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// Copyright 2012 Google, Inc. All rights reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the LICENSE file in the root of the source
// tree.
package reassembly
import (
"encoding/binary"
"fmt"
"github.com/gopacket/gopacket"
"github.com/gopacket/gopacket/layers"
)
/*
* Check TCP packet against options (window, MSS)
*/
type tcpStreamOptions struct {
mss int
scale int
receiveWindow uint
}
// TCPOptionCheck contains options for the two directions
type TCPOptionCheck struct {
options [2]tcpStreamOptions
}
func (t *TCPOptionCheck) getOptions(dir TCPFlowDirection) *tcpStreamOptions {
if dir == TCPDirClientToServer {
return &t.options[0]
}
return &t.options[1]
}
// NewTCPOptionCheck creates default options
func NewTCPOptionCheck() TCPOptionCheck {
return TCPOptionCheck{
options: [2]tcpStreamOptions{
tcpStreamOptions{
mss: 0,
scale: -1,
receiveWindow: 0,
}, tcpStreamOptions{
mss: 0,
scale: -1,
receiveWindow: 0,
},
},
}
}
// Accept checks whether the packet should be accepted by checking TCP options
func (t *TCPOptionCheck) Accept(tcp *layers.TCP, ci gopacket.CaptureInfo, dir TCPFlowDirection, nextSeq Sequence, start *bool) error {
options := t.getOptions(dir)
if tcp.SYN {
mss := -1
scale := -1
for _, o := range tcp.Options {
// MSS
if o.OptionType == 2 {
if len(o.OptionData) != 2 {
return fmt.Errorf("MSS option data length expected 2, got %d", len(o.OptionData))
}
mss = int(binary.BigEndian.Uint16(o.OptionData[:2]))
}
// Window scaling
if o.OptionType == 3 {
if len(o.OptionData) != 1 {
return fmt.Errorf("Window scaling length expected: 1, got %d", len(o.OptionData))
}
scale = int(o.OptionData[0])
}
}
options.mss = mss
options.scale = scale
} else {
if nextSeq != invalidSequence {
revOptions := t.getOptions(dir.Reverse())
length := len(tcp.Payload)
// Check packet is in the correct window
diff := nextSeq.Difference(Sequence(tcp.Seq))
if diff == -1 && (length == 1 || length == 0) {
// This is probably a Keep-alive
// TODO: check byte is ok
} else if diff < 0 {
return fmt.Errorf("Re-emitted packet (diff:%d,seq:%d,rev-ack:%d)", diff,
tcp.Seq, nextSeq)
} else if revOptions.mss > 0 && length > revOptions.mss {
return fmt.Errorf("%d > mss (%d)", length, revOptions.mss)
} else if revOptions.receiveWindow != 0 && revOptions.scale < 0 && diff > int(revOptions.receiveWindow) {
return fmt.Errorf("%d > receiveWindow(%d)", diff, revOptions.receiveWindow)
}
}
}
// Compute receiveWindow
options.receiveWindow = uint(tcp.Window)
if options.scale > 0 {
options.receiveWindow = options.receiveWindow << (uint(options.scale))
}
return nil
}
// TCPSimpleFSM implements a very simple TCP state machine
//
// Usage:
// When implementing a Stream interface and to avoid to consider packets that
// would be rejected due to client/server's TCP stack, the Accept() can call
// TCPSimpleFSM.CheckState().
//
// Limitations:
// - packet should be received in-order.
// - no check on sequence number is performed
// - no RST
type TCPSimpleFSM struct {
dir TCPFlowDirection
state int
options TCPSimpleFSMOptions
}
// TCPSimpleFSMOptions holds options for TCPSimpleFSM
type TCPSimpleFSMOptions struct {
SupportMissingEstablishment bool // Allow missing SYN, SYN+ACK, ACK
}
// Internal values of state machine
const (
TCPStateClosed = 0
TCPStateSynSent = 1
TCPStateEstablished = 2
TCPStateCloseWait = 3
TCPStateLastAck = 4
TCPStateReset = 5
)
// NewTCPSimpleFSM creates a new TCPSimpleFSM
func NewTCPSimpleFSM(options TCPSimpleFSMOptions) *TCPSimpleFSM {
return &TCPSimpleFSM{
state: TCPStateClosed,
options: options,
}
}
func (t *TCPSimpleFSM) String() string {
switch t.state {
case TCPStateClosed:
return "Closed"
case TCPStateSynSent:
return "SynSent"
case TCPStateEstablished:
return "Established"
case TCPStateCloseWait:
return "CloseWait"
case TCPStateLastAck:
return "LastAck"
case TCPStateReset:
return "Reset"
}
return "?"
}
// CheckState returns false if tcp is invalid wrt current state or update the state machine's state
func (t *TCPSimpleFSM) CheckState(tcp *layers.TCP, dir TCPFlowDirection) bool {
if t.state == TCPStateClosed && t.options.SupportMissingEstablishment && !(tcp.SYN && !tcp.ACK) {
/* try to figure out state */
switch true {
case tcp.SYN && tcp.ACK:
t.state = TCPStateSynSent
t.dir = dir.Reverse()
case tcp.FIN && !tcp.ACK:
t.state = TCPStateEstablished
case tcp.FIN && tcp.ACK:
t.state = TCPStateCloseWait
t.dir = dir.Reverse()
default:
t.state = TCPStateEstablished
}
}
switch t.state {
/* openning connection */
case TCPStateClosed:
if tcp.SYN && !tcp.ACK {
t.dir = dir
t.state = TCPStateSynSent
return true
}
case TCPStateSynSent:
if tcp.RST {
t.state = TCPStateReset
return true
}
if tcp.SYN && tcp.ACK && dir == t.dir.Reverse() {
t.state = TCPStateEstablished
return true
}
if tcp.SYN && !tcp.ACK && dir == t.dir {
// re-transmission
return true
}
/* established */
case TCPStateEstablished:
if tcp.RST {
t.state = TCPStateReset
return true
}
if tcp.FIN {
t.state = TCPStateCloseWait
t.dir = dir
return true
}
// accept any packet
return true
/* closing connection */
case TCPStateCloseWait:
if tcp.RST {
t.state = TCPStateReset
return true
}
if tcp.FIN && tcp.ACK && dir == t.dir.Reverse() {
t.state = TCPStateLastAck
return true
}
if tcp.ACK {
return true
}
case TCPStateLastAck:
if tcp.RST {
t.state = TCPStateReset
return true
}
if tcp.ACK && t.dir == dir {
t.state = TCPStateClosed
return true
}
}
return false
}
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