What is telnet in linux

In Linux, telnet is the user interface of the TELNET protocol. It is a remote login program based on the Telnet protocol. It is used to log in to the remote host and determine whether a certain port of the remote server can be accessed. The syntax "telnet [ -8acdEfFKLrx][-b][-e][-k][-l][-n][-S< ;Service type>][-X][Host name or IP address]".

##The operating environment of this tutorial: linux7.3 System, Dell G3 computer.

What is telnet

Telnet is a remote login program based on the Telnet protocol, used to log in to remote hosts and manage remote hosts.

The telnet command is the user interface of the TELNET protocol. It supports two modes: command mode and session mode. Although telnet supports many commands, in most cases, we just use it to check whether the target host has opened a certain Port (default is 23).

telnet uses clear text to transmit messages and has poor security. Many Linux servers do not open the telnet service and instead use the more secure ssh method. But there are still many other systems that may use telnet to provide remote login, so it is still necessary to figure out how to use telnet.

The telnet command can also be used for other purposes, such as determining whether a certain port of the remote server can be accessed.

In addition, the Telnet protocol is a member of the TCP/IP protocol suite and is the standard protocol for Internet remote login services. It is an application layer protocol and implements remote login based on the TCP protocol.

Command format

telnet [-8acdEfFKLrx][-b<主机别名>][-e<脱离字符>][-k<域名>][-l<用户名称>][-n<记录文件>][-S<服务类型>][-X<认证形态>][主机名称或IP地址<通信端口>]

Option description

-8 允许使用8位字符资料，包括输入与输出
-a 尝试自动登入远端系统
-b<主机别名> 使用别名指定远端主机名称
-c 不读取用户专属目录里的.telnetrc文件
-d 启动排错模式
-e<脱离字符> 设置脱离字符
-E 滤除脱离字符
-F 使用Kerberos V5认证时，加上此参数可把本地主机的认证数据上传到远端主机
-f 此参数的效果和”-F”参数相同
-k<域名> 使用Kerberos认证时，加上此参数让远端主机采用指定的领域名，而非该主机的域名
-K 不自动登入远端主机
-l<用户名称> 指定要登入远端主机的用户名称
-L 允许输出8位字符资料
-n<记录文件> 指定文件记录相关信息
-r 使用类似rlogin指令的用户界面
-S<服务类型> 设置telnet连线所需IP TOS信息
-x 假设主机有支持数据加密的功能，就使用它
-X<认证形态> 关闭指定的认证形态

Common examples

(1) Log in to the host. The provided port is not displayed and defaults to 23.

telnet 192.168.0.5

(2) Confirm whether a port of the remote server is available. For example, check whether port 22 of the remote server's ssh service is open.

telnet 10.234.178.144 22
Trying 10.234.178.144...
Connected to 10.234.178.144.
Escape character is '^]'.
SSH-2.0-OpenSSH_6.0

The above indicates that port 22 of the remote server’s ssh service has been opened. Note that this does not mean that you can definitely establish an SSH connection with the remote server using the ssh command, which requires authorization from the remote server.

(3) Enter command line interactive mode.

If telnet is called without the host parameter, it will enter command mode with the prompt telnet>.

telnet
telnet>

Commonly used subcommands in interactive mode are:

auth <argument> [...]：查看和管理telnet登录身份验证方式。argument可取值：disable、enable和status
close：关闭当前 Telnet 连接
display：使用 display 命令可以查看 Telnet 客户端的当前设置
logout：类似于cloase命令，用于关闭Telnet连接或者挂起。注：Telnet服务端可能不支持
open <host> [-l user] [[-] port]：建立到指定主机的 Telnet 连接
send <arguments>：使用send命令可以向Telnet服务器发送特定字符串。支持包括但不限于以下命令字符串：
    abort：终止服务命令
    ao：Abort Output，放弃输出命令
    ayt：Are you there命令
    brk：Break命令
    eof：发送End Of File字符
    escape：发送当前的转义字符，初始为分组符^]。
    ip：中断进程命令
    synch：执行Telnet同步操作。
quit：退出 Telnet 客户端。
status：显示当前Telnet连接状态

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