source: code/trunk/downstream.go@ 747

package soju

import (
        "context"
        "crypto/tls"
        "encoding/base64"
        "errors"
        "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"},
        }}
}

// authError is an authentication error.
type authError struct {
        // Internal error cause. This will not be revealed to the user.
        err error
        // Error cause which can safely be sent to the user without compromising
        // security.
        reason string
}

func (err *authError) Error() string {
        return err.err.Error()
}

func (err *authError) Unwrap() error {
        return err.err
}

// authErrorReason returns the user-friendly reason of an authentication
// failure.
func authErrorReason(err error) string {
        if authErr, ok := err.(*authError); ok {
                return authErr.reason
        } else {
                return "Authentication failed"
        }
}

func newInvalidUsernameOrPasswordError(err error) error {
        return &authError{
                err:    err,
                reason: "Invalid username or password",
        }
}

func parseBouncerNetID(subcommand, s string) (int64, error) {
        id, err := strconv.ParseInt(s, 10, 64)
        if err != nil {
                return 0, ircError{&irc.Message{
                        Command: "FAIL",
                        Params:  []string{"BOUNCER", "INVALID_NETID", subcommand, s, "Invalid network ID"},
                }}
        }
        return id, nil
}

func fillNetworkAddrAttrs(attrs irc.Tags, network *Network) {
        u, err := network.URL()
        if err != nil {
                return
        }

        hasHostPort := true
        switch u.Scheme {
        case "ircs":
                attrs["tls"] = irc.TagValue("1")
        case "irc+insecure":
                attrs["tls"] = irc.TagValue("0")
        default: // e.g. unix://
                hasHostPort = false
        }
        if host, port, err := net.SplitHostPort(u.Host); err == nil && hasHostPort {
                attrs["host"] = irc.TagValue(host)
                attrs["port"] = irc.TagValue(port)
        } else if hasHostPort {
                attrs["host"] = irc.TagValue(u.Host)
        }
}

func getNetworkAttrs(network *network) irc.Tags {
        state := "disconnected"
        if uc := network.conn; uc != nil {
                state = "connected"
        }

        attrs := irc.Tags{
                "name":     irc.TagValue(network.GetName()),
                "state":    irc.TagValue(state),
                "nickname": irc.TagValue(GetNick(&network.user.User, &network.Network)),
        }

        if network.Username != "" {
                attrs["username"] = irc.TagValue(network.Username)
        }
        if realname := GetRealname(&network.user.User, &network.Network); realname != "" {
                attrs["realname"] = irc.TagValue(realname)
        }

        fillNetworkAddrAttrs(attrs, &network.Network)

        return attrs
}

func networkAddrFromAttrs(attrs irc.Tags) string {
        host, ok := attrs.GetTag("host")
        if !ok {
                return ""
        }

        addr := host
        if port, ok := attrs.GetTag("port"); ok {
                addr += ":" + port
        }

        if tlsStr, ok := attrs.GetTag("tls"); ok && tlsStr == "0" {
                addr = "irc+insecure://" + tlsStr
        }

        return addr
}

func updateNetworkAttrs(record *Network, attrs irc.Tags, subcommand string) error {
        addrAttrs := irc.Tags{}
        fillNetworkAddrAttrs(addrAttrs, record)

        updateAddr := false
        for k, v := range attrs {
                s := string(v)
                switch k {
                case "host", "port", "tls":
                        updateAddr = true
                        addrAttrs[k] = v
                case "name":
                        record.Name = s
                case "nickname":
                        record.Nick = s
                case "username":
                        record.Username = s
                case "realname":
                        record.Realname = s
                case "pass":
                        record.Pass = s
                default:
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"BOUNCER", "UNKNOWN_ATTRIBUTE", subcommand, k, "Unknown attribute"},
                        }}
                }
        }

        if updateAddr {
                record.Addr = networkAddrFromAttrs(addrAttrs)
                if record.Addr == "" {
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"BOUNCER", "NEED_ATTRIBUTE", subcommand, "host", "Missing required host attribute"},
                        }}
                }
        }

        return nil
}

// ' ' and ':' break the IRC message wire format, '@' and '!' break prefixes,
// '*' and '?' break masks, '$' breaks server masks in PRIVMSG/NOTICE,
// "*" is the reserved nickname for registration
const illegalNickChars = " :@!*?$"

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

        "soju.im/bouncer-networks":        "",
        "soju.im/bouncer-networks-notify": "",
}

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

        "draft/extended-monitor": "",
}

// 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,
        "BOT":           true,
        "CHANLIMIT":     true,
        "CHANMODES":     true,
        "CHANNELLEN":    true,
        "CHANTYPES":     true,
        "CLIENTTAGDENY": true,
        "ELIST":         true,
        "EXCEPTS":       true,
        "EXTBAN":        true,
        "HOSTLEN":       true,
        "INVEX":         true,
        "KICKLEN":       true,
        "MAXLIST":       true,
        "MAXTARGETS":    true,
        "MODES":         true,
        "MONITOR":       true,
        "NAMELEN":       true,
        "NETWORK":       true,
        "NICKLEN":       true,
        "PREFIX":        true,
        "SAFELIST":      true,
        "TARGMAX":       true,
        "TOPICLEN":      true,
        "USERLEN":       true,
        "UTF8ONLY":      true,
        "WHOX":          true,
}

type downstreamSASL struct {
        server                       sasl.Server
        plainUsername, plainPassword string
}

type downstreamConn struct {
        conn

        id uint64

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

        negotiatingCaps bool
        capVersion      int
        supportedCaps   map[string]string
        caps            map[string]bool
        sasl            *downstreamSASL

        lastBatchRef uint64

        monitored casemapMap
}

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,
                nick:          "*",
                nickCM:        "*",
                supportedCaps: make(map[string]string),
                caps:          make(map[string]bool),
                monitored:     newCasemapMap(0),
        }
        dc.hostname = remoteAddr
        if host, _, err := net.SplitHostPort(dc.hostname); err == nil {
                dc.hostname = host
        }
        for k, v := range permanentDownstreamCaps {
                dc.supportedCaps[k] = v
        }
        dc.supportedCaps["sasl"] = "PLAIN"
        // TODO: this is racy, we should only enable chathistory after
        // authentication and then check that user.msgStore implements
        // chatHistoryMessageStore
        if srv.Config().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 if dc.isMultiUpstream {
                dc.user.forEachNetwork(f)
        }
}

func (dc *downstreamConn) forEachUpstream(f func(*upstreamConn)) {
        if dc.network == nil && !dc.isMultiUpstream {
                return
        }
        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(GetNick(&net.user.User, &net.Network))
}

// 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,
        }
}

// unmarshalEntityNetwork converts a downstream entity name (ie. channel or
// nick) into an upstream entity name.
//
// This involves removing the "/<network>" suffix.
func (dc *downstreamConn) unmarshalEntityNetwork(name string) (*network, string, error) {
        if dc.network != nil {
                return dc.network, name, nil
        }
        if !dc.isMultiUpstream {
                return nil, "", ircError{&irc.Message{
                        Command: irc.ERR_NOSUCHCHANNEL,
                        Params:  []string{dc.nick, name, "Cannot interact with channels and users on the bouncer connection. Did you mean to use a specific network?"},
                }}
        }

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

                for _, n := range dc.user.networks {
                        if network == n.GetName() {
                                net = n
                                break
                        }
                }
        }

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

        return net, name, nil
}

// unmarshalEntity is the same as unmarshalEntityNetwork, but returns the
// upstream connection and fails if the upstream is disconnected.
func (dc *downstreamConn) unmarshalEntity(name string) (*upstreamConn, string, error) {
        net, name, err := dc.unmarshalEntityNetwork(name)
        if err != nil {
                return nil, "", err
        }

        if net.conn == nil {
                return nil, "", ircError{&irc.Message{
                        Command: irc.ERR_NOSUCHCHANNEL,
                        Params:  []string{dc.nick, name, "Disconnected from upstream network"},
                }}
        }

        return net.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) ReadMessage() (*irc.Message, error) {
        msg, err := dc.conn.ReadMessage()
        if err != nil {
                return nil, err
        }
        dc.srv.metrics.downstreamInMessagesTotal.Inc()
        return msg, nil
}

func (dc *downstreamConn) readMessages(ch chan<- event) error {
        for {
                msg, err := dc.ReadMessage()
                if errors.Is(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"]
                        case "account":
                                supported = dc.caps["account"]
                        }
                        if !supported {
                                delete(msg.Tags, name)
                        }
                }
        }
        if !dc.caps["batch"] && msg.Tags["batch"] != "" {
                msg = msg.Copy()
                delete(msg.Tags, "batch")
        }
        if msg.Command == "JOIN" && !dc.caps["extended-join"] {
                msg.Params = msg.Params[:1]
        }
        if msg.Command == "SETNAME" && !dc.caps["setname"] {
                return
        }
        if msg.Command == "AWAY" && !dc.caps["away-notify"] {
                return
        }
        if msg.Command == "ACCOUNT" && !dc.caps["account-notify"] {
                return
        }

        dc.srv.metrics.downstreamOutMessagesTotal.Inc()
        dc.conn.SendMessage(msg)
}

func (dc *downstreamConn) SendBatch(typ string, params []string, tags irc.Tags, f func(batchRef irc.TagValue)) {
        dc.lastBatchRef++
        ref := fmt.Sprintf("%v", dc.lastBatchRef)

        if dc.caps["batch"] {
                dc.SendMessage(&irc.Message{
                        Tags:    tags,
                        Prefix:  dc.srv.prefix(),
                        Command: "BATCH",
                        Params:  append([]string{"+" + ref, typ}, params...),
                })
        }

        f(irc.TagValue(ref))

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

// 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.messageSupportsBacklog(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.messageSupportsBacklog(msg) {
                return
        }

        dc.sendPing(id)
}

// ackMsgID acknowledges that a message has been received.
func (dc *downstreamConn) ackMsgID(id string) {
        netID, entity, err := parseMsgID(id, nil)
        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
        }

        network.delivered.StoreID(entity, dc.clientName, id)
}

func (dc *downstreamConn) sendPing(msgID string) {
        token := "soju-msgid-" + 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
        }
        msgID := strings.TrimPrefix(token, "soju-msgid-")
        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 messages which
// may only appear in single-upstream mode.
func (dc *downstreamConn) marshalMessage(msg *irc.Message, net *network) *irc.Message {
        msg = msg.Copy()
        msg.Prefix = dc.marshalUserPrefix(net, msg.Prefix)

        if dc.network != nil {
                return msg
        }

        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", "SETNAME":
                // This space is intentionally left blank
        default:
                panic(fmt.Sprintf("unexpected %q message", msg.Command))
        }

        return msg
}

func (dc *downstreamConn) handleMessage(ctx context.Context, msg *irc.Message) error {
        ctx, cancel := dc.conn.NewContext(ctx)
        defer cancel()

        ctx, cancel = context.WithTimeout(ctx, handleDownstreamMessageTimeout)
        defer cancel()

        switch msg.Command {
        case "QUIT":
                return dc.Close()
        default:
                if dc.registered {
                        return dc.handleMessageRegistered(ctx, msg)
                } else {
                        return dc.handleMessageUnregistered(ctx, msg)
                }
        }
}

func (dc *downstreamConn) handleMessageUnregistered(ctx context.Context, msg *irc.Message) error {
        switch msg.Command {
        case "NICK":
                var nick string
                if err := parseMessageParams(msg, &nick); err != nil {
                        return err
                }
                if nick == "" || 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":
                credentials, err := dc.handleAuthenticateCommand(msg)
                if err != nil {
                        return err
                } else if credentials == nil {
                        break
                }

                if err := dc.authenticate(ctx, credentials.plainUsername, credentials.plainPassword); err != nil {
                        dc.logger.Printf("SASL authentication error for user %q: %v", credentials.plainUsername, err)
                        dc.endSASL(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.ERR_SASLFAIL,
                                Params:  []string{dc.nick, authErrorReason(err)},
                        })
                        break
                }

                // Technically we should send RPL_LOGGEDIN here. However we use
                // RPL_LOGGEDIN to mirror the upstream connection status. Let's
                // see how many clients that breaks. See:
                // https://github.com/ircv3/ircv3-specifications/pull/476
                dc.endSASL(nil)
        case "BOUNCER":
                var subcommand string
                if err := parseMessageParams(msg, &subcommand); err != nil {
                        return err
                }

                switch strings.ToUpper(subcommand) {
                case "BIND":
                        var idStr string
                        if err := parseMessageParams(msg, nil, &idStr); err != nil {
                                return err
                        }

                        if dc.user == nil {
                                return ircError{&irc.Message{
                                        Command: "FAIL",
                                        Params:  []string{"BOUNCER", "ACCOUNT_REQUIRED", "BIND", "Authentication needed to bind to bouncer network"},
                                }}
                        }

                        id, err := parseBouncerNetID(subcommand, idStr)
                        if err != nil {
                                return err
                        }

                        var match *network
                        dc.user.forEachNetwork(func(net *network) {
                                if net.ID == id {
                                        match = net
                                }
                        })
                        if match == nil {
                                return ircError{&irc.Message{
                                        Command: "FAIL",
                                        Params:  []string{"BOUNCER", "INVALID_NETID", idStr, "Unknown network ID"},
                                }}
                        }

                        dc.networkName = match.GetName()
                }
        default:
                dc.logger.Printf("unhandled message: %v", msg)
                return newUnknownCommandError(msg.Command)
        }
        if dc.rawUsername != "" && dc.nick != "*" && !dc.negotiatingCaps {
                return dc.register(ctx)
        }
        return nil
}

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

        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{dc.nick, "LS", strings.Join(caps, " ")},
                })

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

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

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

                if !dc.registered {
                        dc.negotiatingCaps = true
                }
        case "END":
                dc.negotiatingCaps = false
        default:
                return ircError{&irc.Message{
                        Command: err_invalidcapcmd,
                        Params:  []string{dc.nick, cmd, "Unknown CAP command"},
                }}
        }
        return nil
}

func (dc *downstreamConn) handleAuthenticateCommand(msg *irc.Message) (result *downstreamSASL, err error) {
        defer func() {
                if err != nil {
                        dc.sasl = nil
                }
        }()

        if !dc.caps["sasl"] {
                return nil, ircError{&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: irc.ERR_SASLFAIL,
                        Params:  []string{"*", "AUTHENTICATE requires the \"sasl\" capability to be enabled"},
                }}
        }
        if len(msg.Params) == 0 {
                return nil, ircError{&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: irc.ERR_SASLFAIL,
                        Params:  []string{"*", "Missing AUTHENTICATE argument"},
                }}
        }
        if msg.Params[0] == "*" {
                return nil, ircError{&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: irc.ERR_SASLABORTED,
                        Params:  []string{"*", "SASL authentication aborted"},
                }}
        }

        var resp []byte
        if dc.sasl == nil {
                mech := strings.ToUpper(msg.Params[0])
                var server sasl.Server
                switch mech {
                case "PLAIN":
                        server = sasl.NewPlainServer(sasl.PlainAuthenticator(func(identity, username, password string) error {
                                dc.sasl.plainUsername = username
                                dc.sasl.plainPassword = password
                                return nil
                        }))
                default:
                        return nil, ircError{&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.ERR_SASLFAIL,
                                Params:  []string{"*", fmt.Sprintf("Unsupported SASL mechanism %q", mech)},
                        }}
                }

                dc.sasl = &downstreamSASL{server: server}
        } else {
                // TODO: multi-line messages
                if msg.Params[0] == "+" {
                        resp = nil
                } else if resp, err = base64.StdEncoding.DecodeString(msg.Params[0]); err != nil {
                        return nil, ircError{&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.ERR_SASLFAIL,
                                Params:  []string{"*", "Invalid base64-encoded response"},
                        }}
                }
        }

        challenge, done, err := dc.sasl.server.Next(resp)
        if err != nil {
                return nil, err
        } else if done {
                return dc.sasl, nil
        } 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},
                })
                return nil, nil
        }
}

func (dc *downstreamConn) endSASL(msg *irc.Message) {
        if dc.sasl == nil {
                return
        }

        dc.sasl = nil

        if msg != nil {
                dc.SendMessage(msg)
        } else {
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: irc.RPL_SASLSUCCESS,
                        Params:  []string{dc.nick, "SASL authentication successful"},
                })
        }
}

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
        }

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

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

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

        if uc := dc.upstream(); uc != nil && uc.supportsSASL("PLAIN") {
                dc.setSupportedCap("sasl", "PLAIN")
        } else if dc.network != nil {
                dc.unsetSupportedCap("sasl")
        }

        if uc := dc.upstream(); uc != nil && uc.caps["draft/account-registration"] {
                // Strip "before-connect", because we require downstreams to be fully
                // connected before attempting account registration.
                values := strings.Split(uc.supportedCaps["draft/account-registration"], ",")
                for i, v := range values {
                        if v == "before-connect" {
                                values = append(values[:i], values[i+1:]...)
                                break
                        }
                }
                dc.setSupportedCap("draft/account-registration", strings.Join(values, ","))
        } else {
                dc.unsetSupportedCap("draft/account-registration")
        }

        if _, ok := dc.user.msgStore.(chatHistoryMessageStore); ok && dc.network != nil {
                dc.setSupportedCap("draft/event-playback", "")
        } else {
                dc.unsetSupportedCap("draft/event-playback")
        }
}

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 (dc *downstreamConn) updateRealname() {
        if uc := dc.upstream(); uc != nil && uc.realname != dc.realname && dc.caps["setname"] {
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.prefix(),
                        Command: "SETNAME",
                        Params:  []string{uc.realname},
                })
                dc.realname = uc.realname
        }
}

func (dc *downstreamConn) updateAccount() {
        var account string
        if dc.network == nil {
                account = dc.user.Username
        } else if uc := dc.upstream(); uc != nil {
                account = uc.account
        } else {
                return
        }

        if dc.account == account || !dc.caps["sasl"] {
                return
        }

        if account != "" {
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: irc.RPL_LOGGEDIN,
                        Params:  []string{dc.nick, dc.prefix().String(), account, "You are logged in as " + account},
                })
        } else {
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: irc.RPL_LOGGEDOUT,
                        Params:  []string{dc.nick, dc.prefix().String(), "You are logged out"},
                })
        }

        dc.account = account
}

func sanityCheckServer(ctx context.Context, addr string) error {
        ctx, cancel := context.WithTimeout(ctx, 15*time.Second)
        defer cancel()

        conn, err := new(tls.Dialer).DialContext(ctx, "tcp", addr)
        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(ctx context.Context, username, password string) error {
        username, clientName, networkName := unmarshalUsername(username)

        u, err := dc.srv.db.GetUser(ctx, username)
        if err != nil {
                return newInvalidUsernameOrPasswordError(fmt.Errorf("user not found: %w", err))
        }

        // Password auth disabled
        if u.Password == "" {
                return newInvalidUsernameOrPasswordError(fmt.Errorf("password auth disabled"))
        }

        err = bcrypt.CompareHashAndPassword([]byte(u.Password), []byte(password))
        if err != nil {
                return newInvalidUsernameOrPasswordError(fmt.Errorf("wrong password"))
        }

        dc.user = dc.srv.getUser(username)
        if dc.user == nil {
                return fmt.Errorf("user not active")
        }
        dc.clientName = clientName
        dc.networkName = networkName
        return nil
}

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

        if dc.sasl != nil {
                dc.endSASL(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: irc.ERR_SASLABORTED,
                        Params:  []string{"*", "SASL authentication aborted"},
                })
        }

        password := dc.password
        dc.password = ""
        if dc.user == nil {
                if err := dc.authenticate(ctx, dc.rawUsername, password); err != nil {
                        dc.logger.Printf("PASS authentication error for user %q: %v", dc.rawUsername, err)
                        return ircError{&irc.Message{
                                Command: irc.ERR_PASSWDMISMATCH,
                                Params:  []string{"*", authErrorReason(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(ctx context.Context) 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(ctx, 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(ctx, &Network{
                        Addr:    dc.networkName,
                        Nick:    nick,
                        Enabled: true,
                })
                if err != nil {
                        return err
                }
        }

        dc.network = network
        return nil
}

func (dc *downstreamConn) welcome(ctx context.Context) 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(ctx); err != nil {
                return err
        }

        if dc.network == nil && !dc.caps["soju.im/bouncer-networks"] && dc.srv.Config().MultiUpstream {
                dc.isMultiUpstream = true
        }

        dc.updateSupportedCaps()

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

        if dc.network != nil {
                isupport = append(isupport, fmt.Sprintf("BOUNCER_NETID=%v", dc.network.ID))
        }
        if title := dc.srv.Config().Title; dc.network == nil && title != "" {
                isupport = append(isupport, "NETWORK="+encodeISUPPORT(title))
        }
        if dc.network == nil && !dc.isMultiUpstream {
                isupport = append(isupport, "WHOX")
        }

        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.Config().Hostname},
        })
        dc.SendMessage(&irc.Message{
                Prefix:  dc.srv.prefix(),
                Command: irc.RPL_MYINFO,
                Params:  []string{dc.nick, dc.srv.Config().Hostname, "soju", "aiwroO", "OovaimnqpsrtklbeI"},
        })
        for _, msg := range generateIsupport(dc.srv.prefix(), dc.nick, isupport) {
                dc.SendMessage(msg)
        }
        if uc := dc.upstream(); uc != nil {
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: irc.RPL_UMODEIS,
                        Params:  []string{dc.nick, "+" + string(uc.modes)},
                })
        }
        if dc.network == nil && !dc.isMultiUpstream && dc.user.Admin {
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: irc.RPL_UMODEIS,
                        Params:  []string{dc.nick, "+o"},
                })
        }

        dc.updateNick()
        dc.updateRealname()
        dc.updateAccount()

        if motd := dc.user.srv.Config().MOTD; motd != "" && dc.network == nil {
                for _, msg := range generateMOTD(dc.srv.prefix(), dc.nick, motd) {
                        dc.SendMessage(msg)
                }
        } else {
                motdHint := "No MOTD"
                if dc.network != nil {
                        motdHint = "Use /motd to read the message of the day"
                }
                dc.SendMessage(&irc.Message{
                        Prefix:  dc.srv.prefix(),
                        Command: irc.ERR_NOMOTD,
                        Params:  []string{dc.nick, motdHint},
                })
        }

        if dc.caps["soju.im/bouncer-networks-notify"] {
                dc.SendBatch("soju.im/bouncer-networks", nil, nil, func(batchRef irc.TagValue) {
                        dc.user.forEachNetwork(func(network *network) {
                                idStr := fmt.Sprintf("%v", network.ID)
                                attrs := getNetworkAttrs(network)
                                dc.SendMessage(&irc.Message{
                                        Tags:    irc.Tags{"batch": batchRef},
                                        Prefix:  dc.srv.prefix(),
                                        Command: "BOUNCER",
                                        Params:  []string{"NETWORK", idStr, attrs.String()},
                                })
                        })
                })
        }

        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) {
                if dc.caps["draft/chathistory"] || dc.user.msgStore == nil {
                        return
                }

                // Only send history if we're the first connected client with that name
                // for the network
                firstClient := true
                dc.user.forEachDownstream(func(c *downstreamConn) {
                        if c != dc && c.clientName == dc.clientName && c.network == dc.network {
                                firstClient = false
                        }
                })
                if firstClient {
                        net.delivered.ForEachTarget(func(target string) {
                                lastDelivered := net.delivered.LoadID(target, dc.clientName)
                                if lastDelivered == "" {
                                        return
                                }

                                dc.sendTargetBacklog(ctx, net, target, lastDelivered)

                                // Fast-forward history to last message
                                targetCM := net.casemap(target)
                                lastID, err := dc.user.msgStore.LastMsgID(&net.Network, targetCM, time.Now())
                                if err != nil {
                                        dc.logger.Printf("failed to get last message ID: %v", err)
                                        return
                                }
                                net.delivered.StoreID(target, dc.clientName, lastID)
                        })
                }
        })

        return nil
}

// messageSupportsBacklog checks whether the provided message can be sent as
// part of an history batch.
func (dc *downstreamConn) messageSupportsBacklog(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.
        switch msg.Command {
        case "PRIVMSG", "NOTICE":
                return true
        }
        return false
}

func (dc *downstreamConn) sendTargetBacklog(ctx context.Context, net *network, target, msgID string) {
        if dc.caps["draft/chathistory"] || dc.user.msgStore == nil {
                return
        }

        ch := net.channels.Value(target)

        ctx, cancel := context.WithTimeout(ctx, backlogTimeout)
        defer cancel()

        targetCM := net.casemap(target)
        history, err := dc.user.msgStore.LoadLatestID(ctx, &net.Network, targetCM, msgID, backlogLimit)
        if err != nil {
                dc.logger.Printf("failed to send backlog for %q: %v", target, err)
                return
        }

        dc.SendBatch("chathistory", []string{dc.marshalEntity(net, target)}, nil, func(batchRef irc.TagValue) {
                for _, msg := range history {
                        if ch != nil && ch.Detached {
                                if net.detachedMessageNeedsRelay(ch, msg) {
                                        dc.relayDetachedMessage(net, msg)
                                }
                        } else {
                                msg.Tags["batch"] = batchRef
                                dc.SendMessage(dc.marshalMessage(msg, net))
                        }
                }
        })
}

func (dc *downstreamConn) relayDetachedMessage(net *network, msg *irc.Message) {
        if msg.Command != "PRIVMSG" && msg.Command != "NOTICE" {
                return
        }

        sender := msg.Prefix.Name
        target, text := msg.Params[0], msg.Params[1]
        if net.isHighlight(msg) {
                sendServiceNOTICE(dc, fmt.Sprintf("highlight in %v: <%v> %v", dc.marshalEntity(net, target), sender, text))
        } else {
                sendServiceNOTICE(dc, fmt.Sprintf("message in %v: <%v> %v", dc.marshalEntity(net, target), sender, text))
        }
}

func (dc *downstreamConn) runUntilRegistered() error {
        ctx, cancel := context.WithTimeout(context.TODO(), downstreamRegisterTimeout)
        defer cancel()

        // Close the connection with an error if the deadline is exceeded
        go func() {
                <-ctx.Done()
                if err := ctx.Err(); err == context.DeadlineExceeded {
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: "ERROR",
                                Params:  []string{"Connection registration timed out"},
                        })
                        dc.Close()
                }
        }()

        for !dc.registered {
                msg, err := dc.ReadMessage()
                if err != nil {
                        return fmt.Errorf("failed to read IRC command: %w", err)
                }

                err = dc.handleMessage(ctx, 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(ctx context.Context, 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]
                }
                hostname := dc.srv.Config().Hostname
                if destination != "" && destination != 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{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 nick == "" || 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(ctx, 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 && 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 "SETNAME":
                var realname string
                if err := parseMessageParams(msg, &realname); err != nil {
                        return err
                }

                // If the client just resets to the default, just wipe the per-network
                // preference
                storeRealname := realname
                if realname == dc.user.Realname {
                        storeRealname = ""
                }

                var storeErr error
                var needUpdate []Network
                dc.forEachNetwork(func(n *network) {
                        // We only need to call updateNetwork for upstreams that don't
                        // support setname
                        if uc := n.conn; uc != nil && uc.caps["setname"] {
                                uc.SendMessageLabeled(dc.id, &irc.Message{
                                        Command: "SETNAME",
                                        Params:  []string{realname},
                                })

                                n.Realname = storeRealname
                                if err := dc.srv.db.StoreNetwork(ctx, dc.user.ID, &n.Network); err != nil {
                                        dc.logger.Printf("failed to store network realname: %v", err)
                                        storeErr = err
                                }
                                return
                        }

                        record := n.Network // copy network record because we'll mutate it
                        record.Realname = storeRealname
                        needUpdate = append(needUpdate, record)
                })

                // Walk the network list as a second step, because updateNetwork
                // mutates the original list
                for _, record := range needUpdate {
                        if _, err := dc.user.updateNetwork(ctx, &record); err != nil {
                                dc.logger.Printf("failed to update network realname: %v", err)
                                storeErr = err
                        }
                }
                if storeErr != nil {
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"SETNAME", "CANNOT_CHANGE_REALNAME", "Failed to update realname"},
                        }}
                }

                if dc.upstream() == nil {
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.prefix(),
                                Command: "SETNAME",
                                Params:  []string{realname},
                        })
                }
        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]
                        }

                        if !uc.isChannel(upstreamName) {
                                dc.SendMessage(&irc.Message{
                                        Prefix:  dc.srv.prefix(),
                                        Command: irc.ERR_NOSUCHCHANNEL,
                                        Params:  []string{name, "Not a channel name"},
                                })
                                continue
                        }

                        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(ctx, 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(ctx, 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(ctx, 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 != "" {
                                if uc := dc.upstream(); uc != nil {
                                        uc.SendMessageLabeled(dc.id, &irc.Message{
                                                Command: "MODE",
                                                Params:  []string{uc.nick, modeStr},
                                        })
                                } else {
                                        dc.SendMessage(&irc.Message{
                                                Prefix:  dc.srv.prefix(),
                                                Command: irc.ERR_UMODEUNKNOWNFLAG,
                                                Params:  []string{dc.nick, "Cannot change user mode in multi-upstream mode"},
                                        })
                                }
                        } else {
                                var userMode string
                                if uc := dc.upstream(); uc != nil {
                                        userMode = string(uc.modes)
                                }

                                dc.SendMessage(&irc.Message{
                                        Prefix:  dc.srv.prefix(),
                                        Command: irc.RPL_UMODEIS,
                                        Params:  []string{dc.nick, "+" + userMode},
                                })
                        }
                        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":
                network := dc.network
                if network == nil && len(msg.Params) > 0 {
                        var err error
                        network, msg.Params[0], err = dc.unmarshalEntityNetwork(msg.Params[0])
                        if err != nil {
                                return err
                        }
                }
                if network == nil {
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_LISTEND,
                                Params:  []string{dc.nick, "LIST without a network suffix is not supported in multi-upstream mode"},
                        })
                        return nil
                }

                uc := network.conn
                if uc == nil {
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_LISTEND,
                                Params:  []string{dc.nick, "Disconnected from upstream server"},
                        })
                        return nil
                }

                uc.enqueueCommand(dc, msg)
        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},
                                })
                        }
                }
        // For WHOX docs, see:
        // - http://faerion.sourceforge.net/doc/irc/whox.var
        // - https://github.com/quakenet/snircd/blob/master/doc/readme.who
        // Note, many features aren't widely implemented, such as flags and mask2
        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
                }

                // Clients will use the first mask to match RPL_ENDOFWHO
                endOfWhoToken := msg.Params[0]

                // TODO: add support for WHOX mask2
                mask := msg.Params[0]
                var options string
                if len(msg.Params) > 1 {
                        options = msg.Params[1]
                }

                optionsParts := strings.SplitN(options, "%", 2)
                // TODO: add support for WHOX flags in optionsParts[0]
                var fields, whoxToken string
                if len(optionsParts) == 2 {
                        optionsParts := strings.SplitN(optionsParts[1], ",", 2)
                        fields = strings.ToLower(optionsParts[0])
                        if len(optionsParts) == 2 && strings.Contains(fields, "t") {
                                whoxToken = optionsParts[1]
                        }
                }

                // TODO: support mixed bouncer/upstream WHO queries
                maskCM := casemapASCII(mask)
                if dc.network == nil && maskCM == dc.nickCM {
                        // TODO: support AWAY (H/G) in self WHO reply
                        flags := "H"
                        if dc.user.Admin {
                                flags += "*"
                        }
                        info := whoxInfo{
                                Token:    whoxToken,
                                Username: dc.user.Username,
                                Hostname: dc.hostname,
                                Server:   dc.srv.Config().Hostname,
                                Nickname: dc.nick,
                                Flags:    flags,
                                Account:  dc.user.Username,
                                Realname: dc.realname,
                        }
                        dc.SendMessage(generateWHOXReply(dc.srv.prefix(), dc.nick, fields, &info))
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_ENDOFWHO,
                                Params:  []string{dc.nick, endOfWhoToken, "End of /WHO list"},
                        })
                        return nil
                }
                if maskCM == serviceNickCM {
                        info := whoxInfo{
                                Token:    whoxToken,
                                Username: servicePrefix.User,
                                Hostname: servicePrefix.Host,
                                Server:   dc.srv.Config().Hostname,
                                Nickname: serviceNick,
                                Flags:    "H*",
                                Account:  serviceNick,
                                Realname: serviceRealname,
                        }
                        dc.SendMessage(generateWHOXReply(dc.srv.prefix(), dc.nick, fields, &info))
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_ENDOFWHO,
                                Params:  []string{dc.nick, endOfWhoToken, "End of /WHO list"},
                        })
                        return nil
                }

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

                params := []string{upstreamMask}
                if options != "" {
                        params = append(params, options)
                }

                uc.enqueueCommand(dc, &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 dc.network == nil && 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.Config().Hostname, "soju"},
                        })
                        if dc.user.Admin {
                                dc.SendMessage(&irc.Message{
                                        Prefix:  dc.srv.prefix(),
                                        Command: irc.RPL_WHOISOPERATOR,
                                        Params:  []string{dc.nick, dc.nick, "is a bouncer administrator"},
                                })
                        }
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: rpl_whoisaccount,
                                Params:  []string{dc.nick, dc.nick, dc.user.Username, "is logged in as"},
                        })
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_ENDOFWHOIS,
                                Params:  []string{dc.nick, dc.nick, "End of /WHOIS list"},
                        })
                        return nil
                }
                if casemapASCII(mask) == serviceNickCM {
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_WHOISUSER,
                                Params:  []string{dc.nick, serviceNick, servicePrefix.User, servicePrefix.Host, "*", serviceRealname},
                        })
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_WHOISSERVER,
                                Params:  []string{dc.nick, serviceNick, dc.srv.Config().Hostname, "soju"},
                        })
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_WHOISOPERATOR,
                                Params:  []string{dc.nick, serviceNick, "is the bouncer service"},
                        })
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: rpl_whoisaccount,
                                Params:  []string{dc.nick, serviceNick, serviceNick, "is logged in as"},
                        })
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_ENDOFWHOIS,
                                Params:  []string{dc.nick, serviceNick, "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", "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, ",") {
                        if name == "$"+dc.srv.Config().Hostname || (name == "$*" && dc.network == nil) {
                                // "$" means a server mask follows. If it's the bouncer's
                                // hostname, broadcast the message to all bouncer users.
                                if !dc.user.Admin {
                                        return ircError{&irc.Message{
                                                Prefix:  dc.srv.prefix(),
                                                Command: irc.ERR_BADMASK,
                                                Params:  []string{dc.nick, name, "Permission denied to broadcast message to all bouncer users"},
                                        }}
                                }

                                dc.logger.Printf("broadcasting bouncer-wide %v: %v", msg.Command, text)

                                broadcastTags := tags.Copy()
                                broadcastTags["time"] = irc.TagValue(time.Now().UTC().Format(serverTimeLayout))
                                broadcastMsg := &irc.Message{
                                        Tags:    broadcastTags,
                                        Prefix:  servicePrefix,
                                        Command: msg.Command,
                                        Params:  []string{name, text},
                                }
                                dc.srv.forEachUser(func(u *user) {
                                        u.events <- eventBroadcast{broadcastMsg}
                                })
                                continue
                        }

                        if dc.network == nil && casemapASCII(name) == dc.nickCM {
                                dc.SendMessage(&irc.Message{
                                        Tags:    msg.Tags.Copy(),
                                        Prefix:  dc.prefix(),
                                        Command: msg.Command,
                                        Params:  []string{name, text},
                                })
                                continue
                        }

                        if msg.Command == "PRIVMSG" && casemapASCII(name) == serviceNickCM {
                                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: msg.Command,
                                                Params:  []string{name, text},
                                        })
                                }
                                handleServicePRIVMSG(ctx, dc, text)
                                continue
                        }

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

                        if msg.Command == "PRIVMSG" && uc.network.casemap(upstreamName) == "nickserv" {
                                dc.handleNickServPRIVMSG(ctx, uc, text)
                        }

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

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

                        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, ",") {
                        if dc.network == nil && casemapASCII(name) == dc.nickCM {
                                dc.SendMessage(&irc.Message{
                                        Tags:    msg.Tags.Copy(),
                                        Prefix:  dc.prefix(),
                                        Command: "TAGMSG",
                                        Params:  []string{name},
                                })
                                continue
                        }

                        if casemapASCII(name) == serviceNickCM {
                                continue
                        }

                        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 "AUTHENTICATE":
                // Post-connection-registration AUTHENTICATE is unsupported in
                // multi-upstream mode, or if the upstream doesn't support SASL
                uc := dc.upstream()
                if uc == nil || !uc.caps["sasl"] {
                        return ircError{&irc.Message{
                                Command: irc.ERR_SASLFAIL,
                                Params:  []string{dc.nick, "Upstream network authentication not supported"},
                        }}
                }

                credentials, err := dc.handleAuthenticateCommand(msg)
                if err != nil {
                        return err
                }

                if credentials != nil {
                        if uc.saslClient != nil {
                                dc.endSASL(&irc.Message{
                                        Prefix:  dc.srv.prefix(),
                                        Command: irc.ERR_SASLFAIL,
                                        Params:  []string{dc.nick, "Another authentication attempt is already in progress"},
                                })
                                return nil
                        }

                        uc.logger.Printf("starting post-registration SASL PLAIN authentication with username %q", credentials.plainUsername)
                        uc.saslClient = sasl.NewPlainClient("", credentials.plainUsername, credentials.plainPassword)
                        uc.enqueueCommand(dc, &irc.Message{
                                Command: "AUTHENTICATE",
                                Params:  []string{"PLAIN"},
                        })
                }
        case "REGISTER", "VERIFY":
                // Check number of params here, since we'll use that to save the
                // credentials on command success
                if (msg.Command == "REGISTER" && len(msg.Params) < 3) || (msg.Command == "VERIFY" && len(msg.Params) < 2) {
                        return newNeedMoreParamsError(msg.Command)
                }

                uc := dc.upstream()
                if uc == nil || !uc.caps["draft/account-registration"] {
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{msg.Command, "TEMPORARILY_UNAVAILABLE", "*", "Upstream network account registration not supported"},
                        }}
                }

                uc.logger.Printf("starting %v with account name %v", msg.Command, msg.Params[0])
                uc.enqueueCommand(dc, msg)
        case "MONITOR":
                // MONITOR is unsupported in multi-upstream mode
                uc := dc.upstream()
                if uc == nil {
                        return newUnknownCommandError(msg.Command)
                }
                if _, ok := uc.isupport["MONITOR"]; !ok {
                        return newUnknownCommandError(msg.Command)
                }

                var subcommand string
                if err := parseMessageParams(msg, &subcommand); err != nil {
                        return err
                }

                switch strings.ToUpper(subcommand) {
                case "+", "-":
                        var targets string
                        if err := parseMessageParams(msg, nil, &targets); err != nil {
                                return err
                        }
                        for _, target := range strings.Split(targets, ",") {
                                if subcommand == "+" {
                                        // Hard limit, just to avoid having downstreams fill our map
                                        if len(dc.monitored.innerMap) >= 1000 {
                                                dc.SendMessage(&irc.Message{
                                                        Prefix:  dc.srv.prefix(),
                                                        Command: irc.ERR_MONLISTFULL,
                                                        Params:  []string{dc.nick, "1000", target, "Bouncer monitor list is full"},
                                                })
                                                continue
                                        }

                                        dc.monitored.SetValue(target, nil)

                                        if uc.monitored.Has(target) {
                                                cmd := irc.RPL_MONOFFLINE
                                                if online := uc.monitored.Value(target); online {
                                                        cmd = irc.RPL_MONONLINE
                                                }

                                                dc.SendMessage(&irc.Message{
                                                        Prefix:  dc.srv.prefix(),
                                                        Command: cmd,
                                                        Params:  []string{dc.nick, target},
                                                })
                                        }
                                } else {
                                        dc.monitored.Delete(target)
                                }
                        }
                        uc.updateMonitor()
                case "C": // clear
                        dc.monitored = newCasemapMap(0)
                        uc.updateMonitor()
                case "L": // list
                        // TODO: be less lazy and pack the list
                        for _, entry := range dc.monitored.innerMap {
                                dc.SendMessage(&irc.Message{
                                        Prefix:  dc.srv.prefix(),
                                        Command: irc.RPL_MONLIST,
                                        Params:  []string{dc.nick, entry.originalKey},
                                })
                        }
                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: irc.RPL_ENDOFMONLIST,
                                Params:  []string{dc.nick, "End of MONITOR list"},
                        })
                case "S": // status
                        // TODO: be less lazy and pack the lists
                        for _, entry := range dc.monitored.innerMap {
                                target := entry.originalKey

                                cmd := irc.RPL_MONOFFLINE
                                if online := uc.monitored.Value(target); online {
                                        cmd = irc.RPL_MONONLINE
                                }

                                dc.SendMessage(&irc.Message{
                                        Prefix:  dc.srv.prefix(),
                                        Command: cmd,
                                        Params:  []string{dc.nick, target},
                                })
                        }
                }
        case "CHATHISTORY":
                var subcommand string
                if err := parseMessageParams(msg, &subcommand); err != nil {
                        return err
                }
                var target, limitStr string
                var boundsStr [2]string
                switch subcommand {
                case "AFTER", "BEFORE", "LATEST":
                        if err := parseMessageParams(msg, nil, &target, &boundsStr[0], &limitStr); err != nil {
                                return err
                        }
                case "BETWEEN":
                        if err := parseMessageParams(msg, nil, &target, &boundsStr[0], &boundsStr[1], &limitStr); err != nil {
                                return err
                        }
                case "TARGETS":
                        if dc.network == nil {
                                // Either an unbound bouncer network, in which case we should return no targets,
                                // or a multi-upstream downstream, but we don't support CHATHISTORY TARGETS for those yet.
                                dc.SendBatch("draft/chathistory-targets", nil, nil, func(batchRef irc.TagValue) {})
                                return nil
                        }
                        if err := parseMessageParams(msg, nil, &boundsStr[0], &boundsStr[1], &limitStr); err != nil {
                                return err
                        }
                default:
                        // TODO: support AROUND
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"CHATHISTORY", "INVALID_PARAMS", subcommand, "Unknown command"},
                        }}
                }

                // We don't save history for our service
                if casemapASCII(target) == serviceNickCM {
                        dc.SendBatch("chathistory", []string{target}, nil, func(batchRef irc.TagValue) {})
                        return nil
                }

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

                network, entity, err := dc.unmarshalEntityNetwork(target)
                if err != nil {
                        return err
                }
                entity = network.casemap(entity)

                // TODO: support msgid criteria
                var bounds [2]time.Time
                bounds[0] = parseChatHistoryBound(boundsStr[0])
                if subcommand == "LATEST" && boundsStr[0] == "*" {
                        bounds[0] = time.Now()
                } else if bounds[0].IsZero() {
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"CHATHISTORY", "INVALID_PARAMS", subcommand, boundsStr[0], "Invalid first bound"},
                        }}
                }

                if boundsStr[1] != "" {
                        bounds[1] = parseChatHistoryBound(boundsStr[1])
                        if bounds[1].IsZero() {
                                return ircError{&irc.Message{
                                        Command: "FAIL",
                                        Params:  []string{"CHATHISTORY", "INVALID_PARAMS", subcommand, boundsStr[1], "Invalid second bound"},
                                }}
                        }
                }

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

                eventPlayback := dc.caps["draft/event-playback"]

                var history []*irc.Message
                switch subcommand {
                case "BEFORE", "LATEST":
                        history, err = store.LoadBeforeTime(ctx, &network.Network, entity, bounds[0], time.Time{}, limit, eventPlayback)
                case "AFTER":
                        history, err = store.LoadAfterTime(ctx, &network.Network, entity, bounds[0], time.Now(), limit, eventPlayback)
                case "BETWEEN":
                        if bounds[0].Before(bounds[1]) {
                                history, err = store.LoadAfterTime(ctx, &network.Network, entity, bounds[0], bounds[1], limit, eventPlayback)
                        } else {
                                history, err = store.LoadBeforeTime(ctx, &network.Network, entity, bounds[0], bounds[1], limit, eventPlayback)
                        }
                case "TARGETS":
                        // TODO: support TARGETS in multi-upstream mode
                        targets, err := store.ListTargets(ctx, &network.Network, bounds[0], bounds[1], limit, eventPlayback)
                        if err != nil {
                                dc.logger.Printf("failed fetching targets for chathistory: %v", err)
                                return ircError{&irc.Message{
                                        Command: "FAIL",
                                        Params:  []string{"CHATHISTORY", "MESSAGE_ERROR", subcommand, "Failed to retrieve targets"},
                                }}
                        }

                        dc.SendBatch("draft/chathistory-targets", nil, nil, func(batchRef irc.TagValue) {
                                for _, target := range targets {
                                        if ch := network.channels.Value(target.Name); ch != nil && ch.Detached {
                                                continue
                                        }

                                        dc.SendMessage(&irc.Message{
                                                Tags:    irc.Tags{"batch": batchRef},
                                                Prefix:  dc.srv.prefix(),
                                                Command: "CHATHISTORY",
                                                Params:  []string{"TARGETS", target.Name, target.LatestMessage.UTC().Format(serverTimeLayout)},
                                        })
                                }
                        })

                        return nil
                }
                if err != nil {
                        dc.logger.Printf("failed fetching %q messages for chathistory: %v", target, err)
                        return newChatHistoryError(subcommand, target)
                }

                dc.SendBatch("chathistory", []string{target}, nil, func(batchRef irc.TagValue) {
                        for _, msg := range history {
                                msg.Tags["batch"] = batchRef
                                dc.SendMessage(dc.marshalMessage(msg, network))
                        }
                })
        case "BOUNCER":
                var subcommand string
                if err := parseMessageParams(msg, &subcommand); err != nil {
                        return err
                }

                switch strings.ToUpper(subcommand) {
                case "BIND":
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"BOUNCER", "REGISTRATION_IS_COMPLETED", "BIND", "Cannot bind to a network after registration"},
                        }}
                case "LISTNETWORKS":
                        dc.SendBatch("soju.im/bouncer-networks", nil, nil, func(batchRef irc.TagValue) {
                                dc.user.forEachNetwork(func(network *network) {
                                        idStr := fmt.Sprintf("%v", network.ID)
                                        attrs := getNetworkAttrs(network)
                                        dc.SendMessage(&irc.Message{
                                                Tags:    irc.Tags{"batch": batchRef},
                                                Prefix:  dc.srv.prefix(),
                                                Command: "BOUNCER",
                                                Params:  []string{"NETWORK", idStr, attrs.String()},
                                        })
                                })
                        })
                case "ADDNETWORK":
                        var attrsStr string
                        if err := parseMessageParams(msg, nil, &attrsStr); err != nil {
                                return err
                        }
                        attrs := irc.ParseTags(attrsStr)

                        record := &Network{Nick: dc.nick, Enabled: true}
                        if err := updateNetworkAttrs(record, attrs, subcommand); err != nil {
                                return err
                        }

                        if record.Nick == dc.user.Username {
                                record.Nick = ""
                        }
                        if record.Realname == dc.user.Realname {
                                record.Realname = ""
                        }

                        network, err := dc.user.createNetwork(ctx, record)
                        if err != nil {
                                return ircError{&irc.Message{
                                        Command: "FAIL",
                                        Params:  []string{"BOUNCER", "UNKNOWN_ERROR", subcommand, fmt.Sprintf("Failed to create network: %v", err)},
                                }}
                        }

                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: "BOUNCER",
                                Params:  []string{"ADDNETWORK", fmt.Sprintf("%v", network.ID)},
                        })
                case "CHANGENETWORK":
                        var idStr, attrsStr string
                        if err := parseMessageParams(msg, nil, &idStr, &attrsStr); err != nil {
                                return err
                        }
                        id, err := parseBouncerNetID(subcommand, idStr)
                        if err != nil {
                                return err
                        }
                        attrs := irc.ParseTags(attrsStr)

                        net := dc.user.getNetworkByID(id)
                        if net == nil {
                                return ircError{&irc.Message{
                                        Command: "FAIL",
                                        Params:  []string{"BOUNCER", "INVALID_NETID", subcommand, idStr, "Invalid network ID"},
                                }}
                        }

                        record := net.Network // copy network record because we'll mutate it
                        if err := updateNetworkAttrs(&record, attrs, subcommand); err != nil {
                                return err
                        }

                        if record.Nick == dc.user.Username {
                                record.Nick = ""
                        }
                        if record.Realname == dc.user.Realname {
                                record.Realname = ""
                        }

                        _, err = dc.user.updateNetwork(ctx, &record)
                        if err != nil {
                                return ircError{&irc.Message{
                                        Command: "FAIL",
                                        Params:  []string{"BOUNCER", "UNKNOWN_ERROR", subcommand, fmt.Sprintf("Failed to update network: %v", err)},
                                }}
                        }

                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: "BOUNCER",
                                Params:  []string{"CHANGENETWORK", idStr},
                        })
                case "DELNETWORK":
                        var idStr string
                        if err := parseMessageParams(msg, nil, &idStr); err != nil {
                                return err
                        }
                        id, err := parseBouncerNetID(subcommand, idStr)
                        if err != nil {
                                return err
                        }

                        net := dc.user.getNetworkByID(id)
                        if net == nil {
                                return ircError{&irc.Message{
                                        Command: "FAIL",
                                        Params:  []string{"BOUNCER", "INVALID_NETID", subcommand, idStr, "Invalid network ID"},
                                }}
                        }

                        if err := dc.user.deleteNetwork(ctx, net.ID); err != nil {
                                return err
                        }

                        dc.SendMessage(&irc.Message{
                                Prefix:  dc.srv.prefix(),
                                Command: "BOUNCER",
                                Params:  []string{"DELNETWORK", idStr},
                        })
                default:
                        return ircError{&irc.Message{
                                Command: "FAIL",
                                Params:  []string{"BOUNCER", "UNKNOWN_COMMAND", subcommand, "Unknown subcommand"},
                        }}
                }
        default:
                dc.logger.Printf("unhandled message: %v", msg)

                // Only forward unknown commands in single-upstream mode
                uc := dc.upstream()
                if uc == nil {
                        return newUnknownCommandError(msg.Command)
                }

                uc.SendMessageLabeled(dc.id, msg)
        }
        return nil
}

func (dc *downstreamConn) handleNickServPRIVMSG(ctx context.Context, uc *upstreamConn, text string) {
        username, password, ok := parseNickServCredentials(text, uc.nick)
        if ok {
                uc.network.autoSaveSASLPlain(ctx, username, password)
        }
}

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
}