog for server msgs
	operators      map[string]bool // TLS certificate fingerprints for IRCops
)

// Forcefully tear down all connections.
func forceQuit(reason string) {
	if !quitting {
		exitFatal("forceQuit called without initiateQuit")
	}

	printStatus("forced shutdown (%s)", reason)
	for c := range clients {
		// initiateQuit has already unregistered the client.
		c.kill("Shutting down")
	}
}

// Initiate a clean shutdown of the whole daemon.
func initiateQuit() {
	printStatus("shutting down")
	for _, ln := range listeners {
		if err := ln.Close(); err != nil {
			printError("%s", err)
		}
	}
	for c := range clients {
		c.closeLink("Shutting down")
	}

	quitTimer = time.After(5 * time.Second)
	quitting = true
}

func ircChannelCreate(name string) *channel {
	ch := &channel{
		name:      name,
		userLimit: -1,
		created:   time.Now(),
		userModes: make(map[*client]uint),
	}
	channels[ircToCanon(name)] = ch
	return ch
}

func ircChannelDestroyIfEmpty(ch *channel) {
	if len(ch.userModes) == 0 {
		delete(channels, ircToCanon(ch.name))
	}
}

func ircNotifyRoommates(c *client, message string) {
	targets := make(map[*client]bool)
	for _, ch := range channels {
		_, present := ch.userModes[c]
		if !present || 0 != ch.modes&ircChanModeQuiet {
			continue
		}
		for client := range ch.userModes {
			targets[client] = true
		}
	}

	for roommate := range targets {
		if roommate != c {
			roommate.send(message)
		}
	}
}

// --- Clients (continued) -----------------------------------------------------

func (c *client) printDebug(format string, args ...interface{}) {
	if debugMode {
		printDebug("(%s) %s", c.address, fmt.Sprintf(format, args...))
	}
}

func ircAppendClientModes(m uint, mode []byte) []byte {
	if 0 != m&ircUserModeInvisible {
		mode = append(mode, 'i')
	}
	if 0 != m&ircUserModeRxWallops {
		mode = append(mode, 'w')
	}
	if 0 != m&ircUserModeRestricted {
		mode = append(mode, 'r')
	}
	if 0 != m&ircUserModeOperator {
		mode = append(mode, 'o')
	}
	if 0 != m&ircUserModeRxServerNotices {
		mode = append(mode, 's')
	}
	return mode
}

func (c *client) getMode() string {
	var mode []byte
	if c.awayMessage != "" {
		mode = append(mode, 'a')
	}
	return string(ircAppendClientModes(c.mode, mode))
}

func (c *client) send(line string) {