diff --git a/common/errors/errors.go b/common/errors/errors.go
deleted file mode 100644
index 0d19b69b8..000000000
--- a/common/errors/errors.go
+++ /dev/null
@@ -1,133 +0,0 @@
-package errors
-
-import (
-	"fmt"
-)
-
-func HasCode(err error, code int) bool {
-	if errWithCode, ok := err.(ErrorWithCode); ok {
-		return errWithCode.Code() == code
-	}
-	return false
-}
-
-type ErrorWithCode interface {
-	error
-	Code() int
-}
-
-type ErrorCode int
-
-func (code ErrorCode) Code() int {
-	return int(code)
-}
-
-func (code ErrorCode) Prefix() string {
-	return fmt.Sprintf("[Error 0x%04X] ", code.Code())
-}
-
-type AuthenticationError struct {
-	ErrorCode
-	AuthDetail interface{}
-}
-
-func NewAuthenticationError(detail interface{}) AuthenticationError {
-	return AuthenticationError{
-		ErrorCode:  1,
-		AuthDetail: detail,
-	}
-}
-
-func (err AuthenticationError) Error() string {
-	return fmt.Sprintf("%sInvalid auth %v", err.Prefix(), err.AuthDetail)
-}
-
-type ProtocolVersionError struct {
-	ErrorCode
-	Version int
-}
-
-func NewProtocolVersionError(version int) ProtocolVersionError {
-	return ProtocolVersionError{
-		ErrorCode: 2,
-		Version:   version,
-	}
-}
-
-func (err ProtocolVersionError) Error() string {
-	return fmt.Sprintf("%sInvalid version %d", err.Prefix(), err.Version)
-}
-
-type CorruptedPacketError struct {
-	ErrorCode
-}
-
-var corruptedPacketErrorInstance = CorruptedPacketError{ErrorCode: 3}
-
-func NewCorruptedPacketError() CorruptedPacketError {
-	return corruptedPacketErrorInstance
-}
-
-func (err CorruptedPacketError) Error() string {
-	return err.Prefix() + "Corrupted packet."
-}
-
-type IPFormatError struct {
-	ErrorCode
-	IP []byte
-}
-
-func NewIPFormatError(ip []byte) IPFormatError {
-	return IPFormatError{
-		ErrorCode: 4,
-		IP:        ip,
-	}
-}
-
-func (err IPFormatError) Error() string {
-	return fmt.Sprintf("%sInvalid IP %v", err.Prefix(), err.IP)
-}
-
-type ConfigurationError struct {
-	ErrorCode
-}
-
-var configurationErrorInstance = ConfigurationError{ErrorCode: 5}
-
-func NewConfigurationError() ConfigurationError {
-	return configurationErrorInstance
-}
-
-func (r ConfigurationError) Error() string {
-	return r.Prefix() + "Invalid configuration."
-}
-
-type InvalidOperationError struct {
-	ErrorCode
-	Operation string
-}
-
-func NewInvalidOperationError(operation string) InvalidOperationError {
-	return InvalidOperationError{
-		ErrorCode: 6,
-		Operation: operation,
-	}
-}
-
-func (r InvalidOperationError) Error() string {
-	return r.Prefix() + "Invalid operation: " + r.Operation
-}
-
-type BadConfigurationError struct {
-	ErrorCode
-}
-
-var badConfigurationErrorInstance = BadConfigurationError{ErrorCode: 6}
-
-func NewBadConfigurationError() BadConfigurationError {
-	return badConfigurationErrorInstance
-}
-
-func (r BadConfigurationError) Error() string {
-	return r.Prefix() + "Bad configuration."
-}
diff --git a/common/errors/errors_test.go b/common/errors/errors_test.go
deleted file mode 100644
index 5228f8bc1..000000000
--- a/common/errors/errors_test.go
+++ /dev/null
@@ -1,23 +0,0 @@
-package errors_test
-
-import (
-	"testing"
-
-	"github.com/v2ray/v2ray-core/common/errors"
-	"github.com/v2ray/v2ray-core/testing/unit"
-)
-
-type MockError struct {
-	errors.ErrorCode
-}
-
-func (err MockError) Error() string {
-	return "This is a fake error."
-}
-
-func TestHasCode(t *testing.T) {
-	assert := unit.Assert(t)
-
-	err := MockError{ErrorCode: 101}
-	assert.Error(err).HasCode(101)
-}
diff --git a/common/net/address.go b/common/net/address.go
index f344a2228..14b1bc164 100644
--- a/common/net/address.go
+++ b/common/net/address.go
@@ -4,7 +4,6 @@ import (
 	"net"
 	"strconv"
 
-	"github.com/v2ray/v2ray-core/common/errors"
 	"github.com/v2ray/v2ray-core/common/log"
 )
 
@@ -42,7 +41,7 @@ func IPAddress(ip []byte, port uint16) Address {
 			},
 		}
 	default:
-		log.Error(errors.NewIPFormatError(ip).Error())
+		log.Error("Invalid IP format: %v", ip)
 		return nil
 	}
 }
diff --git a/config/errors.go b/config/errors.go
new file mode 100644
index 000000000..e6f1057ee
--- /dev/null
+++ b/config/errors.go
@@ -0,0 +1,9 @@
+package config
+
+import (
+	"errors"
+)
+
+var (
+	BadConfiguration = errors.New("Bad configuration.")
+)
diff --git a/point.go b/point.go
index dbb74f3ce..1364b7dd8 100644
--- a/point.go
+++ b/point.go
@@ -1,7 +1,6 @@
 package core
 
 import (
-	"github.com/v2ray/v2ray-core/common/errors"
 	"github.com/v2ray/v2ray-core/common/log"
 	v2net "github.com/v2ray/v2ray-core/common/net"
 	"github.com/v2ray/v2ray-core/common/retry"
@@ -41,7 +40,7 @@ func NewPoint(pConfig config.PointConfig) (*Point, error) {
 	ichFactory, ok := inboundFactories[pConfig.InboundConfig().Protocol()]
 	if !ok {
 		log.Error("Unknown inbound connection handler factory %s", pConfig.InboundConfig().Protocol())
-		return nil, errors.NewBadConfigurationError()
+		return nil, config.BadConfiguration
 	}
 	ichConfig := pConfig.InboundConfig().Settings(config.TypeInbound)
 	ich, err := ichFactory.Create(vpoint, ichConfig)
@@ -54,7 +53,7 @@ func NewPoint(pConfig config.PointConfig) (*Point, error) {
 	ochFactory, ok := outboundFactories[pConfig.OutboundConfig().Protocol()]
 	if !ok {
 		log.Error("Unknown outbound connection handler factory %s", pConfig.OutboundConfig().Protocol())
-		return nil, errors.NewBadConfigurationError()
+		return nil, config.BadConfiguration
 	}
 	ochConfig := pConfig.OutboundConfig().Settings(config.TypeOutbound)
 	och, err := ochFactory.Create(vpoint, ochConfig)
@@ -88,7 +87,7 @@ type OutboundConnectionHandler interface {
 func (vp *Point) Start() error {
 	if vp.port <= 0 {
 		log.Error("Invalid port %d", vp.port)
-		return errors.NewBadConfigurationError()
+		return config.BadConfiguration
 	}
 
 	return retry.Timed(100 /* times */, 100 /* ms */).On(func() error {
diff --git a/proxy/errors.go b/proxy/errors.go
new file mode 100644
index 000000000..4eb105696
--- /dev/null
+++ b/proxy/errors.go
@@ -0,0 +1,10 @@
+package proxy
+
+import (
+	"errors"
+)
+
+var (
+	InvalidAuthentication  = errors.New("Invalid authentication.")
+	InvalidProtocolVersion = errors.New("Invalid protocol version.")
+)
diff --git a/proxy/socks/protocol/socks.go b/proxy/socks/protocol/socks.go
index d6a34b069..38d8a0070 100644
--- a/proxy/socks/protocol/socks.go
+++ b/proxy/socks/protocol/socks.go
@@ -5,9 +5,10 @@ import (
 	"io"
 
 	"github.com/v2ray/v2ray-core/common/alloc"
-	"github.com/v2ray/v2ray-core/common/errors"
 	"github.com/v2ray/v2ray-core/common/log"
 	v2net "github.com/v2ray/v2ray-core/common/net"
+	"github.com/v2ray/v2ray-core/proxy"
+	"github.com/v2ray/v2ray-core/transport"
 )
 
 const (
@@ -49,7 +50,7 @@ func ReadAuthentication(reader io.Reader) (auth Socks5AuthenticationRequest, aut
 	}
 	if nBytes < 2 {
 		log.Info("Socks expected 2 bytes read, but only %d bytes read", nBytes)
-		err = errors.NewCorruptedPacketError()
+		err = transport.CorruptedPacket
 		return
 	}
 
@@ -58,26 +59,27 @@ func ReadAuthentication(reader io.Reader) (auth Socks5AuthenticationRequest, aut
 		auth4.Command = buffer.Value[1]
 		auth4.Port = binary.BigEndian.Uint16(buffer.Value[2:4])
 		copy(auth4.IP[:], buffer.Value[4:8])
-		err = NewSocksVersion4Error()
+		err = Socks4Downgrade
 		return
 	}
 
 	auth.version = buffer.Value[0]
 	if auth.version != socksVersion {
-		err = errors.NewProtocolVersionError(int(auth.version))
+		log.Warning("Unknown protocol version %d", auth.version)
+		err = proxy.InvalidProtocolVersion
 		return
 	}
 
 	auth.nMethods = buffer.Value[1]
 	if auth.nMethods <= 0 {
 		log.Info("Zero length of authentication methods")
-		err = errors.NewCorruptedPacketError()
+		err = transport.CorruptedPacket
 		return
 	}
 
 	if nBytes-2 != int(auth.nMethods) {
 		log.Info("Unmatching number of auth methods, expecting %d, but got %d", auth.nMethods, nBytes)
-		err = errors.NewCorruptedPacketError()
+		err = transport.CorruptedPacket
 		return
 	}
 	copy(auth.authMethods[:], buffer.Value[2:nBytes])
@@ -194,7 +196,7 @@ func ReadRequest(reader io.Reader) (request *Socks5Request, err error) {
 		return
 	}
 	if nBytes < 4 {
-		err = errors.NewCorruptedPacketError()
+		err = transport.CorruptedPacket
 		return
 	}
 	request = &Socks5Request{
@@ -210,7 +212,7 @@ func ReadRequest(reader io.Reader) (request *Socks5Request, err error) {
 			return
 		}
 		if nBytes != 4 {
-			err = errors.NewCorruptedPacketError()
+			err = transport.CorruptedPacket
 			return
 		}
 	case AddrTypeDomain:
@@ -226,7 +228,7 @@ func ReadRequest(reader io.Reader) (request *Socks5Request, err error) {
 
 		if nBytes != int(domainLength) {
 			log.Info("Unable to read domain with %d bytes, expecting %d bytes", nBytes, domainLength)
-			err = errors.NewCorruptedPacketError()
+			err = transport.CorruptedPacket
 			return
 		}
 		request.Domain = string(buffer.Value[:domainLength])
@@ -236,12 +238,12 @@ func ReadRequest(reader io.Reader) (request *Socks5Request, err error) {
 			return
 		}
 		if nBytes != 16 {
-			err = errors.NewCorruptedPacketError()
+			err = transport.CorruptedPacket
 			return
 		}
 	default:
 		log.Info("Unexpected address type %d", request.AddrType)
-		err = errors.NewCorruptedPacketError()
+		err = transport.CorruptedPacket
 		return
 	}
 
@@ -250,7 +252,7 @@ func ReadRequest(reader io.Reader) (request *Socks5Request, err error) {
 		return
 	}
 	if nBytes != 2 {
-		err = errors.NewCorruptedPacketError()
+		err = transport.CorruptedPacket
 		return
 	}
 
diff --git a/proxy/socks/protocol/socks4.go b/proxy/socks/protocol/socks4.go
index 0b5fa841d..a1c42217b 100644
--- a/proxy/socks/protocol/socks4.go
+++ b/proxy/socks/protocol/socks4.go
@@ -1,23 +1,14 @@
 package protocol
 
 import (
+	"errors"
+
 	"github.com/v2ray/v2ray-core/common/alloc"
-	"github.com/v2ray/v2ray-core/common/errors"
 )
 
-type SocksVersion4Error struct {
-	errors.ErrorCode
-}
-
-var socksVersion4ErrorInstance = SocksVersion4Error{ErrorCode: 1000}
-
-func NewSocksVersion4Error() SocksVersion4Error {
-	return socksVersion4ErrorInstance
-}
-
-func (err SocksVersion4Error) Error() string {
-	return err.Prefix() + "Request is socks version 4."
-}
+var (
+	Socks4Downgrade = errors.New("Downgraded to Socks 4.")
+)
 
 type Socks4AuthenticationRequest struct {
 	Version byte
diff --git a/proxy/socks/protocol/socks4_test.go b/proxy/socks/protocol/socks4_test.go
index eb45aecba..41119d556 100644
--- a/proxy/socks/protocol/socks4_test.go
+++ b/proxy/socks/protocol/socks4_test.go
@@ -18,7 +18,7 @@ func TestSocks4AuthenticationRequestRead(t *testing.T) {
 		0x72, 0x72, 0x72, 0x72,
 	}
 	_, request4, err := ReadAuthentication(bytes.NewReader(rawRequest))
-	assert.Error(err).HasCode(1000)
+	assert.Error(err).Equals(Socks4Downgrade)
 	assert.Byte(request4.Version).Named("Version").Equals(0x04)
 	assert.Byte(request4.Command).Named("Command").Equals(0x01)
 	assert.Uint16(request4.Port).Named("Port").Equals(53)
diff --git a/proxy/socks/socks.go b/proxy/socks/socks.go
index 29bd0a28c..35d8001e7 100644
--- a/proxy/socks/socks.go
+++ b/proxy/socks/socks.go
@@ -1,6 +1,7 @@
 package socks
 
 import (
+	"errors"
 	"io"
 	"net"
 	"strconv"
@@ -9,13 +10,18 @@ import (
 
 	"github.com/v2ray/v2ray-core"
 	"github.com/v2ray/v2ray-core/common/alloc"
-	"github.com/v2ray/v2ray-core/common/errors"
 	"github.com/v2ray/v2ray-core/common/log"
 	v2net "github.com/v2ray/v2ray-core/common/net"
+	"github.com/v2ray/v2ray-core/proxy"
 	jsonconfig "github.com/v2ray/v2ray-core/proxy/socks/config/json"
 	"github.com/v2ray/v2ray-core/proxy/socks/protocol"
 )
 
+var (
+	UnsupportedSocksCommand = errors.New("Unsupported socks command.")
+	UnsupportedAuthMethod   = errors.New("Unsupported auth method.")
+)
+
 // SocksServer is a SOCKS 5 proxy server
 type SocksServer struct {
 	accepting bool
@@ -61,12 +67,12 @@ func (server *SocksServer) HandleConnection(connection net.Conn) error {
 	reader := v2net.NewTimeOutReader(120, connection)
 
 	auth, auth4, err := protocol.ReadAuthentication(reader)
-	if err != nil && !errors.HasCode(err, 1000) {
+	if err != nil && err != protocol.Socks4Downgrade {
 		log.Error("Socks failed to read authentication: %v", err)
 		return err
 	}
 
-	if err != nil && errors.HasCode(err, 1000) {
+	if err != nil && err == protocol.Socks4Downgrade {
 		return server.handleSocks4(reader, connection, auth4)
 	} else {
 		return server.handleSocks5(reader, connection, auth)
@@ -87,7 +93,7 @@ func (server *SocksServer) handleSocks5(reader *v2net.TimeOutReader, writer io.W
 			return err
 		}
 		log.Warning("Socks client doesn't support allowed any auth methods.")
-		return errors.NewInvalidOperationError("Unsupported auth methods.")
+		return UnsupportedAuthMethod
 	}
 
 	authResponse := protocol.NewAuthenticationResponse(expectedAuthMethod)
@@ -113,9 +119,8 @@ func (server *SocksServer) handleSocks5(reader *v2net.TimeOutReader, writer io.W
 			return err
 		}
 		if status != byte(0) {
-			err = errors.NewAuthenticationError(upRequest.AuthDetail())
-			log.Warning(err.Error())
-			return err
+			log.Warning("Invalid user account: %s", upRequest.AuthDetail())
+			return proxy.InvalidAuthentication
 		}
 	}
 
@@ -143,7 +148,7 @@ func (server *SocksServer) handleSocks5(reader *v2net.TimeOutReader, writer io.W
 			return err
 		}
 		log.Warning("Unsupported socks command %d", request.Command)
-		return errors.NewInvalidOperationError("Socks command " + strconv.Itoa(int(request.Command)))
+		return UnsupportedSocksCommand
 	}
 
 	response.Error = protocol.ErrorSuccess
@@ -228,7 +233,8 @@ func (server *SocksServer) handleSocks4(reader io.Reader, writer io.Writer, auth
 	responseBuffer.Release()
 
 	if result == protocol.Socks4RequestRejected {
-		return errors.NewInvalidOperationError("Socks4 command " + strconv.Itoa(int(auth.Command)))
+		log.Warning("Unsupported socks 4 command %d", auth.Command)
+		return UnsupportedSocksCommand
 	}
 
 	dest := v2net.NewTCPDestination(v2net.IPAddress(auth.IP[:], auth.Port))
diff --git a/proxy/vmess/protocol/vmess.go b/proxy/vmess/protocol/vmess.go
index cb8545295..4d2d11e76 100644
--- a/proxy/vmess/protocol/vmess.go
+++ b/proxy/vmess/protocol/vmess.go
@@ -9,10 +9,12 @@ import (
 	"io"
 	"time"
 
-	"github.com/v2ray/v2ray-core/common/errors"
 	v2io "github.com/v2ray/v2ray-core/common/io"
+	"github.com/v2ray/v2ray-core/common/log"
 	v2net "github.com/v2ray/v2ray-core/common/net"
+	"github.com/v2ray/v2ray-core/proxy"
 	"github.com/v2ray/v2ray-core/proxy/vmess/protocol/user"
+	"github.com/v2ray/v2ray-core/transport"
 )
 
 const (
@@ -73,7 +75,7 @@ func (r *VMessRequestReader) Read(reader io.Reader) (*VMessRequest, error) {
 
 	userId, timeSec, valid := r.vUserSet.GetUser(buffer[:nBytes])
 	if !valid {
-		return nil, errors.NewAuthenticationError(buffer[:nBytes])
+		return nil, proxy.InvalidAuthentication
 	}
 
 	aesCipher, err := aes.NewCipher(userId.CmdKey())
@@ -99,7 +101,8 @@ func (r *VMessRequestReader) Read(reader io.Reader) (*VMessRequest, error) {
 	}
 
 	if request.Version != Version {
-		return nil, errors.NewProtocolVersionError(int(request.Version))
+		log.Warning("Invalid protocol version %d", request.Version)
+		return nil, proxy.InvalidProtocolVersion
 	}
 
 	request.RequestIV = buffer[1:17]       // 16 bytes
@@ -149,7 +152,7 @@ func (r *VMessRequestReader) Read(reader io.Reader) (*VMessRequest, error) {
 	expectedHash := binary.BigEndian.Uint32(buffer[bufferLen : bufferLen+4])
 
 	if actualHash != expectedHash {
-		return nil, errors.NewCorruptedPacketError()
+		return nil, transport.CorruptedPacket
 	}
 
 	return request, nil
diff --git a/testing/unit/errorsubject.go b/testing/unit/errorsubject.go
index 7c6dae549..e4612a225 100644
--- a/testing/unit/errorsubject.go
+++ b/testing/unit/errorsubject.go
@@ -1,11 +1,5 @@
 package unit
 
-import (
-	"fmt"
-
-	"github.com/v2ray/v2ray-core/common/errors"
-)
-
 type ErrorSubject struct {
 	*Subject
 	value error
@@ -42,9 +36,3 @@ func (subject *ErrorSubject) IsNil() {
 		subject.FailWithMessage("Not true that " + subject.DisplayString() + " is nil.")
 	}
 }
-
-func (subject *ErrorSubject) HasCode(code int) {
-	if !errors.HasCode(subject.value, code) {
-		subject.FailWithMessage(fmt.Sprintf("Not ture that %s has error code 0x%04X.", subject.DisplayString(), code))
-	}
-}
diff --git a/transport/errors.go b/transport/errors.go
new file mode 100644
index 000000000..557d730c0
--- /dev/null
+++ b/transport/errors.go
@@ -0,0 +1,9 @@
+package transport
+
+import (
+	"errors"
+)
+
+var (
+	CorruptedPacket = errors.New("Packet is corrupted.")
+)