/src/PcapPlusPlus/Packet++/src/TelnetLayer.cpp

Source
#define LOG_MODULE PacketLogModuleTelnetLayer

#include "TelnetLayer.h"
#include "Logger.h"
#include "GeneralUtils.h"
#include "AssertionUtils.h"
#include <cstring>
#include <iterator>
#include <algorithm>

namespace pcpp
{
  namespace
  {
    /// @brief An enum representing the type of a Telnet sequence
    enum class TelnetSequenceType
    {
      /// @brief An unknown sequence type. Usually means parsing error.
      /// Commonly happens when an IAC symbol is found at the end of the buffer without a following byte.
      Unknown,
      /// @brief A telnet command sequence. Starts with IAC followed by a command code.
      Command,
      /// @brief A telnet data sequence. Either does not start with IAC or starts with IAC IAC.
      UserData,
    };

    /// @brief Checks if a given sequence matches Telnet command or data pattern.
    /// @param first Start of the sequence to check
    /// @param maxCount Maximum number of bytes to check
    /// @return The type of the Telnet sequence. Unknown if the sequence does not match either command or data
    /// pattern or if parsing error occurs.
    TelnetSequenceType getTelnetSequenceType(uint8_t const* first, size_t maxCount)
    {
      if (first == nullptr || maxCount == 0)
      {
        PCPP_LOG_DEBUG("Checking empty or null buffer for telnet sequence type");
        return TelnetSequenceType::Unknown;
      }

      // If first byte is not "FF" it's data
      if (*first != static_cast<int>(TelnetLayer::TelnetCommand::InterpretAsCommand))
      {
        return TelnetSequenceType::UserData;
      }

      // IAC must be followed by another octet
      if (maxCount <= 1)
      {
        PCPP_LOG_DEBUG("Telnet Parse Error: IAC (FF) must always be followed by another octet");
        return TelnetSequenceType::Unknown;
      }

      if (first[1] == static_cast<int>(TelnetLayer::TelnetCommand::InterpretAsCommand))
      {
        // "FF FF" means data continue
        return TelnetSequenceType::UserData;
      }

      // "FF X" where X != "FF" means command
      return TelnetSequenceType::Command;
    }

    /// @brief Checks if a given sequence matches Telnet command pattern.
    /// @param first Start of the sequence to check
    /// @param maxCount Maximum number of bytes to check
    /// @return True if the buffer matches Telnet command pattern, false otherwise
    bool isTelnetCommand(uint8_t const* first, size_t maxCount)
    {
      return getTelnetSequenceType(first, maxCount) == TelnetSequenceType::Command;
    }

    /// @brief Finds the next IAC symbol in a data stream ignoring the first byte.
    /// @param[in] first Iterator to the start of the data stream to check.
    /// @param[in] last Iterator one past the end of the data stream to check.
    /// @return The next encountered IAC symbol after the first.
    uint8_t* findNextIAC(uint8_t* first, uint8_t* last)
    {
      // FF FF pattern is FF literal
      // FF non-FF pattern is IAC op code
      // Sample seq: FF AB CD 0F FF FF A4 B5 [FF] 2D DA

      // Requires at least 2 elements.
      if (first + 1 >= last)
        return last;

      constexpr int IAC = static_cast<int>(TelnetLayer::TelnetCommand::InterpretAsCommand);

      // Start the search from the second byte.
      auto it = first + 1;
      while (it != last)
      {
        // Find the next IAC symbol.
        it = std::find(it, last, IAC);

        // Reached the end of the sequence.
        if (it == last)
          return last;

        auto itNext = std::next(it);
        // Reached the end of the sequence.
        // IAC at the end of the sequence is invalid.
        if (itNext == last)
          return last;

        // If the next symbol is not IAC, this isn't an escaped sequence.
        if (*itNext != IAC)
          return it;

        // Escaped sequence "FF FF", move it to 1 past the second FF, to skip the view "FF [[FF XX]]".
        it = std::next(itNext);
      }

      return last;
    }
  }  // namespace

  size_t TelnetLayer::distanceToNextIAC(uint8_t* startPos, size_t maxLength)
  {
    auto beginIt = startPos;
    auto endIt = startPos + maxLength;
    auto nextIacIt = findNextIAC(beginIt, endIt);
    return std::distance(beginIt, nextIacIt);
  }

  size_t TelnetLayer::getFieldLen(uint8_t* startPos, size_t maxLength)
  {
    // Check first byte is IAC
    if (startPos && (startPos[0] == static_cast<int>(TelnetCommand::InterpretAsCommand)) && (maxLength >= 2))
    {
      // If subnegotiation parse until next IAC
      if (startPos[1] == static_cast<int>(TelnetCommand::Subnegotiation))
        return distanceToNextIAC(startPos, maxLength);
      // Only WILL, WONT, DO, DONT have option. Ref http://pcmicro.com/netfoss/telnet.html
      else if (startPos[1] >= static_cast<int>(TelnetCommand::WillPerform) &&
               startPos[1] <= static_cast<int>(TelnetCommand::DontPerform))
        return std::min<size_t>(3, maxLength);
      return 2;
    }
    return distanceToNextIAC(startPos, maxLength);
  }

  uint8_t* TelnetLayer::getNextDataField(uint8_t* pos, size_t len)
  {
    // This assumes `pos` points to the start of a valid field.
    auto const endIt = pos + len;

    // Advance to the next field, as we are skipping the current one from the search.
    pos += getFieldLen(pos, len);

    while (pos < endIt)
    {
      // Check if the current field is data
      switch (getTelnetSequenceType(pos, std::distance(pos, endIt)))
      {
      case TelnetSequenceType::Unknown:
      {
        PCPP_LOG_DEBUG("Telnet Parse Error: Unknown sequence found during data field search.");
        return nullptr;
      }
      case TelnetSequenceType::UserData:
        return pos;
      default:
        break;  // continue searching
      }

      // If not data, move to next field
      pos += getFieldLen(pos, std::distance(pos, endIt));
    }

    // If we got here, no data field has been found before the end of the buffer
    return nullptr;
  }

  uint8_t* TelnetLayer::getNextCommandField(uint8_t* pos, size_t len)
  {
    // This assumes `pos` points to the start of a valid field.
    auto const endIt = pos + len;

    // Advance to the next field, as we are skipping the current one from the search.
    pos += getFieldLen(pos, len);

    while (pos < endIt)
    {
      // Check if the current field is command
      switch (getTelnetSequenceType(pos, std::distance(pos, endIt)))
      {
      case TelnetSequenceType::Unknown:
      {
        PCPP_LOG_DEBUG("Telnet Parse Error: Unknown sequence found during command field search.");
        return nullptr;
      }
      case TelnetSequenceType::Command:
        return pos;
      default:
        break;  // continue searching
      }

      // If not command, move to next field
      pos += getFieldLen(pos, std::distance(pos, endIt));
    }

    // If we got here, no command field has been found before the end of the buffer
    return nullptr;
  }

  int16_t TelnetLayer::getSubCommand(uint8_t* pos, size_t len)
  {
    if (len < 3 || pos[1] < static_cast<int>(TelnetCommand::Subnegotiation))
      return static_cast<int>(TelnetOption::TelnetOptionNoOption);
    return pos[2];
  }

  uint8_t* TelnetLayer::getCommandData(uint8_t* pos, size_t& len)
  {
    if (pos[1] == static_cast<int>(TelnetCommand::Subnegotiation) && len > 3)
    {
      len -= 3;
      return &pos[3];
    }
    len = 0;
    return nullptr;
  }

  std::string TelnetLayer::getDataAsString(bool removeEscapeCharacters)
  {
    if (m_Data == nullptr)
    {
      PCPP_LOG_DEBUG("Layer does not have data");
      return {};
    }

    // Convert to string
    if (removeEscapeCharacters)
    {
      uint8_t* dataPos = nullptr;
      switch (getTelnetSequenceType(m_Data, m_DataLen))
      {
      case TelnetSequenceType::Unknown:
      {
        PCPP_LOG_DEBUG("Telnet Parse Error: Unknown sequence found during data string extraction.");
        return {};
      }
      case TelnetSequenceType::UserData:
        dataPos = m_Data;
        break;
      case TelnetSequenceType::Command:
        dataPos = getNextDataField(m_Data, m_DataLen);
        break;
      default:
        throw std::logic_error("Unsupported sequence type");
      }

      if (!dataPos)
      {
        PCPP_LOG_DEBUG("Packet does not have a data field");
        return std::string();
      }

      PCPP_ASSERT(dataPos >= m_Data && dataPos < (m_Data + m_DataLen),
                  "Data position is out of bounds, this should never happen!");

      // End of range is corrected by the advance offset.
      auto const* beginIt = dataPos;
      auto const* endIt = dataPos + m_DataLen - std::distance(m_Data, dataPos);

      std::string result;
      std::copy_if(beginIt, endIt, std::back_inserter(result), [](char ch) -> bool {
        return ch > 31 && ch < 127;  // From SPACE to ~
      });
      return result;
    }
    return std::string(reinterpret_cast<char*>(m_Data), m_DataLen);
  }

  size_t TelnetLayer::getTotalNumberOfCommands()
  {
    size_t ctr = 0;
    if (isTelnetCommand(m_Data, m_DataLen))
      ++ctr;

    uint8_t* pos = m_Data;
    while (pos != nullptr)
    {
      size_t offset = pos - m_Data;
      pos = getNextCommandField(pos, m_DataLen - offset);
      if (pos)
        ++ctr;
    }

    return ctr;
  }

  size_t TelnetLayer::getNumberOfCommands(TelnetCommand command)
  {
    if (static_cast<int>(command) < 0)
      return 0;

    size_t ctr = 0;
    if (isTelnetCommand(m_Data, m_DataLen) && m_Data[1] == static_cast<int>(command))
      ++ctr;

    uint8_t* pos = m_Data;
    while (pos != nullptr)
    {
      size_t offset = pos - m_Data;
      pos = getNextCommandField(pos, m_DataLen - offset);
      if (pos && pos[1] == static_cast<int>(command))
        ++ctr;
    }

    return ctr;
  }

  TelnetLayer::TelnetCommand TelnetLayer::getFirstCommand()
  {
    // If starts with command
    if (isTelnetCommand(m_Data, m_DataLen))
      return static_cast<TelnetCommand>(m_Data[1]);

    // Check is there any command
    uint8_t* pos = getNextCommandField(m_Data, m_DataLen);
    if (pos)
      return static_cast<TelnetCommand>(pos[1]);
    return TelnetCommand::TelnetCommandEndOfPacket;
  }

  TelnetLayer::TelnetCommand TelnetLayer::getNextCommand()
  {
    if (lastPositionOffset == SIZE_MAX)
    {
      lastPositionOffset = 0;
      if (isTelnetCommand(m_Data, m_DataLen))
        return static_cast<TelnetLayer::TelnetCommand>(m_Data[1]);
    }

    uint8_t* pos = getNextCommandField(&m_Data[lastPositionOffset], m_DataLen - lastPositionOffset);
    if (pos)
    {
      lastPositionOffset = pos - m_Data;
      return static_cast<TelnetLayer::TelnetCommand>(pos[1]);
    }
    lastPositionOffset = SIZE_MAX;
    return TelnetCommand::TelnetCommandEndOfPacket;
  }

  TelnetLayer::TelnetOption TelnetLayer::getOption()
  {
    if (lastPositionOffset < m_DataLen)
      return static_cast<TelnetOption>(getSubCommand(
          &m_Data[lastPositionOffset], getFieldLen(&m_Data[lastPositionOffset], m_DataLen - lastPositionOffset)));
    return TelnetOption::TelnetOptionNoOption;
  }

  TelnetLayer::TelnetOption TelnetLayer::getOption(TelnetCommand command)
  {
    // Check input
    if (static_cast<int>(command) < 0)
    {
      PCPP_LOG_ERROR("Command type can't be negative");
      return TelnetOption::TelnetOptionNoOption;
    }

    if (isTelnetCommand(m_Data, m_DataLen) && m_Data[1] == static_cast<int>(command))
      return static_cast<TelnetOption>(getSubCommand(m_Data, getFieldLen(m_Data, m_DataLen)));

    uint8_t* pos = m_Data;
    while (pos != nullptr)
    {
      size_t offset = pos - m_Data;
      pos = getNextCommandField(pos, m_DataLen - offset);

      if (pos && pos[1] == static_cast<int>(command))
      {
        offset = pos - m_Data;
        return static_cast<TelnetOption>(getSubCommand(pos, getFieldLen(pos, m_DataLen - offset)));
      }
    }

    PCPP_LOG_DEBUG("Can't find requested command");
    return TelnetOption::TelnetOptionNoOption;
  }

  uint8_t* TelnetLayer::getOptionData(size_t& length)
  {
    if (lastPositionOffset < m_DataLen)
    {
      size_t lenBuffer = getFieldLen(&m_Data[lastPositionOffset], m_DataLen - lastPositionOffset);
      uint8_t* posBuffer = getCommandData(&m_Data[lastPositionOffset], lenBuffer);

      length = lenBuffer;
      return posBuffer;
    }
    return nullptr;
  }

  uint8_t* TelnetLayer::getOptionData(TelnetCommand command, size_t& length)
  {
    // Check input
    if (static_cast<int>(command) < 0)
    {
      PCPP_LOG_ERROR("Command type can't be negative");
      length = 0;
      return nullptr;
    }

    if (isTelnetCommand(m_Data, m_DataLen) && m_Data[1] == static_cast<int>(command))
    {
      size_t lenBuffer = getFieldLen(m_Data, m_DataLen);
      uint8_t* posBuffer = getCommandData(m_Data, lenBuffer);

      length = lenBuffer;
      return posBuffer;
    }

    uint8_t* pos = m_Data;
    while (pos != nullptr)
    {
      size_t offset = pos - m_Data;
      pos = getNextCommandField(pos, m_DataLen - offset);

      if (pos && pos[1] == static_cast<int>(command))
      {
        offset = pos - m_Data;
        size_t lenBuffer = getFieldLen(pos, m_DataLen - offset);
        uint8_t* posBuffer = getCommandData(pos, lenBuffer);

        length = lenBuffer;
        return posBuffer;
      }
    }

    PCPP_LOG_DEBUG("Can't find requested command");
    length = 0;
    return nullptr;
  }

  std::string TelnetLayer::getTelnetCommandAsString(TelnetCommand val)
  {
    switch (val)
    {
    case TelnetCommand::TelnetCommandEndOfPacket:
      return "Reached end of packet while parsing";
    case TelnetCommand::EndOfFile:
      return "End of File";
    case TelnetCommand::Suspend:
      return "Suspend current process";
    case TelnetCommand::Abort:
      return "Abort Process";
    case TelnetCommand::EndOfRecordCommand:
      return "End of Record";
    case TelnetCommand::SubnegotiationEnd:
      return "Subnegotiation End";
    case TelnetCommand::NoOperation:
      return "No Operation";
    case TelnetCommand::DataMark:
      return "Data Mark";
    case TelnetCommand::Break:
      return "Break";
    case TelnetCommand::InterruptProcess:
      return "Interrupt Process";
    case TelnetCommand::AbortOutput:
      return "Abort Output";
    case TelnetCommand::AreYouThere:
      return "Are You There";
    case TelnetCommand::EraseCharacter:
      return "Erase Character";
    case TelnetCommand::EraseLine:
      return "Erase Line";
    case TelnetCommand::GoAhead:
      return "Go Ahead";
    case TelnetCommand::Subnegotiation:
      return "Subnegotiation";
    case TelnetCommand::WillPerform:
      return "Will Perform";
    case TelnetCommand::WontPerform:
      return "Wont Perform";
    case TelnetCommand::DoPerform:
      return "Do Perform";
    case TelnetCommand::DontPerform:
      return "Dont Perform";
    case TelnetCommand::InterpretAsCommand:
      return "Interpret As Command";
    default:
      return "Unknown Command";
    }
  }

  std::string TelnetLayer::getTelnetOptionAsString(TelnetOption val)
  {
    switch (val)
    {
    case TelnetOption::TelnetOptionNoOption:
      return "No option for this command";
    case TelnetOption::TransmitBinary:
      return "Binary Transmission";
    case TelnetOption::Echo:
      return "Echo";
    case TelnetOption::Reconnection:
      return "Reconnection";
    case TelnetOption::SuppressGoAhead:
      return "Suppress Go Ahead";
    case TelnetOption::ApproxMsgSizeNegotiation:
      return "Negotiate approximate message size";
    case TelnetOption::Status:
      return "Status";
    case TelnetOption::TimingMark:
      return "Timing Mark";
    case TelnetOption::RemoteControlledTransAndEcho:
      return "Remote Controlled Transmission and Echo";
    case TelnetOption::OutputLineWidth:
      return "Output Line Width";
    case TelnetOption::OutputPageSize:
      return "Output Page Size";
    case TelnetOption::OutputCarriageReturnDisposition:
      return "Negotiate About Output Carriage-Return Disposition";
    case TelnetOption::OutputHorizontalTabStops:
      return "Negotiate About Output Horizontal Tabstops";
    case TelnetOption::OutputHorizontalTabDisposition:
      return "Negotiate About Output Horizontal Tab Disposition";
    case TelnetOption::OutputFormfeedDisposition:
      return "Negotiate About Output Formfeed Disposition";
    case TelnetOption::OutputVerticalTabStops:
      return "Negotiate About Vertical Tabstops";
    case TelnetOption::OutputVerticalTabDisposition:
      return "Negotiate About Output Vertcial Tab Disposition";
    case TelnetOption::OutputLinefeedDisposition:
      return "Negotiate About Output Linefeed Disposition";
    case TelnetOption::ExtendedASCII:
      return "Extended ASCII";
    case TelnetOption::Logout:
      return "Logout";
    case TelnetOption::ByteMacro:
      return "Byte Macro";
    case TelnetOption::DataEntryTerminal:
      return "Data Entry Terminal";
    case TelnetOption::SUPDUP:
      return "SUPDUP";
    case TelnetOption::SUPDUPOutput:
      return "SUPDUP Output";
    case TelnetOption::SendLocation:
      return "Send Location";
    case TelnetOption::TerminalType:
      return "Terminal Type";
    case TelnetOption::EndOfRecordOption:
      return "End Of Record";
    case TelnetOption::TACACSUserIdentification:
      return "TACACS User Identification";
    case TelnetOption::OutputMarking:
      return "Output Marking";
    case TelnetOption::TerminalLocationNumber:
      return "Terminal Location Number";
    case TelnetOption::Telnet3270Regime:
      return "Telnet 3270 Regime";
    case TelnetOption::X3Pad:
      return "X3 Pad";
    case TelnetOption::NegotiateAboutWindowSize:
      return "Negotiate About Window Size";
    case TelnetOption::TerminalSpeed:
      return "Terminal Speed";
    case TelnetOption::RemoteFlowControl:
      return "Remote Flow Control";
    case TelnetOption::Linemode:
      return "Line mode";
    case TelnetOption::XDisplayLocation:
      return "X Display Location";
    case TelnetOption::EnvironmentOption:
      return "Environment Option";
    case TelnetOption::AuthenticationOption:
      return "Authentication Option";
    case TelnetOption::EncryptionOption:
      return "Encryption Option";
    case TelnetOption::NewEnvironmentOption:
      return "New Environment Option";
    case TelnetOption::TN3270E:
      return "TN3270E";
    case TelnetOption::XAuth:
      return "X Server Authentication";
    case TelnetOption::Charset:
      return "Charset";
    case TelnetOption::TelnetRemoteSerialPort:
      return "Telnet Remote Serial Port";
    case TelnetOption::ComPortControlOption:
      return "Com Port Control Option";
    case TelnetOption::TelnetSuppressLocalEcho:
      return "Telnet Suppress Local Echo";
    case TelnetOption::TelnetStartTLS:
      return "Telnet Start TLS";
    case TelnetOption::Kermit:
      return "Kermit";
    case TelnetOption::SendURL:
      return "Send URL";
    case TelnetOption::ForwardX:
      return "Forward X Server";
    case TelnetOption::TelOptPragmaLogon:
      return "Telnet Option Pragma Logon";
    case TelnetOption::TelOptSSPILogon:
      return "Telnet Option SSPI Logon";
    case TelnetOption::TelOptPragmaHeartbeat:
      return "Telnet Option Pragma Heartbeat";
    case TelnetOption::ExtendedOptions:
      return "Extended option list";
    default:
      return "Unknown Option";
    }
  }

  std::string TelnetLayer::toString() const
  {
    // TODO: Perhaps print the entire sequence of commands and data?
    switch (getTelnetSequenceType(m_Data, m_DataLen))
    {
    case TelnetSequenceType::Unknown:
      return "Telnet Unknown";
    case TelnetSequenceType::Command:
      return "Telnet Control";
    case TelnetSequenceType::UserData:
      return "Telnet Data";
    default:
      throw std::logic_error("Unsupported sequence type");
    }
  }

}  // namespace pcpp

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Created: 2026-08-13 07:36