modbus/server.go

package modbus

import (
	"crypto/tls"
	"crypto/x509"
	"encoding/asn1"
	"errors"
	"fmt"
	"log"
	"net"
	"strings"
	"sync"
	"time"
)

// Modbus Role PEM OID (see R-21 of the MBAPS spec)
var modbusRoleOID asn1.ObjectIdentifier = asn1.ObjectIdentifier{
	1, 3, 6, 1, 4, 1, 50316, 802, 1,
}

// Server configuration object.
type ServerConfiguration struct {
	// URL defines where to listen at e.g. tcp://[::]:502
	URL           string
	// Timeout sets the idle session timeout (client connections will
	// be closed if idle for this long)
	Timeout	      time.Duration
	// MaxClients sets the maximum number of concurrent client connections
	MaxClients    uint
	// TLSServerCert sets the server-side TLS key pair (tcp+tls only)
	TLSServerCert *tls.Certificate
	// TLSClientCAs sets the list of CA certificates used to authenticate
	// client connections (tcp+tls only). Leaf (i.e. client) certificates can
	// also be used in case of self-signed certs, or if cert pinning is required.
	TLSClientCAs  *x509.CertPool
	// Logger provides a custom sink for log messages.
	// If nil, messages will be written to stdout.
	Logger        *log.Logger
}

// Request object passed to the coil handler.
type CoilsRequest struct {
	ClientAddr string  // the source (client) IP address
	ClientRole string  // the client role as encoded in the client certificate (tcp+tls only)
	UnitId     uint8   // the requested unit id (slave id)
	Addr       uint16  // the base coil address requested
	Quantity   uint16  // the number of consecutive coils covered by this request
	                   // (first address: Addr, last address: Addr + Quantity - 1)
	IsWrite    bool    // true if the request is a write, false if a read
	Args       []bool  // a slice of bool values of the coils to be set, ordered
	                   // from Addr to Addr + Quantity - 1 (for writes only)
}

// Request object passed to the discrete input handler.
type DiscreteInputsRequest struct {
	ClientAddr string  // the source (client) IP address
	ClientRole string  // the client role as encoded in the client certificate (tcp+tls only)
	UnitId     uint8   // the requested unit id (slave id)
	Addr       uint16  // the base discrete input address requested
	Quantity   uint16  // the number of consecutive discrete inputs covered by this request
}

// Request object passed to the holding register handler.
type HoldingRegistersRequest struct {
	ClientAddr string   // the source (client) IP address
	ClientRole string   // the client role as encoded in the client certificate (tcp+tls only)
	UnitId     uint8    // the requested unit id (slave id)
	Addr       uint16   // the base register address requested
	Quantity   uint16   // the number of consecutive registers covered by this request
	IsWrite    bool     // true if the request is a write, false if a read
	Args       []uint16 // a slice of register values to be set, ordered from
	                    // Addr to Addr + Quantity - 1 (for writes only)
}

// Request object passed to the input register handler.
type InputRegistersRequest struct {
	ClientAddr string   // the source (client) IP address
	ClientRole string   // the client role as encoded in the client certificate (tcp+tls only)
	UnitId     uint8    // the requested unit id (slave id)
	Addr       uint16   // the base register address requested
	Quantity   uint16   // the number of consecutive registers covered by this request
}

// The RequestHandler interface should be implemented by the handler
// object passed to NewServer (see reqHandler in NewServer()).
// After decoding and validating an incoming request, the server will
// invoke the appropriate handler function, depending on the function code
// of the request.
type RequestHandler interface {
	// HandleCoils handles the read coils (0x01), write single coil (0x05)
	// and write multiple coils (0x0f) function codes.
	// A CoilsRequest object is passed to the handler (see above).
	//
	// Expected return values:
	// - res:	a slice of bools containing the coil values to be sent to back
	//		to the client (only sent for reads),
	// - err:	either nil if no error occurred, a modbus error (see
	//		mapErrorToExceptionCode() in modbus.go for a complete list),
	//		or any other error.
	//		If nil, a positive modbus response is sent back to the client
	//		along with the returned data.
	//		If non-nil, a negative modbus response is sent back, with the
	//		exception code set depending on the error
	//		(again, see mapErrorToExceptionCode()).
	HandleCoils	(req *CoilsRequest) (res []bool, err error)

	// HandleDiscreteInputs handles the read discrete inputs (0x02) function code.
	// A DiscreteInputsRequest oibject is passed to the handler (see above).
	//
	// Expected return values:
	// - res:	a slice of bools containing the discrete input values to be
	//		sent back to the client,
	// - err:	either nil if no error occurred, a modbus error (see
	//		mapErrorToExceptionCode() in modbus.go for a complete list),
	//		or any other error.
	HandleDiscreteInputs	(req *DiscreteInputsRequest) (res []bool, err error)

	// HandleHoldingRegisters handles the read holding registers (0x03),
	// write single register (0x06) and write multiple registers (0x10).
	// A HoldingRegistersRequest object is passed to the handler (see above).
	//
	// Expected return values:
	// - res:	a slice of uint16 containing the register values to be sent
	//		to back to the client (only sent for reads),
	// - err:	either nil if no error occurred, a modbus error (see
	//		mapErrorToExceptionCode() in modbus.go for a complete list),
	//		or any other error.
	HandleHoldingRegisters	(req *HoldingRegistersRequest) (res []uint16, err error)

	// HandleInputRegisters handles the read input registers (0x04) function code.
	// An InputRegistersRequest object is passed to the handler (see above).
	//
	// Expected return values:
	// - res:	a slice of uint16 containing the register values to be sent
	//		back to the client,
	// - err:	either nil if no error occurred, a modbus error (see
	//		mapErrorToExceptionCode() in modbus.go for a complete list),
	//		or any other error.
	HandleInputRegisters	(req *InputRegistersRequest) (res []uint16, err error)
}

// Modbus server object.
type ModbusServer struct {
	conf		ServerConfiguration
	logger		*logger
	lock		sync.Mutex
	started		bool
	handler		RequestHandler
	tcpListener	net.Listener
	tcpClients	[]net.Conn
	transportType	transportType
}

// Returns a new modbus server.
// reqHandler should be a user-provided handler object satisfying the RequestHandler
// interface.
func NewServer(conf *ServerConfiguration, reqHandler RequestHandler) (
	ms *ModbusServer, err error) {
	var serverType string
	var splitURL   []string

	ms = &ModbusServer{
		conf:		*conf,
		handler:	reqHandler,
	}

	splitURL = strings.SplitN(ms.conf.URL, "://", 2)
	if len(splitURL) == 2 {
		serverType  = splitURL[0]
		ms.conf.URL = splitURL[1]
	}

	ms.logger = newLogger(
		fmt.Sprintf("modbus-server(%s)", ms.conf.URL), ms.conf.Logger)

	if ms.conf.URL == "" {
		ms.logger.Errorf("missing host part in URL '%s'", conf.URL)
		err = ErrConfigurationError
		return
	}

	switch serverType {
	case "tcp":
		if ms.conf.Timeout == 0 {
			ms.conf.Timeout = 120 * time.Second
		}

		if ms.conf.MaxClients == 0 {
			ms.conf.MaxClients = 10
		}

		ms.transportType	= modbusTCP

	case "tcp+tls":
		if ms.conf.Timeout == 0 {
			ms.conf.Timeout = 120 * time.Second
		}

		if ms.conf.MaxClients == 0 {
			ms.conf.MaxClients = 10
		}

		// expect a server-side certificate
		if ms.conf.TLSServerCert == nil {
			ms.logger.Errorf("missing server certificate")
			err = ErrConfigurationError
			return
		}

		// expect a CertPool object containing at least 1 CA or
		// leaf certificate to validate client-side certificates
		if ms.conf.TLSClientCAs == nil {
			ms.logger.Errorf("missing CA/client certificates")
			err = ErrConfigurationError
			return
		}

		ms.transportType	= modbusTCPOverTLS

	default:
		err	= ErrConfigurationError
		return
	}

	return
}

// Starts accepting client connections.
func (ms *ModbusServer) Start() (err error) {
	ms.lock.Lock()
	defer ms.lock.Unlock()

	if ms.started {
		return
	}

	switch ms.transportType {
	case modbusTCP, modbusTCPOverTLS:
		// bind to a TCP socket
		ms.tcpListener, err	= net.Listen("tcp", ms.conf.URL)
		if err != nil {
			return
		}

		// accept client connections in a goroutine
		go ms.acceptTCPClients()

	default:
		err = ErrConfigurationError
		return
	}

	ms.started = true

	return
}

// Stops accepting new client connections and closes any active session.
func (ms *ModbusServer) Stop() (err error) {
	ms.lock.Lock()
	defer ms.lock.Unlock()

	if !ms.started {
		return
	}

	ms.started = false

	if ms.transportType == modbusTCP || ms.transportType == modbusTCPOverTLS {
		// close the server socket if we're listening over TCP
		err	= ms.tcpListener.Close()

		// close all active TCP clients
		for _, sock := range ms.tcpClients{
			sock.Close()
		}
	}

	return
}

// Accepts new client connections if the configured connection limit allows it.
// Each connection is served from a dedicated goroutine to allow for concurrent
// connections.
func (ms *ModbusServer) acceptTCPClients() {
	var sock     net.Conn
	var err      error
	var accepted bool

	for {
		sock, err = ms.tcpListener.Accept()
		if err != nil {
			// if the server socket has just been closed, return here as
			// this goroutine isn't going to see any new client connection
			if errors.Is(err, net.ErrClosed) {
				return
			}
			ms.logger.Warningf("failed to accept client connection: %v", err)
			continue
		}

		ms.lock.Lock()
		// apply a connection limit
		if ms.started && uint(len(ms.tcpClients)) < ms.conf.MaxClients {
			accepted = true
			// add the new client connection to the pool
			ms.tcpClients = append(ms.tcpClients, sock)
		} else {
			accepted = false
		}
		ms.lock.Unlock()

		if accepted {
			// spin a client handler goroutine to serve the new client
			go ms.handleTCPClient(sock)
		} else {
			ms.logger.Warningf("max. number of concurrent connections " +
					   "reached, rejecting %v", sock.RemoteAddr())
			// discard the connection
			sock.Close()
		}
	}

	// never reached
	return
}

// Handles a TCP client connection.
// Once handleTransport() returns (i.e. the connection has either closed, timed
// out, or an unrecoverable error happened), the TCP socket is closed and removed
// from the list of active client connections.
func (ms *ModbusServer) handleTCPClient(sock net.Conn) {
	var err        error
	var clientRole string
	var tlsSock    net.Conn

	switch ms.transportType {
	case modbusTCP:
		// serve modbus requests over the raw TCP connection
		ms.handleTransport(
			newTCPTransport(sock, ms.conf.Timeout, ms.conf.Logger),
			sock.RemoteAddr().String(), "")

	case modbusTCPOverTLS:
		// start TLS negotiation over the raw TCP connection
		tlsSock, clientRole, err = ms.startTLS(sock)
		if err != nil {
			ms.logger.Warningf("TLS handshake with %s failed: %v",
				sock.RemoteAddr().String(), err)
		} else {
			// serve modbus requests over the TLS tunnel
			ms.handleTransport(
				newTCPTransport(tlsSock, ms.conf.Timeout, ms.conf.Logger),
				sock.RemoteAddr().String(), clientRole)
		}

	default:
		ms.logger.Errorf("unimplemented transport type %v", ms.transportType)
	}

	// once done, remove our connection from the list of active client conns
	ms.lock.Lock()
	for i := range ms.tcpClients {
		if ms.tcpClients[i] == sock {
			ms.tcpClients[i] = ms.tcpClients[len(ms.tcpClients)-1]
			ms.tcpClients	 = ms.tcpClients[:len(ms.tcpClients)-1]
			break
		}
	}
	ms.lock.Unlock()

	// close the connection
	sock.Close()

	return
}

// For each request read from the transport, performs decoding and validation,
// calls the user-provided handler, then encodes and writes the response
// to the transport.
func (ms *ModbusServer) handleTransport(t transport, clientAddr string, clientRole string) {
	var req		*pdu
	var res		*pdu
	var err		error
	var addr	uint16
	var quantity	uint16

	for {
		req, err = t.ReadRequest()
		if err != nil {
			return
		}

		switch req.functionCode {
		case fcReadCoils, fcReadDiscreteInputs:
			var coils	[]bool
			var resCount	int

			if len(req.payload) != 4 {
				err = ErrProtocolError
				break
			}

			// decode address and quantity fields
			addr		= bytesToUint16(BIG_ENDIAN, req.payload[0:2])
			quantity	= bytesToUint16(BIG_ENDIAN, req.payload[2:4])

			// ensure the reply never exceeds the maximum PDU length and we
			// never read past 0xffff
			if quantity > 2000 || quantity == 0 {
				err	= ErrProtocolError
				break
			}
			if uint32(addr) + uint32(quantity) - 1 > 0xffff {
				err	= ErrIllegalDataAddress
				break
			}

			// invoke the appropriate handler
			if req.functionCode == fcReadCoils {
				coils, err	= ms.handler.HandleCoils(&CoilsRequest{
					ClientAddr: clientAddr,
					ClientRole: clientRole,
					UnitId:     req.unitId,
					Addr:       addr,
					Quantity:   quantity,
					IsWrite:    false,
					Args:       nil,
				})
			} else {
				coils, err	= ms.handler.HandleDiscreteInputs(
					&DiscreteInputsRequest{
						ClientAddr: clientAddr,
						ClientRole: clientRole,
						UnitId:     req.unitId,
						Addr:       addr,
						Quantity:   quantity,
					})
			}
			resCount	= len(coils)

			// make sure the handler returned the expected number of items
			if err == nil && resCount != int(quantity) {
				ms.logger.Errorf("handler returned %v bools, " +
					         "expected %v", resCount, quantity)
				err = ErrServerDeviceFailure
				break
			}

			if err != nil {
				break
			}

			// assemble a response PDU
			res = &pdu{
				unitId:		req.unitId,
				functionCode:	req.functionCode,
				payload:	[]byte{0},
			}

			// byte count (1 byte for 8 coils)
			res.payload[0]	= uint8(resCount / 8)
			if resCount % 8 != 0 {
				res.payload[0]++
			}

			// coil values
			res.payload	= append(res.payload, encodeBools(coils)...)

		case fcWriteSingleCoil:
			if len(req.payload) != 4 {
				err = ErrProtocolError
				break
			}

			// decode the address field
			addr	= bytesToUint16(BIG_ENDIAN, req.payload[0:2])

			// validate the value field (should be either 0xff00 or 0x0000)
			if ((req.payload[2] != 0xff && req.payload[2] != 0x00) ||
			    req.payload[3] != 0x00) {
				err = ErrProtocolError
				break
			}

			// invoke the coil handler
			_, err	= ms.handler.HandleCoils(&CoilsRequest{
				ClientAddr: clientAddr,
				ClientRole: clientRole,
				UnitId:     req.unitId,
				Addr:       addr,
				Quantity:   1, // request for a single coil
				IsWrite:    true, // this is a write request
				Args:       []bool{(req.payload[2] == 0xff)},
			})

			if err != nil {
				break
			}

			// assemble a response PDU
			res = &pdu{
				unitId:		req.unitId,
				functionCode:	req.functionCode,
			}

			// echo the address and value in the response
			res.payload	= append(res.payload,
						 uint16ToBytes(BIG_ENDIAN, addr)...)
			res.payload	= append(res.payload,
						 req.payload[2], req.payload[3])

		case fcWriteMultipleCoils:
			var expectedLen	int

			if len(req.payload) < 6 {
				err = ErrProtocolError
				break
			}

			// decode address and quantity fields
			addr		= bytesToUint16(BIG_ENDIAN, req.payload[0:2])
			quantity	= bytesToUint16(BIG_ENDIAN, req.payload[2:4])

			// ensure the reply never exceeds the maximum PDU length and we
			// never read past 0xffff
			if quantity > 0x7b0 || quantity == 0 {
				err	= ErrProtocolError
				break
			}
			if uint32(addr) + uint32(quantity) - 1 > 0xffff {
				err	= ErrIllegalDataAddress
				break
			}

			// validate the byte count field (1 byte for 8 coils)
			expectedLen	= int(quantity) / 8
			if quantity % 8 != 0 {
				expectedLen++
			}

			if req.payload[4] != uint8(expectedLen) {
				err	= ErrProtocolError
				break
			}

			// make sure we have enough bytes
			if len(req.payload) - 5 != expectedLen {
				err	= ErrProtocolError
				break
			}

			// invoke the coil handler
			_, err	= ms.handler.HandleCoils(&CoilsRequest{
				ClientAddr: clientAddr,
				ClientRole: clientRole,
				UnitId:     req.unitId,
				Addr:       addr,
				Quantity:   quantity,
				IsWrite:    true, // this is a write request
				Args:       decodeBools(quantity, req.payload[5:]),
			})

			if err != nil {
				break
			}

			// assemble a response PDU
			res = &pdu{
				unitId:		req.unitId,
				functionCode:	req.functionCode,
			}

			// echo the address and quantity in the response
			res.payload	= append(res.payload,
						 uint16ToBytes(BIG_ENDIAN, addr)...)
			res.payload	= append(res.payload,
						 uint16ToBytes(BIG_ENDIAN, quantity)...)

		case fcReadHoldingRegisters, fcReadInputRegisters:
			var regs	[]uint16
			var resCount	int

			if len(req.payload) != 4 {
				err = ErrProtocolError
				break
			}

			// decode address and quantity fields
			addr		= bytesToUint16(BIG_ENDIAN, req.payload[0:2])
			quantity	= bytesToUint16(BIG_ENDIAN, req.payload[2:4])

			// ensure the reply never exceeds the maximum PDU length and we
			// never read past 0xffff
			if quantity > 0x007d || quantity == 0 {
				err	= ErrProtocolError
				break
			}
			if uint32(addr) + uint32(quantity) - 1 > 0xffff {
				err	= ErrIllegalDataAddress
				break
			}

			// invoke the appropriate handler
			if req.functionCode == fcReadHoldingRegisters {
				regs, err	= ms.handler.HandleHoldingRegisters(
					&HoldingRegistersRequest{
						ClientAddr: clientAddr,
						ClientRole: clientRole,
						UnitId:     req.unitId,
						Addr:       addr,
						Quantity:   quantity,
						IsWrite:    false,
						Args:       nil,
					})
			} else {
				regs, err	= ms.handler.HandleInputRegisters(
					&InputRegistersRequest{
						ClientAddr: clientAddr,
						ClientRole: clientRole,
						UnitId:     req.unitId,
						Addr:       addr,
						Quantity:   quantity,
					})
			}
			resCount	= len(regs)

			// make sure the handler returned the expected number of items
			if err == nil && resCount != int(quantity) {
				ms.logger.Errorf("handler returned %v 16-bit values, " +
					         "expected %v", resCount, quantity)
				err = ErrServerDeviceFailure
				break
			}

			if err != nil {
				break
			}

			// assemble a response PDU
			res = &pdu{
				unitId:		req.unitId,
				functionCode:	req.functionCode,
				payload:	[]byte{0},
			}

			// byte count (2 bytes per register)
			res.payload[0]	= uint8(resCount * 2)

			// register values
			res.payload	= append(res.payload,
						 uint16sToBytes(BIG_ENDIAN, regs)...)

		case fcWriteSingleRegister:
			var value	uint16

			if len(req.payload) != 4 {
				err = ErrProtocolError
				break
			}

			// decode address and value fields
			addr	= bytesToUint16(BIG_ENDIAN, req.payload[0:2])
			value	= bytesToUint16(BIG_ENDIAN, req.payload[2:4])

			// invoke the handler
			_, err	= ms.handler.HandleHoldingRegisters(
				&HoldingRegistersRequest{
					ClientAddr: clientAddr,
					ClientRole: clientRole,
					UnitId:     req.unitId,
					Addr:       addr,
					Quantity:   1, // request for a single register
					IsWrite:    true, // request is a write
					Args:       []uint16{value},
				})

			if err != nil {
				break
			}

			// assemble a response PDU
			res = &pdu{
				unitId:		req.unitId,
				functionCode:	req.functionCode,
			}

			// echo the address and value in the response
			res.payload	= append(res.payload,
						 uint16ToBytes(BIG_ENDIAN, addr)...)
			res.payload	= append(res.payload,
						 uint16ToBytes(BIG_ENDIAN, value)...)

		case fcWriteMultipleRegisters:
			var expectedLen	int

			if len(req.payload) < 6 {
				err = ErrProtocolError
				break
			}

			// decode address and quantity fields
			addr		= bytesToUint16(BIG_ENDIAN, req.payload[0:2])
			quantity	= bytesToUint16(BIG_ENDIAN, req.payload[2:4])

			// ensure the reply never exceeds the maximum PDU length and we
			// never read past 0xffff
			if quantity > 0x007b || quantity == 0 {
				err	= ErrProtocolError
				break
			}
			if uint32(addr) + uint32(quantity) - 1 > 0xffff {
				err	= ErrIllegalDataAddress
				break
			}

			// validate the byte count field (2 bytes per register)
			expectedLen	= int(quantity) * 2

			if req.payload[4] != uint8(expectedLen) {
				err	= ErrProtocolError
				break
			}

			// make sure we have enough bytes
			if len(req.payload) - 5 != expectedLen {
				err	= ErrProtocolError
				break
			}

			// invoke the holding register handler
			_, err		= ms.handler.HandleHoldingRegisters(
				&HoldingRegistersRequest{
					ClientAddr: clientAddr,
					ClientRole: clientRole,
					UnitId:     req.unitId,
					Addr:       addr,
					Quantity:   quantity,
					IsWrite:    true, // this is a write request
					Args:       bytesToUint16s(BIG_ENDIAN, req.payload[5:]),
				})
			if err != nil {
				break
			}

			// assemble a response PDU
			res = &pdu{
				unitId:		req.unitId,
				functionCode:	req.functionCode,
			}

			// echo the address and quantity in the response
			res.payload	= append(res.payload,
						 uint16ToBytes(BIG_ENDIAN, addr)...)
			res.payload	= append(res.payload,
						 uint16ToBytes(BIG_ENDIAN, quantity)...)

		default:
			res = &pdu{
				// reply with the request target unit ID
				unitId:		req.unitId,
				// set the error bit
				functionCode:	(0x80 | req.functionCode),
				// set the exception code to illegal function to indicate that
				// the server does not know how to handle this function code.
				payload:	[]byte{exIllegalFunction},
			}
		}

		// if there was no error processing the request but the response is nil
		// (which should never happen), emit a server failure exception code
		// and log an error
		if err == nil && res == nil {
			err = ErrServerDeviceFailure
			ms.logger.Errorf("internal server error (req: %v, res: %v, err: %v)",
					 req, res, err)
		}

		// map go errors to modbus errors, unless the error is a protocol error,
		// in which case close the transport and return.
		if err != nil {
			if err == ErrProtocolError {
				ms.logger.Warningf(
					"protocol error, closing link (client address: '%s')",
					clientAddr)
				t.Close()
				return
			} else {
				res = &pdu{
					unitId:		req.unitId,
					functionCode:	(0x80 | req.functionCode),
					payload:	[]byte{mapErrorToExceptionCode(err)},
				}
			}
		}

		// write the response to the transport
		err	= t.WriteResponse(res)
		if err != nil {
			ms.logger.Warningf("failed to write response: %v", err)
		}

		// avoid holding on to stale data
		req	= nil
		res	= nil
	}

	// never reached
	return
}

// startTLS performs a full TLS handshake (with client authentication) on tcpSock
// and returns a 'wrapped' clear-text socket suitable for use by the TCP transport.
func (ms *ModbusServer) startTLS(tcpSock net.Conn) (
	tlsSock *tls.Conn, clientRole string, err error) {
	var connState  tls.ConnectionState

	// set a 30s timeout for the TLS handshake to complete
	err = tcpSock.SetDeadline(time.Now().Add(30 * time.Second))
	if err != nil {
		return
	}

	// start TLS negotiation over the raw TCP connection
	tlsSock = tls.Server(tcpSock, &tls.Config{
		Certificates: []tls.Certificate{
			*ms.conf.TLSServerCert,
		},
		ClientCAs:    ms.conf.TLSClientCAs,
		// require a valid (verified) certificate from the client
		// (see R-06, R-08 and R-10 of the MBAPS spec)
		ClientAuth:   tls.RequireAndVerifyClientCert,
		// mandate TLSv1.2 or higher (see R-01 of the MBAPS spec)
		MinVersion:   tls.VersionTLS12,
	})

	// complete the full TLS handshake (with client cert validation)
	err = tlsSock.Handshake()
	if err != nil {
		return
	}

	// look for and extract the client's role, if any
	connState = tlsSock.ConnectionState()
	if len(connState.PeerCertificates) == 0 {
		err = errors.New("no client certificate received")
		return
	}
	// From the tls.ConnectionState doc:
	// "The first element is the leaf certificate that the connection is
	// verified against."
	clientRole = ms.extractRole(connState.PeerCertificates[0])

	return
}

// extractRole looks for Modbus Role extensions in a certificate and returns the
// role as a string.
// If no role extension is found, a nil string is returned (R-23).
// If multiple or invalid role extensions are found, a nil string is returned (R-65, R-22).
func (ms *ModbusServer) extractRole(cert *x509.Certificate) (role string) {
	var err     error
	var found   bool
	var badCert bool

	// walk through all extensions looking for Modbus Role OIDs
	for _, ext := range cert.Extensions {
		if ext.Id.Equal(modbusRoleOID) {

			// there must be only one role extension per cert (R-65)
			if found {
				ms.logger.Warning("client certificate contains more than one role OIDs")
				badCert = true
				break
			}
			found = true

			// the role extension must use UTF8String encoding (R-22)
			// (the ASN1 tag for UTF8String is 0x0c)
			if len(ext.Value) < 2 || ext.Value[0] != 0x0c {
				badCert = true
				break
			}

			// extract the ASN1 string
			_, err = asn1.Unmarshal(ext.Value, &role)
			if err != nil {
				ms.logger.Warningf("failed to decode Modbus Role extension: %v", err)
				badCert = true
				break
			}
		}
	}

	// blank the role if we found more than one Role extension
	if badCert {
		role = ""
	}

	return
}