TELNET

A protocol used to establish a dumb terminal（哑终端，只输入输出） session to another computer on the Internet

所有指令不在本机执行

An important Internet application for remote access

The purpose of the TELNET Protocol is to provide a general, bi-directional, byte oriented communications facility.

允许使用一种标准方法将终端设备和面向终端的进程相互连接起来。

预计该协议还可以用于终端与终端之间的通信(“链接”)和进程与进程之间的通信(分布式计算)。

TELNET vs. telnet

大写的TELNET表示的是协议；小写的telnet表示的是程序/应用

• TELNET is a protocol that provides “a general, bi-directional, eight-bit byte oriented communications facility” TELNET是一种提供“通用的、双向的、面向8位字节的通信工具”的协议。

• 建立在TCP协议之上

• Many application protocols are built upon the TELNET protocol. 许多应用协议建立在TELNET协议之上（映证TELNET提供general通信能力）

The idea of option negotiation was a very good design feature

采用选项协商，在建立连接前确定可选项。不改变最初的定义但是增加了功能

Enables telnet to evolve to meet new demands without endless new versions of basic protocol

Concept Of Remote / Virtual Terminal

1）远程终端访问

• Early motivation for networks was remote access to interactive systems 网络的早期激励是互动系统的远程访问
• Dumb terminals 哑终端
  • Keyboard and screen with primitive communication hardware 键盘和屏幕与原始通信硬件
  • Local host computer establish connection to remote host 本地主机建立连接到远程主机
• The challenge is that terminals and host systems were not standardized 终端和主机系统的挑战是没有标准化
  • local terminal was not speaking the same language as the remote host 本地终端并不是说同一种语言作为远程主机

产生一些问题，不同os间的字符编码方式可能不同，字符集无法公用

Network Virtual Terminal

所以使用NVT转换为同一字符集

Transform local characteristics into standardized form

◼ Network virtual terminal (NVT)

TELNET客户端和服务器在native format和NVT format之间做转换

TELNET operations 操作

Connection management

◼ Connection request, establish and terminate

◼ Telnet uses TCP (port 23) by default　23端口

Option Negotiation 选项协商（在连接之后，交换数据之前）

To determine mutually agreeable set of characteristics and options 确定双方同意的一组特征和选项（找到双方共同支持的能力和特性，在此基础上才能进行交互）

TELNET protocol 协议

特征

• TCP connection: directed toward port 23 of the server being asked to perform a service TCP连接:针对端口23的服务器被要求执行一个服务
• Data and control multiplexed over the same connection 数据和控制多路复用在同一连接上（在TELNET的client和server之间只会建立一个connection，双方之间交互的data和control都是混合在一个连接上的）
• NVT-representation of a generic terminal 通用终端的网络虚拟终端表示
• Negotiated options-Enabling Telnet to evolve to meet new demands without endless new versions of basic protocol 协商选项Telnet进行改进,以满足新的要求,没有没完没了的新版本的基本协议
• A symmetric view of terminals and processes 对称视图和过程

Transmission Of Data

• Data path from the user’s keyboard to the remote system 从用户键盘到远程系统的数据路径

• Underlying TCP full duplex 底层TCP全双工
  • The underlying network is intrinsically full duplex 底层网络本质上是全双工的
• Data sent half duplex 数据发送半双工
  • The communication between terminal and process is one direction at a time. 终端和进程之间的通信每次都是一个方向的。
  • 这两句话的意思就是说，TCP是全双工的，但是基于TCP的TELNET在应用上数据的传输是半双工的，因为控制权交接的问题，在交接的时候client和server只有一方能发数据
• Data sent as stream of 8-bit bytes 数据都是以8位字节流的形式进行发送的（实际上发的都是字节流）
  • No other formatting 没有其他格式化
• Control signals and other non-data information sent as Telnet commands 控制信息和其他非数据信息以Telnet command 的形式发送
  • Byte strings embedded in data stream 每一个字符串都是由若干个字节来构成的
  • User control signals, commands between Telnet processes as part of protocol and option negotiation and subnegotiation 用户控制信号，作为协议和选项协商和子协商的一部分的Telnet进程之间的命令

Control Functions

• TELNET includes support for a series of control functions commonly supported by servers TELNET包括对一系列通常由服务器支持的控制功能的支持

• This provides a uniform mechanism for communication of (the supported) control functions 这为(受支持的)控制功能的通信提供了统一的机制

• You can imagine them as some extra virtual keys in the NVT keyboard 您可以将它们想象成NVT键盘中的一些额外的虚拟键

• **Interrupt Process(IP)**：suspend/interrupt/abort/terminate process 终止一个进程

• Abort Output (AO)：allow a process, which is generating output, to run to completion but without sending the output to the user’s terminal 允许一个进程产生输出，并且允许它运行直到结束，但是并不将输出的内容发回给用户的命令行

• **Are You There (AYT)**：check to see if system is still running 检查系统是否正在运行

• **Erase Character (EC)**：delete last character sent；typically used to edit keyboard input 删除一个字符

• **Erase Line (EL)**：delete all input in current line；typically used to edit keyboard input 删除一行

IAC：把跟在后面的下一个字节解释为命令

蓝色字四个：选项协商（Control Functions），表达支持意愿

IAC

TELNET command structure

◼ at least a two byte sequence: the IAC (Interpret as Command) escape character followed by the code for the command至少要有两个字节作为命令，前一个字节是IAC标识后面那个字节是命令不是数据

如 255 254：DONT

​ 254：254

• The IAC code is 255

标识数据255，则要写两次ICA

​ 255 255：255

Control Functions

will是 sender问receiver：自己是否可以干XXX

DO是 sender问receiver：你是否可以干ＸＸＸ

TELNET Options Negotiation

为什么需要：

All NVTs support a minimal set of capabilities（只支持最小功能集）

想添加一些其他功能选项： The two endpoints negotiate a set of mutually acceptable options

The set of options is not part of the TELNET protocol, so that new terminal features can be incorporated without changing the TELNET protocol 组选项不是TELNET协议的一部分,所以新的终端功能可以合并不改变TELNET协议

• Each option is assigned a byte value 每个选项都分配了byte value

• The DO, DONT, WILL, and WONT commands are used to negotiate options

• Options negotiation is symmetric 都可以发起

• Steps must be taken to avoid option processing loops防止反复协商

• Subnegotiations（多次协商时候使用） are used when more information is needed, such as when negotiating terminal type, window size, etc 除了支不支持之外，还有些需要更详细的信息来完成选项协商

• 举例;

  IAC DO ECHO

  IAC WILL ECHO

使用wireshark 来了解发送和接收的情况

wireshark（xming启动）

可以在指定的网络接口上抓取接收到的或者发送出去的数据，然后按照各种协议规范解析并进行图形化的展示，可用于故障分析，协议开发等等

抓到的包的列表（各自的信息，可以了解到通信流程）；按照协议规范解析信息；抓到的原始二进制数据

SB SE代表子协商的开始和结束

设为character mode时候，一行发送一个字符

回车也分为两个字符

5）总结

① 优点

• Use Internet accounts you may have on remote computers
  • you need an account (login ID) and password on the remote computer to permit access
• Use free services accessible with telnet, e.g.
  • library catalogues
  • databases
  • BBS (Bulletin Board System)
  • Router/switch configuration

② 缺点 局限性

• Poor user interface 界面低级
  • Based on dumb terminal
  • Text-only display 只有字符
  • Monochrome
    • One color for text, one for background
  • Have to type command-line commands
    • Often have complex syntax
  • Not very secure, SSH made enhancement 不安全！！
    • TELNET does not encrypt any data sent over the connection (including passwords)

6. Other remote access technologies 其他远程访问技术

1）其他远程访问技术

• Remote loginin text-based system
  • telnet SSH Rlogin
• Remote desktopin windowing system 远程桌面
  • VNC (Virtual Network Computing)
  • RDP (Remote Desktop Protocol)

2）ssh

① 简介

• Secure Shell
• Command line terminal connection tool
• All traffic encrypted 加密传输
• Both ends authenticate themselves to the other end 双方验证
• Ability to carry and encrypt non-terminal traffic
• Private key kept on client, public key stored on server
• Now, it is an IETF standard
  • RFC4251, The Secure Shell (SSH) Protocol Architecture

② 相较于telnet的两点提升

• Providing secure communications 提供安全通信
• Providing users with the ability to perform additional, independent data transfer over the same connection that is used for remote login 为用户提供在用于远程登录的相同连接上执行额外的、独立的数据传输的能力

③ 三个主要机制

• A transport layer protocolthat provides sever authentication, data confidentiality, and data integrity with perfect forward secrecy 一种传输层协议，提供服务器认证、数据机密性和数据完整性，并提供完全的前向保密性
• A user authentication protocolthat authenticates the user to the server 用户认证协议，用于认证用户到服务器
• A connection protocolthat multiplexes multiple logical communications channels over a single underlying SSH connection 一种连接协议，在一个底层SSH连接上复用多个逻辑通信通道

④ 使用举例