source: code/trunk/downstream.go@ 478

package soju

import (
        "crypto/tls"
        "encoding/base64"
        "fmt"
        "io"
        "net"
        "strconv"
        "strings"
        "time"

        "github.com/emersion/go-sasl"
        "golang.org/x/crypto/bcrypt"
        "gopkg.in/irc.v3"
)

type ircError struct {
        Message *irc.Message
}

func (err ircError) Error() string {
        return err.Message.String()
}

func newUnknownCommandError(cmd string) ircError {
        return ircError{&irc.Message{
                Command: irc.ERR_UNKNOWNCOMMAND,
                Params: []string{
                        "*",
                        cmd,
                        "Unknown command",
                },
        }}
}

func newNeedMoreParamsError(cmd string) ircError {
        return ircError{&irc.Message{
                Command: irc.ERR_NEEDMOREPARAMS,
                Params: []string{
                        "*",
                        cmd,
                        "Not enough parameters",
                },
        }}
}

func newChatHistoryError(subcommand string, target string) ircError {
        return ircError{&irc.Message{
                Command: "FAIL",
                Params:  []string{"CHATHISTORY", "MESSAGE_ERROR", subcommand, target, "Messages could not be retrieved"},
        }}
}

var errAuthFailed = ircError{&irc.Message{
        Command: irc.ERR_PASSWDMISMATCH,
        Params:  []string{"*", "Invalid username or password"},
}}

// ' ' and ':' break the IRC message wire format, '@' and '!' break prefixes,
// '*' and '?' break masks
const illegalNickChars = " :@!*?"

// permanentDownstreamCaps is the list of always-supported downstream
// capabilities.
var permanentDownstreamCaps = map[string]string{
        "batch":         "",
        "cap-notify":    "",
        "echo-message":  "",
        "invite-notify": "",
        "message-tags":  "",
        "sasl":          "PLAIN",
        "server-time":   "",
}

// needAllDownstreamCaps is the list of downstream capabilities that
// require support from all upstreams to be enabled
var needAllDownstreamCaps = map[string]string{
        "away-notify":   "",
        "extended-join": "",
        "multi-prefix":  "",
}

// passthroughIsupport is the set of ISUPPORT tokens that are directly passed
// through from the upstream server to downstream clients.
//
// This is only effective in single-upstream mode.
var passthroughIsupport = map[string]bool{
        "AWAYLEN":    true,
        "CHANLIMIT":  true,
        "CHANMODES":  true,
        "CHANNELLEN": true,
        "CHANTYPES":  true,
        "EXCEPTS":    true,
        "EXTBAN":     true,
        "HOSTLEN":    true,
        "INVEX":      true,
        "KICKLEN":    true,
        "MAXLIST":    true,
        "MAXTARGETS": true,
        "MODES":      true,
        "NETWORK":    true,
        "NICKLEN":    true,
        "PREFIX":     true,
        "SAFELIST":   true,
        "TARGMAX":    true,
        "TOPICLEN":   true,
        "USERLEN":    true,
}

type downstreamConn struct {
        conn

        id uint64

        registered  bool
        user        *user
        nick        string
        nickCM      string
        rawUsername string
        networkName string
        clientName  string
        realname    string
        hostname    string
        password    string   // empty after authentication
        network     *network // can be nil

        negociatingCaps bool
        capVersion      int
        supportedCaps   map[string]string
        caps            map[string]bool

        saslServer sasl.Server
}

func newDownstreamConn(srv *Server, ic ircConn, id uint64) *downstreamConn {
        remoteAddr := ic.RemoteAddr().String()
        logger := &prefixLogger{srv.Logger, fmt.Sprintf("downstream %q: ", remoteAddr)}
        options := connOptions{Logger: logger}
        dc := &downstreamConn{
                conn:          *newConn(srv, ic, &options),
                id:            id,
                supportedCaps: make(map[string]string),
                caps:          make(map[string]bool),
        }
        dc.hostname = remoteAddr
        if host, _, err := net.SplitHostPort(dc.hostname); err == nil {
                dc.hostname = host
        }
        for k, v := range permanentDownstreamCaps {
                dc.supportedCaps[k] = v
        }
        if srv.LogPath != "" {
                dc.supportedCaps["draft/chathistory"] = ""
        }
        return dc
}

func (dc *downstreamConn) prefix() *irc.Prefix {
        return &irc.Prefix{
                Name: dc.nick,
                User: dc.user.Username,
                Host: dc.hostname,
        }
}

func (dc *downstreamConn) forEachNetwork(f func(*network)) {
        if dc.network != nil {
                f(dc.network)
        } else {
                dc.user.forEachNetwork(f)
        }
}

func (dc *downstreamConn) forEachUpstream(f func(*upstreamConn)) {
        dc.user.forEachUpstream(func(uc *upstreamConn) {
                if dc.network != nil && uc.network != dc.network {
                        return
                }
                f(uc)
        })
}

// upstream returns the upstream connection, if any. If there are zero or if
// there are multiple upstream connections, it returns nil.
func (dc *downstreamConn) upstream() *upstreamConn {
        if dc.network == nil {
                return nil
        }
        return dc.network.conn
}

func isOurNick(net *network, nick string) bool {
        // TODO: this doesn't account for nick changes
        if net.conn != nil {
                return net.casemap(nick) == net.conn.nickCM
        }
        // We're not currently connected to the upstream connection, so we don't
        // know whether this name is our nickname. Best-effort: use the network's
        // configured nickname and hope it was the one being used when we were
        // connected.
        return net.casemap(nick) == net.casemap(net.Nick)
}

// marshalEntity converts an upstream entity name (ie. channel or nick) into a
// downstream entity name.
//
// This involves adding a "/<network>" suffix if the entity isn't the current
// user.
func (dc *downstreamConn) marshalEntity(net *network, name string) string {
        if isOurNick(net, name) {
                return dc.nick
        }
        name = partialCasemap(net.casemap, name)
        if dc.network != nil {
                if dc.network != net {
                        panic("soju: tried to marshal an entity for another network")
                }
                return name
        }
        return name + "/" + net.GetName()
}

func (dc *downstreamConn) marshalUserPrefix(net *network, prefix *irc.Prefix) *irc.Prefix {
        if isOurNick(net, prefix.Name) {
                return dc.prefix()
        }
        prefix.Name = partialCasemap(net.casemap, prefix.Name)
        if dc.network != nil {
                if dc.network != net {
                        panic("soju: tried to marshal a user prefix for another network")
                }
                return prefix
        }
        return &irc.Prefix{
                Name: prefix.Name + "/" + net.GetName(),
                User: prefix.User,
                Host: prefix.Host,
        }
}

// unmarshalEntity converts a downstream entity name (ie. channel or nick) into
// an upstream entity name.
//
// This involves removing the "/<network>" suffix.
func (dc *downstreamConn) unmarshalEntity(name string) (*upstreamConn, string, error) {
        if uc := dc.upstream(); uc != nil {
                return uc, name, nil
        }
        if dc.network != nil {
                return nil, "", ircError{&irc.Message{
                        Command: irc.ERR_NOSUCHCHANNEL,
                        Params:  []string{name, "Disconnected from upstream network"},
                }}
        }

        var conn *upstreamConn
        if i := strings.LastIndexByte(name, '/'); i >= 0 {
                network := name[i+1:]
                name = name[:i]

                dc.forEachUpstream(func(uc *upstreamConn) {
                        if network != uc.network.GetName() {
                                return
                        }
                        conn = uc
                })
        }

        if conn == nil {
                return nil, "", ircError{&irc.Message{
                        Command: irc.ERR_NOSUCHCHANNEL,
                        Params:  []string{name, "Missing network suffix in channel name"},
                }}
        }
        return conn, name, nil
}

func (dc *downstreamConn) unmarshalText(uc *upstreamConn, text string) string {
        if dc.upstream() != nil {
                return text
        }
        // TODO: smarter parsing that ignores URLs
        return strings.ReplaceAll(text, "/"+uc.network.GetName(), "")
}

func (dc *downstreamConn) readMessages(ch chan<- event) error {
        for {
                msg, err := dc.ReadMessage()
                if err == io.EOF {
                        break
                } else if err != nil {
                        return fmt.Errorf("failed to read IRC command: %v", err)
                }

                ch <- eventDownstreamMessage{msg, dc}
        }

        return nil
}

// SendMessage sends an outgoing message.
//
// This can only called from the user goroutine.
func (dc *downstreamConn) SendMessage(msg *irc.Message) {
        if !dc.caps["message-tags"] {
                if msg.Command == "TAGMSG" {
                        return
                }
                msg = msg.Copy()
                for name := range msg.Tags {
                        supported := false
                        switch name {
                        case "time":
                                supported = dc.caps["server-time"]
                        }
                        if !supported {
                                delete(msg.Tags, name)
                        }
                }
        }
        if msg.Command == "JOIN" && !dc.caps["extended-join"] {
                msg.Params = msg.Params[:1]
        }

        dc.conn.SendMessage(msg)
}

// sendMessageWithID sends an outgoing message with the specified internal ID.
func (dc *downstreamConn) sendMessageWithID(msg *irc.Message, id string) {
        dc.SendMessage(msg)

        if id == "" || !dc.messageSupportsHistory(msg) {
                return
        }

        dc.sendPing(id)
}

// advanceMessageWithID advances history to the specified message ID without
// sending a message. This is useful e.g. for self-messages when echo-message
// isn't enabled.
func (dc *downstreamConn) advanceMessageWithID(msg *irc.Message, id string) {
        if id == "" || !dc.messageSupportsHistory(msg) {
                return
        }

        dc.sendPing(id)
}

// ackMsgID acknowledges that a message has been received.
func (dc *downstreamConn) ackMsgID(id string) {
        netID, entity, _, err := parseMsgID(id)
        if err != nil {
                dc.logger.Printf("failed to ACK message ID %q: %v", id, err)
                return
        }

        network := dc.user.getNetworkByID(netID)
        if network == nil {
                return
        }

        delivered := network.delivered.Value(entity)
        if delivered == nil {
                return
        }

        delivered[dc.clientName] = id
}

func (dc *downstreamConn) sendPing(msgID string) {
        token := "soju-msgid-" + base64.RawURLEncoding.EncodeToString([]byte(msgID))
        dc.SendMessage(&irc.Message{
                Command: "PING",
                Params:  []string{token},
        })
}

func (dc *downstreamConn) handlePong(token string) {
        if !strings.HasPrefix(token, "soju-msgid-") {
                dc.logger.Printf("received unrecognized PONG token %q", token)
                return
        }
        token = strings.TrimPrefix(token, "soju-msgid-")
        b, err := base64.RawURLEncoding.DecodeString(token)
        if err != nil {
                dc.logger.Printf("received malformed PONG token: %v", err)
                return
        }
        msgID := string(b)

        dc.ackMsgID(msgID)
}

// marshalMessage re-formats a message coming from an upstream connection so
// that it's suitable for being sent on this downstream connection. Only
// messages that may appear in logs are supported, except MODE.
func (dc *downstreamConn) marshalMessage(msg *irc.Message, net *network) *irc.Message {
        msg = msg.Copy()
        msg.Prefix = dc.marshalUserPrefix(net, msg.Prefix)

        switch msg.Command {
        case "PRIVMSG", "NOTICE", "TAGMSG":
                msg.Params[0] = dc.marshalEntity(net, msg.Params[0])
        case "NICK":
                // Nick change for another user
                msg.Params[0] = dc.marshalEntity(net, msg.Params[0])
        case "JOIN", "PART":
                msg.Params[0] = dc.marshalEntity(net, msg.Params[0])
        case "KICK":
                msg.Params[0] = dc.marshalEntity(net, msg.Params[0])
                msg.Params[1] = dc.marshalEntity(net, msg.Params[1])
        case "TOPIC":
                msg.Params[0] = dc.marshalEntity(net, msg.Params[0])
        case "QUIT":
                // This space is intentionally left blank
        default:
                panic(fmt.Sprintf("unexpected %q message", msg.Command))
        }

        return msg
}

func (dc *downstreamConn) handleMessage(msg *irc.Message) error {
        switch msg.Command {
        case "QUIT":
                return dc.Close()
        default:
                if dc.registered {
                        return dc.handleMessageRegistered(msg)
                } else {
                        return dc.handleMessageUnregistered(msg)
                }
        }
}

func (dc *downstreamConn) handleMessageUnregistered(msg *irc.Message) error {
        switch msg.Command {
        case "NICK":
                var nick string
                if err := parseMessageParams(msg, &nick); err != nil {
                        return err
                }
                if strings.ContainsAny(nick, illegalNickChars) {
                        return ircError{&irc.Message{
                                Command: irc.ERR_ERRONEUSNICKNAME,
                                Params:  []string{dc.nick, nick, "contains illegal characters"},
                        }}
                }
                nickCM := casemapASCII(nick)
                if nickCM == serviceNickCM {
                        return ircError{&irc.Message{
                                Command: irc.ERR_NICKNAMEINUSE,
                                Params:  []string{dc.nick, nick, "Nickname reserved for bouncer service"},
                        }}
                }
                dc.nick = nick
                dc.nickCM = nickCM
        case "USER":
                if err := parseMessageParams(msg, &dc.rawUsername, nil, nil, &dc.realname); err != nil {
                        return err
                }
        case "PASS":
                if err := parseMessageParams(msg, &dc.password); err != nil {
                        return err
                }
        case "CAP":
                var subCmd string
                if err := parseMessageParams(msg, &subCmd); err != nil {
                        return err
                }
                if err := dc.handleCapCommand(subCmd, msg.Params[1:]); err != nil {
                        return err
                }
        case "AUTHENTICATE":
                if !dc.caps["sasl"] {
                        return ircError{&irc.Message{
                                Command: irc.ERR_SASLFAIL,
                                Params:  []string{"*", "AUTHENTICATE requires the \"sasl\" capability to be enabled"},
                        }}
                }
                if len(msg.Params) == 0 {
                        return ircError{&irc.Message{
                                Command: irc.ERR_SASLFAIL,
                                Params:  []string{"*", "Missing AUTHENTICATE argument"},
                        }}
                }
                if dc.nick == "" {
                        return ircError{&irc.Message{
                                Command: irc.ERR_SASLFAIL,
                                Params:  []string{"*", "Expected NICK command before AUTHENTICATE"},
                        }}
                }

                var resp []byte
                if dc.saslServer == nil {
                        mech := strings.ToUpper(msg.Params[0])
                        switch mech {
                        case "PLAIN":
                                dc.saslServer = sasl.NewPlainServer(sasl.PlainAuthenticator(func(identity, username, password string) error {
                                        return dc.authenticate(username, password)
                                }))
                        default:
                                return ircError{&irc.Message{
                                        Command: irc.ERR_SASLFAIL,
                                        Params:  []string{"*", fmt.Sprintf("Unsupported SASL mechanism %q", mech)},
                                }}
                        }
                } else if msg.Params[0] == "*" {
                        dc.saslServer = nil
                        return ircError{&irc.Message{
                                Command: irc.ERR_SASLABORTED,
                                Params:  []string{"*", "SASL authentication aborted"},
                        }}
                } else if msg.Params[0] == "+" {
                        resp = nil
                } else {
                        // TODO: multi-line messages
                        var err error
                        resp, err = base64.StdEncoding.DecodeString(msg.Params[0])
                        if err != nil {
                                dc.saslServer = nil
                                return ircError{&irc.Message{
                                        Command: irc.ERR_SASLFAIL,
                                        Params:  []string{"*", "Invalid base64-encoded response"},
                                }}
                        }
                }

                challenge, done, err := dc.saslServer.Next(resp)
                if err != nil {
                        dc.saslServer = nil
                        if ircErr, ok := err.(ircError); ok && ircErr.Message.Command == irc.ERR_PASSWDMISMATCH {
                                return ircError{&irc.Message{
                                        Command: irc.ERR_SASLFAIL,
                                        Params:  []string{"*", ircErr.Message.Params[1]},
                                }}
                        }
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.ERR_SASLFAIL,
                                Params:  []string{"*", "SASL error"},
                        })
                        return fmt.Errorf("SASL authentication failed: %v", err)
                } else if done {
                        dc.saslServer = nil
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_LOGGEDIN,
                                Params:  []string{dc.nick, dc.prefix().String(), dc.user.Username, "You are now logged in"},
                        })
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_SASLSUCCESS,
                                Params:  []string{dc.nick, "SASL authentication successful"},
                        })
                } else {
                        challengeStr := "+"
                        if len(challenge) > 0 {
                                challengeStr = base64.StdEncoding.EncodeToString(challenge)
                        }

                        // TODO: multi-line messages
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: "AUTHENTICATE",
                                Params:  []string{challengeStr},
                        })
                }
        default:
                dc.logger.Printf("unhandled message: %v", msg)
                return newUnknownCommandError(msg.Command)
        }
        if dc.rawUsername != "" && dc.nick != "" && !dc.negociatingCaps {
                return dc.register()
        }
        return nil
}

func (dc *downstreamConn) handleCapCommand(cmd string, args []string) error {
        cmd = strings.ToUpper(cmd)

        replyTo := dc.nick
        if !dc.registered {
                replyTo = "*"
        }

        switch cmd {
        case "LS":
                if len(args) > 0 {
                        var err error
                        if dc.capVersion, err = strconv.Atoi(args[0]); err != nil {
                                return err
                        }
                }
                if !dc.registered && dc.capVersion >= 302 {
                        // Let downstream show everything it supports, and trim
                        // down the available capabilities when upstreams are
                        // known.
                        for k, v := range needAllDownstreamCaps {
                                dc.supportedCaps[k] = v
                        }
                }

                caps := make([]string, 0, len(dc.supportedCaps))
                for k, v := range dc.supportedCaps {
                        if dc.capVersion >= 302 && v != "" {
                                caps = append(caps, k+"="+v)
                        } else {
                                caps = append(caps, k)
                        }
                }

                // TODO: multi-line replies
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: "CAP",
                        Params:  []string{replyTo, "LS", strings.Join(caps, " ")},
                })

                if dc.capVersion >= 302 {
                        // CAP version 302 implicitly enables cap-notify
                        dc.caps["cap-notify"] = true
                }

                if !dc.registered {
                        dc.negociatingCaps = true
                }
        case "LIST":
                var caps []string
                for name := range dc.caps {
                        caps = append(caps, name)
                }

                // TODO: multi-line replies
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: "CAP",
                        Params:  []string{replyTo, "LIST", strings.Join(caps, " ")},
                })
        case "REQ":
                if len(args) == 0 {
                        return ircError{&irc.Message{
                                Command: err_invalidcapcmd,
                                Params:  []string{replyTo, cmd, "Missing argument in CAP REQ command"},
                        }}
                }

                // TODO: atomically ack/nak the whole capability set
                caps := strings.Fields(args[0])
                ack := true
                for _, name := range caps {
                        name = strings.ToLower(name)
                        enable := !strings.HasPrefix(name, "-")
                        if !enable {
                                name = strings.TrimPrefix(name, "-")
                        }

                        if enable == dc.caps[name] {
                                continue
                        }

                        _, ok := dc.supportedCaps[name]
                        if !ok {
                                ack = false
                                break
                        }

                        if name == "cap-notify" && dc.capVersion >= 302 && !enable {
                                // cap-notify cannot be disabled with CAP version 302
                                ack = false
                                break
                        }

                        dc.caps[name] = enable
                }

                reply := "NAK"
                if ack {
                        reply = "ACK"
                }
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: "CAP",
                        Params:  []string{replyTo, reply, args[0]},
                })
        case "END":
                dc.negociatingCaps = false
        default:
                return ircError{&irc.Message{
                        Command: err_invalidcapcmd,
                        Params:  []string{replyTo, cmd, "Unknown CAP command"},
                }}
        }
        return nil
}

func (dc *downstreamConn) setSupportedCap(name, value string) {
        prevValue, hasPrev := dc.supportedCaps[name]
        changed := !hasPrev || prevValue != value
        dc.supportedCaps[name] = value

        if !dc.caps["cap-notify"] || !changed {
                return
        }

        replyTo := dc.nick
        if !dc.registered {
                replyTo = "*"
        }

        cap := name
        if value != "" && dc.capVersion >= 302 {
                cap = name + "=" + value
        }

        dc.SendMessage(&irc.Message{
                Prefix:  dc.srv.prefix(),
                Command: "CAP",
                Params:  []string{replyTo, "NEW", cap},
        })
}

func (dc *downstreamConn) unsetSupportedCap(name string) {
        _, hasPrev := dc.supportedCaps[name]
        delete(dc.supportedCaps, name)
        delete(dc.caps, name)

        if !dc.caps["cap-notify"] || !hasPrev {
                return
        }

        replyTo := dc.nick
        if !dc.registered {
                replyTo = "*"
        }

        dc.SendMessage(&irc.Message{
                Prefix:  dc.srv.prefix(),
                Command: "CAP",
                Params:  []string{replyTo, "DEL", name},
        })
}

func (dc *downstreamConn) updateSupportedCaps() {
        supportedCaps := make(map[string]bool)
        for cap := range needAllDownstreamCaps {
                supportedCaps[cap] = true
        }
        dc.forEachUpstream(func(uc *upstreamConn) {
                for cap, supported := range supportedCaps {
                        supportedCaps[cap] = supported && uc.caps[cap]
                }
        })

        for cap, supported := range supportedCaps {
                if supported {
                        dc.setSupportedCap(cap, needAllDownstreamCaps[cap])
                } else {
                        dc.unsetSupportedCap(cap)
                }
        }
}

func (dc *downstreamConn) updateNick() {
        if uc := dc.upstream(); uc != nil && uc.nick != dc.nick {
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.prefix(),
                        Command: "NICK",
                        Params:  []string{uc.nick},
                })
                dc.nick = uc.nick
                dc.nickCM = casemapASCII(dc.nick)
        }
}

func sanityCheckServer(addr string) error {
        dialer := net.Dialer{Timeout: 30 * time.Second}
        conn, err := tls.DialWithDialer(&dialer, "tcp", addr, nil)
        if err != nil {
                return err
        }
        return conn.Close()
}

func unmarshalUsername(rawUsername string) (username, client, network string) {
        username = rawUsername

        i := strings.IndexAny(username, "/@")
        j := strings.LastIndexAny(username, "/@")
        if i >= 0 {
                username = rawUsername[:i]
        }
        if j >= 0 {
                if rawUsername[j] == '@' {
                        client = rawUsername[j+1:]
                } else {
                        network = rawUsername[j+1:]
                }
        }
        if i >= 0 && j >= 0 && i < j {
                if rawUsername[i] == '@' {
                        client = rawUsername[i+1 : j]
                } else {
                        network = rawUsername[i+1 : j]
                }
        }

        return username, client, network
}

func (dc *downstreamConn) authenticate(username, password string) error {
        username, clientName, networkName := unmarshalUsername(username)

        u, err := dc.srv.db.GetUser(username)
        if err != nil {
                dc.logger.Printf("failed authentication for %q: user not found: %v", username, err)
                return errAuthFailed
        }

        // Password auth disabled
        if u.Password == "" {
                return errAuthFailed
        }

        err = bcrypt.CompareHashAndPassword([]byte(u.Password), []byte(password))
        if err != nil {
                dc.logger.Printf("failed authentication for %q: wrong password: %v", username, err)
                return errAuthFailed
        }

        dc.user = dc.srv.getUser(username)
        if dc.user == nil {
                dc.logger.Printf("failed authentication for %q: user not active", username)
                return errAuthFailed
        }
        dc.clientName = clientName
        dc.networkName = networkName
        return nil
}

func (dc *downstreamConn) register() error {
        if dc.registered {
                return fmt.Errorf("tried to register twice")
        }

        password := dc.password
        dc.password = ""
        if dc.user == nil {
                if err := dc.authenticate(dc.rawUsername, password); err != nil {
                        return err
                }
        }

        if dc.clientName == "" && dc.networkName == "" {
                _, dc.clientName, dc.networkName = unmarshalUsername(dc.rawUsername)
        }

        dc.registered = true
        dc.logger.Printf("registration complete for user %q", dc.user.Username)
        return nil
}

func (dc *downstreamConn) loadNetwork() error {
        if dc.networkName == "" {
                return nil
        }

        network := dc.user.getNetwork(dc.networkName)
        if network == nil {
                addr := dc.networkName
                if !strings.ContainsRune(addr, ':') {
                        addr = addr + ":6697"
                }

                dc.logger.Printf("trying to connect to new network %q", addr)
                if err := sanityCheckServer(addr); err != nil {
                        dc.logger.Printf("failed to connect to %q: %v", addr, err)
                        return ircError{&irc.Message{
                                Command: irc.ERR_PASSWDMISMATCH,
                                Params:  []string{"*", fmt.Sprintf("Failed to connect to %q", dc.networkName)},
                        }}
                }

                // Some clients only allow specifying the nickname (and use the
                // nickname as a username too). Strip the network name from the
                // nickname when auto-saving networks.
                nick, _, _ := unmarshalUsername(dc.nick)

                dc.logger.Printf("auto-saving network %q", dc.networkName)
                var err error
                network, err = dc.user.createNetwork(&Network{
                        Addr: dc.networkName,
                        Nick: nick,
                })
                if err != nil {
                        return err
                }
        }

        dc.network = network
        return nil
}

func (dc *downstreamConn) welcome() error {
        if dc.user == nil || !dc.registered {
                panic("tried to welcome an unregistered connection")
        }

        // TODO: doing this might take some time. We should do it in dc.register
        // instead, but we'll potentially be adding a new network and this must be
        // done in the user goroutine.
        if err := dc.loadNetwork(); err != nil {
                return err
        }

        isupport := []string{
                fmt.Sprintf("CHATHISTORY=%v", dc.srv.HistoryLimit),
                "CASEMAPPING=ascii",
        }

        if uc := dc.upstream(); uc != nil {
                for k := range passthroughIsupport {
                        v, ok := uc.isupport[k]
                        if !ok {
                                continue
                        }
                        if v != nil {
                                isupport = append(isupport, fmt.Sprintf("%v=%v", k, *v))
                        } else {
                                isupport = append(isupport, k)
                        }
                }
        }

        dc.SendMessage(&irc.Message{
                Prefix:  dc.srv.prefix(),
                Command: irc.RPL_WELCOME,
                Params:  []string{dc.nick, "Welcome to soju, " + dc.nick},
        })
        dc.SendMessage(&irc.Message{
                Prefix:  dc.srv.prefix(),
                Command: irc.RPL_YOURHOST,
                Params:  []string{dc.nick, "Your host is " + dc.srv.Hostname},
        })
        dc.SendMessage(&irc.Message{
                Prefix:  dc.srv.prefix(),
                Command: irc.RPL_CREATED,
                Params:  []string{dc.nick, "Who cares when the server was created?"},
        })
        dc.SendMessage(&irc.Message{
                Prefix:  dc.srv.prefix(),
                Command: irc.RPL_MYINFO,
                Params:  []string{dc.nick, dc.srv.Hostname, "soju", "aiwroO", "OovaimnqpsrtklbeI"},
        })
        for _, msg := range generateIsupport(dc.srv.prefix(), dc.nick, isupport) {
                dc.SendMessage(msg)
        }
        dc.SendMessage(&irc.Message{
                Prefix:  dc.srv.prefix(),
                Command: irc.ERR_NOMOTD,
                Params:  []string{dc.nick, "No MOTD"},
        })

        dc.updateNick()
        dc.updateSupportedCaps()

        dc.forEachUpstream(func(uc *upstreamConn) {
                for _, entry := range uc.channels.innerMap {
                        ch := entry.value.(*upstreamChannel)
                        if !ch.complete {
                                continue
                        }
                        record := uc.network.channels.Value(ch.Name)
                        if record != nil && record.Detached {
                                continue
                        }

                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.prefix(),
                                Command: "JOIN",
                                Params:  []string{dc.marshalEntity(ch.conn.network, ch.Name)},
                        })

                        forwardChannel(dc, ch)
                }
        })

        dc.forEachNetwork(func(net *network) {
                // Only send history if we're the first connected client with that name
                // for the network
                if _, ok := net.offlineClients[dc.clientName]; ok {
                        dc.sendNetworkBacklog(net)
                        delete(net.offlineClients, dc.clientName)
                }

                // Fast-forward history to last message
                for target, entry := range net.delivered.innerMap {
                        delivered := entry.value.(map[string]string)
                        ch := net.channels.Value(target)
                        if ch != nil && ch.Detached {
                                continue
                        }

                        lastID, err := dc.user.msgStore.LastMsgID(net, target, time.Now())
                        if err != nil {
                                dc.logger.Printf("failed to get last message ID: %v", err)
                                continue
                        }
                        delivered[dc.clientName] = lastID
                }
        })

        return nil
}

// messageSupportsHistory checks whether the provided message can be sent as
// part of an history batch.
func (dc *downstreamConn) messageSupportsHistory(msg *irc.Message) bool {
        // Don't replay all messages, because that would mess up client
        // state. For instance we just sent the list of users, sending
        // PART messages for one of these users would be incorrect.
        // TODO: add support for draft/event-playback
        switch msg.Command {
        case "PRIVMSG", "NOTICE":
                return true
        }
        return false
}

func (dc *downstreamConn) sendNetworkBacklog(net *network) {
        for _, entry := range net.delivered.innerMap {
                target := entry.originalKey
                dc.sendTargetBacklog(net, target)
        }
}

func (dc *downstreamConn) sendTargetBacklog(net *network, target string) {
        if dc.caps["draft/chathistory"] || dc.user.msgStore == nil {
                return
        }
        if ch := net.channels.Value(target); ch != nil && ch.Detached {
                return
        }
        delivered := net.delivered.Value(target)
        if delivered == nil {
                return
        }
        lastDelivered, ok := delivered[dc.clientName]
        if !ok {
                return
        }

        limit := 4000
        history, err := dc.user.msgStore.LoadLatestID(net, target, lastDelivered, limit)
        if err != nil {
                dc.logger.Printf("failed to send implicit history for %q: %v", target, err)
                return
        }

        batchRef := "history"
        if dc.caps["batch"] {
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: "BATCH",
                        Params:  []string{"+" + batchRef, "chathistory", dc.marshalEntity(net, target)},
                })
        }

        for _, msg := range history {
                if !dc.messageSupportsHistory(msg) {
                        continue
                }

                if dc.caps["batch"] {
                        msg.Tags["batch"] = irc.TagValue(batchRef)
                }
                dc.SendMessage(dc.marshalMessage(msg, net))
        }

        if dc.caps["batch"] {
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: "BATCH",
                        Params:  []string{"-" + batchRef},
                })
        }
}

func (dc *downstreamConn) runUntilRegistered() error {
        for !dc.registered {
                msg, err := dc.ReadMessage()
                if err != nil {
                        return fmt.Errorf("failed to read IRC command: %v", err)
                }

                err = dc.handleMessage(msg)
                if ircErr, ok := err.(ircError); ok {
                        ircErr.Message.Prefix = dc.srv.prefix()
                        dc.SendMessage(ircErr.Message)
                } else if err != nil {
                        return fmt.Errorf("failed to handle IRC command %q: %v", msg, err)
                }
        }

        return nil
}

func (dc *downstreamConn) handleMessageRegistered(msg *irc.Message) error {
        switch msg.Command {
        case "CAP":
                var subCmd string
                if err := parseMessageParams(msg, &subCmd); err != nil {
                        return err
                }
                if err := dc.handleCapCommand(subCmd, msg.Params[1:]); err != nil {
                        return err
                }
        case "PING":
                var source, destination string
                if err := parseMessageParams(msg, &source); err != nil {
                        return err
                }
                if len(msg.Params) > 1 {
                        destination = msg.Params[1]
                }
                if destination != "" && destination != dc.srv.Hostname {
                        return ircError{&irc.Message{
                                Command: irc.ERR_NOSUCHSERVER,
                                Params:  []string{dc.nick, destination, "No such server"},
                        }}
                }
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: "PONG",
                        Params:  []string{dc.srv.Hostname, source},
                })
                return nil
        case "PONG":
                if len(msg.Params) == 0 {
                        return newNeedMoreParamsError(msg.Command)
                }
                token := msg.Params[len(msg.Params)-1]
                dc.handlePong(token)
        case "USER":
                return ircError{&irc.Message{
                        Command: irc.ERR_ALREADYREGISTERED,
                        Params:  []string{dc.nick, "You may not reregister"},
                }}
        case "NICK":
                var rawNick string
                if err := parseMessageParams(msg, &rawNick); err != nil {
                        return err
                }

                nick := rawNick
                var upstream *upstreamConn
                if dc.upstream() == nil {
                        uc, unmarshaledNick, err := dc.unmarshalEntity(nick)
                        if err == nil { // NICK nick/network: NICK only on a specific upstream
                                upstream = uc
                                nick = unmarshaledNick
                        }
                }

                if strings.ContainsAny(nick, illegalNickChars) {
                        return ircError{&irc.Message{
                                Command: irc.ERR_ERRONEUSNICKNAME,
                                Params:  []string{dc.nick, rawNick, "contains illegal characters"},
                        }}
                }
                if casemapASCII(nick) == serviceNickCM {
                        return ircError{&irc.Message{
                                Command: irc.ERR_NICKNAMEINUSE,
                                Params:  []string{dc.nick, rawNick, "Nickname reserved for bouncer service"},
                        }}
                }

                var err error
                dc.forEachNetwork(func(n *network) {
                        if err != nil || (upstream != nil && upstream.network != n) {
                                return
                        }
                        n.Nick = nick
                        err = dc.srv.db.StoreNetwork(dc.user.ID, &n.Network)
                })
                if err != nil {
                        return err
                }

                dc.forEachUpstream(func(uc *upstreamConn) {
                        if upstream != nil && upstream != uc {
                                return
                        }
                        uc.SendMessageLabeled(dc.id, &irc.Message{
                                Command: "NICK",
                                Params:  []string{nick},
                        })
                })

                if dc.upstream() == nil && dc.nick != nick {
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.prefix(),
                                Command: "NICK",
                                Params:  []string{nick},
                        })
                        dc.nick = nick
                        dc.nickCM = casemapASCII(dc.nick)
                }
        case "JOIN":
                var namesStr string
                if err := parseMessageParams(msg, &namesStr); err != nil {
                        return err
                }

                var keys []string
                if len(msg.Params) > 1 {
                        keys = strings.Split(msg.Params[1], ",")
                }

                for i, name := range strings.Split(namesStr, ",") {
                        uc, upstreamName, err := dc.unmarshalEntity(name)
                        if err != nil {
                                return err
                        }

                        var key string
                        if len(keys) > i {
                                key = keys[i]
                        }

                        params := []string{upstreamName}
                        if key != "" {
                                params = append(params, key)
                        }
                        uc.SendMessageLabeled(dc.id, &irc.Message{
                                Command: "JOIN",
                                Params:  params,
                        })

                        ch := uc.network.channels.Value(upstreamName)
                        if ch != nil {
                                // Don't clear the channel key if there's one set
                                // TODO: add a way to unset the channel key
                                if key != "" {
                                        ch.Key = key
                                }
                                uc.network.attach(ch)
                        } else {
                                ch = &Channel{
                                        Name: upstreamName,
                                        Key:  key,
                                }
                                uc.network.channels.SetValue(upstreamName, ch)
                        }
                        if err := dc.srv.db.StoreChannel(uc.network.ID, ch); err != nil {
                                dc.logger.Printf("failed to create or update channel %q: %v", upstreamName, err)
                        }
                }
        case "PART":
                var namesStr string
                if err := parseMessageParams(msg, &namesStr); err != nil {
                        return err
                }

                var reason string
                if len(msg.Params) > 1 {
                        reason = msg.Params[1]
                }

                for _, name := range strings.Split(namesStr, ",") {
                        uc, upstreamName, err := dc.unmarshalEntity(name)
                        if err != nil {
                                return err
                        }

                        if strings.EqualFold(reason, "detach") {
                                ch := uc.network.channels.Value(upstreamName)
                                if ch != nil {
                                        uc.network.detach(ch)
                                } else {
                                        ch = &Channel{
                                                Name:     name,
                                                Detached: true,
                                        }
                                        uc.network.channels.SetValue(upstreamName, ch)
                                }
                                if err := dc.srv.db.StoreChannel(uc.network.ID, ch); err != nil {
                                        dc.logger.Printf("failed to create or update channel %q: %v", upstreamName, err)
                                }
                        } else {
                                params := []string{upstreamName}
                                if reason != "" {
                                        params = append(params, reason)
                                }
                                uc.SendMessageLabeled(dc.id, &irc.Message{
                                        Command: "PART",
                                        Params:  params,
                                })

                                if err := uc.network.deleteChannel(upstreamName); err != nil {
                                        dc.logger.Printf("failed to delete channel %q: %v", upstreamName, err)
                                }
                        }
                }
        case "KICK":
                var channelStr, userStr string
                if err := parseMessageParams(msg, &channelStr, &userStr); err != nil {
                        return err
                }

                channels := strings.Split(channelStr, ",")
                users := strings.Split(userStr, ",")

                var reason string
                if len(msg.Params) > 2 {
                        reason = msg.Params[2]
                }

                if len(channels) != 1 && len(channels) != len(users) {
                        return ircError{&irc.Message{
                                Command: irc.ERR_BADCHANMASK,
                                Params:  []string{dc.nick, channelStr, "Bad channel mask"},
                        }}
                }

                for i, user := range users {
                        var channel string
                        if len(channels) == 1 {
                                channel = channels[0]
                        } else {
                                channel = channels[i]
                        }

                        ucChannel, upstreamChannel, err := dc.unmarshalEntity(channel)
                        if err != nil {
                                return err
                        }

                        ucUser, upstreamUser, err := dc.unmarshalEntity(user)
                        if err != nil {
                                return err
                        }

                        if ucChannel != ucUser {
                                return ircError{&irc.Message{
                                        Command: irc.ERR_USERNOTINCHANNEL,
                                        Params:  []string{dc.nick, user, channel, "They are on another network"},
                                }}
                        }
                        uc := ucChannel

                        params := []string{upstreamChannel, upstreamUser}
                        if reason != "" {
                                params = append(params, reason)
                        }
                        uc.SendMessageLabeled(dc.id, &irc.Message{
                                Command: "KICK",
                                Params:  params,
                        })
                }
        case "MODE":
                var name string
                if err := parseMessageParams(msg, &name); err != nil {
                        return err
                }

                var modeStr string
                if len(msg.Params) > 1 {
                        modeStr = msg.Params[1]
                }

                if casemapASCII(name) == dc.nickCM {
                        if modeStr != "" {
                                dc.forEachUpstream(func(uc *upstreamConn) {
                                        uc.SendMessageLabeled(dc.id, &irc.Message{
                                                Command: "MODE",
                                                Params:  []string{uc.nick, modeStr},
                                        })
                                })
                        } else {
                                dc.SendMessage(&irc.Message{
                                        Prefix:  dc.srv.prefix(),
                                        Command: irc.RPL_UMODEIS,
                                        Params:  []string{dc.nick, ""}, // TODO
                                })
                        }
                        return nil
                }

                uc, upstreamName, err := dc.unmarshalEntity(name)
                if err != nil {
                        return err
                }

                if !uc.isChannel(upstreamName) {
                        return ircError{&irc.Message{
                                Command: irc.ERR_USERSDONTMATCH,
                                Params:  []string{dc.nick, "Cannot change mode for other users"},
                        }}
                }

                if modeStr != "" {
                        params := []string{upstreamName, modeStr}
                        params = append(params, msg.Params[2:]...)
                        uc.SendMessageLabeled(dc.id, &irc.Message{
                                Command: "MODE",
                                Params:  params,
                        })
                } else {
                        ch := uc.channels.Value(upstreamName)
                        if ch == nil {
                                return ircError{&irc.Message{
                                        Command: irc.ERR_NOSUCHCHANNEL,
                                        Params:  []string{dc.nick, name, "No such channel"},
                                }}
                        }

                        if ch.modes == nil {
                                // we haven't received the initial RPL_CHANNELMODEIS yet
                                // ignore the request, we will broadcast the modes later when we receive RPL_CHANNELMODEIS
                                return nil
                        }

                        modeStr, modeParams := ch.modes.Format()
                        params := []string{dc.nick, name, modeStr}
                        params = append(params, modeParams...)

                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_CHANNELMODEIS,
                                Params:  params,
                        })
                        if ch.creationTime != "" {
                                dc.SendMessage(&irc.Message{
                                        Prefix:  dc.srv.prefix(),
                                        Command: rpl_creationtime,
                                        Params:  []string{dc.nick, name, ch.creationTime},
                                })
                        }
                }
        case "TOPIC":
                var channel string
                if err := parseMessageParams(msg, &channel); err != nil {
                        return err
                }

                uc, upstreamName, err := dc.unmarshalEntity(channel)
                if err != nil {
                        return err
                }

                if len(msg.Params) > 1 { // setting topic
                        topic := msg.Params[1]
                        uc.SendMessageLabeled(dc.id, &irc.Message{
                                Command: "TOPIC",
                                Params:  []string{upstreamName, topic},
                        })
                } else { // getting topic
                        ch := uc.channels.Value(upstreamName)
                        if ch == nil {
                                return ircError{&irc.Message{
                                        Command: irc.ERR_NOSUCHCHANNEL,
                                        Params:  []string{dc.nick, upstreamName, "No such channel"},
                                }}
                        }
                        sendTopic(dc, ch)
                }
        case "LIST":
                // TODO: support ELIST when supported by all upstreams

                pl := pendingLIST{
                        downstreamID:    dc.id,
                        pendingCommands: make(map[int64]*irc.Message),
                }
                var upstream *upstreamConn
                var upstreamChannels map[int64][]string
                if len(msg.Params) > 0 {
                        uc, upstreamMask, err := dc.unmarshalEntity(msg.Params[0])
                        if err == nil && upstreamMask == "*" { // LIST */network: send LIST only to one network
                                upstream = uc
                        } else {
                                upstreamChannels = make(map[int64][]string)
                                channels := strings.Split(msg.Params[0], ",")
                                for _, channel := range channels {
                                        uc, upstreamChannel, err := dc.unmarshalEntity(channel)
                                        if err != nil {
                                                return err
                                        }
                                        upstreamChannels[uc.network.ID] = append(upstreamChannels[uc.network.ID], upstreamChannel)
                                }
                        }
                }

                dc.user.pendingLISTs = append(dc.user.pendingLISTs, pl)
                dc.forEachUpstream(func(uc *upstreamConn) {
                        if upstream != nil && upstream != uc {
                                return
                        }
                        var params []string
                        if upstreamChannels != nil {
                                if channels, ok := upstreamChannels[uc.network.ID]; ok {
                                        params = []string{strings.Join(channels, ",")}
                                } else {
                                        return
                                }
                        }
                        pl.pendingCommands[uc.network.ID] = &irc.Message{
                                Command: "LIST",
                                Params:  params,
                        }
                        uc.trySendLIST(dc.id)
                })
        case "NAMES":
                if len(msg.Params) == 0 {
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_ENDOFNAMES,
                                Params:  []string{dc.nick, "*", "End of /NAMES list"},
                        })
                        return nil
                }

                channels := strings.Split(msg.Params[0], ",")
                for _, channel := range channels {
                        uc, upstreamName, err := dc.unmarshalEntity(channel)
                        if err != nil {
                                return err
                        }

                        ch := uc.channels.Value(upstreamName)
                        if ch != nil {
                                sendNames(dc, ch)
                        } else {
                                // NAMES on a channel we have not joined, ask upstream
                                uc.SendMessageLabeled(dc.id, &irc.Message{
                                        Command: "NAMES",
                                        Params:  []string{upstreamName},
                                })
                        }
                }
        case "WHO":
                if len(msg.Params) == 0 {
                        // TODO: support WHO without parameters
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_ENDOFWHO,
                                Params:  []string{dc.nick, "*", "End of /WHO list"},
                        })
                        return nil
                }

                // TODO: support WHO masks
                entity := msg.Params[0]
                entityCM := casemapASCII(entity)

                if entityCM == dc.nickCM {
                        // TODO: support AWAY (H/G) in self WHO reply
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_WHOREPLY,
                                Params:  []string{dc.nick, "*", dc.user.Username, dc.hostname, dc.srv.Hostname, dc.nick, "H", "0 " + dc.realname},
                        })
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_ENDOFWHO,
                                Params:  []string{dc.nick, dc.nick, "End of /WHO list"},
                        })
                        return nil
                }
                if entityCM == serviceNickCM {
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_WHOREPLY,
                                Params:  []string{serviceNick, "*", servicePrefix.User, servicePrefix.Host, dc.srv.Hostname, serviceNick, "H", "0 " + serviceRealname},
                        })
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_ENDOFWHO,
                                Params:  []string{dc.nick, serviceNick, "End of /WHO list"},
                        })
                        return nil
                }

                uc, upstreamName, err := dc.unmarshalEntity(entity)
                if err != nil {
                        return err
                }

                var params []string
                if len(msg.Params) == 2 {
                        params = []string{upstreamName, msg.Params[1]}
                } else {
                        params = []string{upstreamName}
                }

                uc.SendMessageLabeled(dc.id, &irc.Message{
                        Command: "WHO",
                        Params:  params,
                })
        case "WHOIS":
                if len(msg.Params) == 0 {
                        return ircError{&irc.Message{
                                Command: irc.ERR_NONICKNAMEGIVEN,
                                Params:  []string{dc.nick, "No nickname given"},
                        }}
                }

                var target, mask string
                if len(msg.Params) == 1 {
                        target = ""
                        mask = msg.Params[0]
                } else {
                        target = msg.Params[0]
                        mask = msg.Params[1]
                }
                // TODO: support multiple WHOIS users
                if i := strings.IndexByte(mask, ','); i >= 0 {
                        mask = mask[:i]
                }

                if casemapASCII(mask) == dc.nickCM {
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_WHOISUSER,
                                Params:  []string{dc.nick, dc.nick, dc.user.Username, dc.hostname, "*", dc.realname},
                        })
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_WHOISSERVER,
                                Params:  []string{dc.nick, dc.nick, dc.srv.Hostname, "soju"},
                        })
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_ENDOFWHOIS,
                                Params:  []string{dc.nick, dc.nick, "End of /WHOIS list"},
                        })
                        return nil
                }

                // TODO: support WHOIS masks
                uc, upstreamNick, err := dc.unmarshalEntity(mask)
                if err != nil {
                        return err
                }

                var params []string
                if target != "" {
                        if target == mask { // WHOIS nick nick
                                params = []string{upstreamNick, upstreamNick}
                        } else {
                                params = []string{target, upstreamNick}
                        }
                } else {
                        params = []string{upstreamNick}
                }

                uc.SendMessageLabeled(dc.id, &irc.Message{
                        Command: "WHOIS",
                        Params:  params,
                })
        case "PRIVMSG":
                var targetsStr, text string
                if err := parseMessageParams(msg, &targetsStr, &text); err != nil {
                        return err
                }
                tags := copyClientTags(msg.Tags)

                for _, name := range strings.Split(targetsStr, ",") {
                        if name == serviceNick {
                                if dc.caps["echo-message"] {
                                        echoTags := tags.Copy()
                                        echoTags["time"] = irc.TagValue(time.Now().UTC().Format(serverTimeLayout))
                                        dc.SendMessage(&irc.Message{
                                                Tags:    echoTags,
                                                Prefix:  dc.prefix(),
                                                Command: "PRIVMSG",
                                                Params:  []string{name, text},
                                        })
                                }
                                handleServicePRIVMSG(dc, text)
                                continue
                        }

                        uc, upstreamName, err := dc.unmarshalEntity(name)
                        if err != nil {
                                return err
                        }

                        if upstreamName == "NickServ" {
                                dc.handleNickServPRIVMSG(uc, text)
                        }

                        unmarshaledText := text
                        if uc.isChannel(upstreamName) {
                                unmarshaledText = dc.unmarshalText(uc, text)
                        }
                        uc.SendMessageLabeled(dc.id, &irc.Message{
                                Tags:    tags,
                                Command: "PRIVMSG",
                                Params:  []string{upstreamName, unmarshaledText},
                        })

                        echoTags := tags.Copy()
                        echoTags["time"] = irc.TagValue(time.Now().UTC().Format(serverTimeLayout))
                        echoMsg := &irc.Message{
                                Tags: echoTags,
                                Prefix: &irc.Prefix{
                                        Name: uc.nick,
                                        User: uc.username,
                                },
                                Command: "PRIVMSG",
                                Params:  []string{upstreamName, text},
                        }
                        uc.produce(upstreamName, echoMsg, dc)

                        uc.updateChannelAutoDetach(upstreamName)
                }
        case "NOTICE":
                var targetsStr, text string
                if err := parseMessageParams(msg, &targetsStr, &text); err != nil {
                        return err
                }
                tags := copyClientTags(msg.Tags)

                for _, name := range strings.Split(targetsStr, ",") {
                        uc, upstreamName, err := dc.unmarshalEntity(name)
                        if err != nil {
                                return err
                        }

                        unmarshaledText := text
                        if uc.isChannel(upstreamName) {
                                unmarshaledText = dc.unmarshalText(uc, text)
                        }
                        uc.SendMessageLabeled(dc.id, &irc.Message{
                                Tags:    tags,
                                Command: "NOTICE",
                                Params:  []string{upstreamName, unmarshaledText},
                        })

                        uc.updateChannelAutoDetach(upstreamName)
                }
        case "TAGMSG":
                var targetsStr string
                if err := parseMessageParams(msg, &targetsStr); err != nil {
                        return err
                }
                tags := copyClientTags(msg.Tags)

                for _, name := range strings.Split(targetsStr, ",") {
                        uc, upstreamName, err := dc.unmarshalEntity(name)
                        if err != nil {
                                return err
                        }
                        if _, ok := uc.caps["message-tags"]; !ok {
                                continue
                        }

                        uc.SendMessageLabeled(dc.id, &irc.Message{
                                Tags:    tags,
                                Command: "TAGMSG",
                                Params:  []string{upstreamName},
                        })

                        uc.updateChannelAutoDetach(upstreamName)
                }
        case "INVITE":
                var user, channel string
                if err := parseMessageParams(msg, &user, &channel); err != nil {
                        return err
                }

                ucChannel, upstreamChannel, err := dc.unmarshalEntity(channel)
                if err != nil {
                        return err
                }

                ucUser, upstreamUser, err := dc.unmarshalEntity(user)
                if err != nil {
                        return err
                }

                if ucChannel != ucUser {
                        return ircError{&irc.Message{
                                Command: irc.ERR_USERNOTINCHANNEL,
                                Params:  []string{dc.nick, user, channel, "They are on another network"},
                        }}
                }
                uc := ucChannel

                uc.SendMessageLabeled(dc.id, &irc.Message{
                        Command: "INVITE",
                        Params:  []string{upstreamUser, upstreamChannel},
                })
        case "CHATHISTORY":
                var subcommand string
                if err := parseMessageParams(msg, &subcommand); err != nil {
                        return err
                }
                var target, criteria, limitStr string
                if err := parseMessageParams(msg, nil, &target, &criteria, &limitStr); err != nil {
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"CHATHISTORY", "NEED_MORE_PARAMS", subcommand, "Missing parameters"},
                        }}
                }

                store, ok := dc.user.msgStore.(chatHistoryMessageStore)
                if !ok {
                        return ircError{&irc.Message{
                                Command: irc.ERR_UNKNOWNCOMMAND,
                                Params:  []string{dc.nick, "CHATHISTORY", "Unknown command"},
                        }}
                }

                uc, entity, err := dc.unmarshalEntity(target)
                if err != nil {
                        return err
                }

                // TODO: support msgid criteria
                criteriaParts := strings.SplitN(criteria, "=", 2)
                if len(criteriaParts) != 2 || criteriaParts[0] != "timestamp" {
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"CHATHISTORY", "INVALID_PARAMS", subcommand, criteria, "Unknown criteria"},
                        }}
                }

                timestamp, err := time.Parse(serverTimeLayout, criteriaParts[1])
                if err != nil {
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"CHATHISTORY", "INVALID_PARAMS", subcommand, criteria, "Invalid criteria"},
                        }}
                }

                limit, err := strconv.Atoi(limitStr)
                if err != nil || limit < 0 || limit > dc.srv.HistoryLimit {
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"CHATHISTORY", "INVALID_PARAMS", subcommand, limitStr, "Invalid limit"},
                        }}
                }

                var history []*irc.Message
                switch subcommand {
                case "BEFORE":
                        history, err = store.LoadBeforeTime(uc.network, entity, timestamp, limit)
                case "AFTER":
                        history, err = store.LoadAfterTime(uc.network, entity, timestamp, limit)
                default:
                        // TODO: support LATEST, BETWEEN
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"CHATHISTORY", "UNKNOWN_COMMAND", subcommand, "Unknown command"},
                        }}
                }
                if err != nil {
                        dc.logger.Printf("failed parsing log messages for chathistory: %v", err)
                        return newChatHistoryError(subcommand, target)
                }

                batchRef := "history"
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: "BATCH",
                        Params:  []string{"+" + batchRef, "chathistory", target},
                })

                for _, msg := range history {
                        msg.Tags["batch"] = irc.TagValue(batchRef)
                        dc.SendMessage(dc.marshalMessage(msg, uc.network))
                }

                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: "BATCH",
                        Params:  []string{"-" + batchRef},
                })
        default:
                dc.logger.Printf("unhandled message: %v", msg)
                return newUnknownCommandError(msg.Command)
        }
        return nil
}

func (dc *downstreamConn) handleNickServPRIVMSG(uc *upstreamConn, text string) {
        username, password, ok := parseNickServCredentials(text, uc.nick)
        if !ok {
                return
        }

        // User may have e.g. EXTERNAL mechanism configured. We do not want to
        // automatically erase the key pair or any other credentials.
        if uc.network.SASL.Mechanism != "" && uc.network.SASL.Mechanism != "PLAIN" {
                return
        }

        dc.logger.Printf("auto-saving NickServ credentials with username %q", username)
        n := uc.network
        n.SASL.Mechanism = "PLAIN"
        n.SASL.Plain.Username = username
        n.SASL.Plain.Password = password
        if err := dc.srv.db.StoreNetwork(dc.user.ID, &n.Network); err != nil {
                dc.logger.Printf("failed to save NickServ credentials: %v", err)
        }
}

func parseNickServCredentials(text, nick string) (username, password string, ok bool) {
        fields := strings.Fields(text)
        if len(fields) < 2 {
                return "", "", false
        }
        cmd := strings.ToUpper(fields[0])
        params := fields[1:]
        switch cmd {
        case "REGISTER":
                username = nick
                password = params[0]
        case "IDENTIFY":
                if len(params) == 1 {
                        username = nick
                        password = params[0]
                } else {
                        username = params[0]
                        password = params[1]
                }
        case "SET":
                if len(params) == 2 && strings.EqualFold(params[0], "PASSWORD") {
                        username = nick
                        password = params[1]
                }
        default:
                return "", "", false
        }
        return username, password, true
}