一、安装telnet服务

yum install xinetd 依赖
yum install telnet 客户端
yum install telnet-server 服务端

1、开启telnet服务

vim /etc/xinetd.d/telnet
service telnet
{
        flags           = REUSE
        socket_type     = stream
        wait            = no
        user            = root
        server          = /usr/sbin/in.telnetd
        log_on_failure  += USERID
        #disable        = yes             【将disable=yes行前加#注释掉，或者把yes改为no】
}

2、允许root以telnet登陆

备份配置：cp /etc/securetty /etc/securetty.bak
修改以下配置文件vim /etc/securetty
在末尾添加（可添加pts/0-11）
pts/0
pts/1

3、启动xinetd、telnet服务

systemctl enable xinetd
systemctl enable telnet.socket
systemctl start xinetd
systemctl start telnet.socket

4、检查端口是否已经开启

[root@jzyue ~]# netstat -antp|grep 23
tcp        0      0 0.0.0.0:23              0.0.0.0:*               LISTEN      1/systemd

5、测试telnet

使用同一网段的计算机使用telnet +IP地址连接服务器，输入用户名和密码，可通过telnet服务远程连接到服务器。

[root@jzyue ~]# telnet 172.26.152.68
Trying 172.26.152.68...
Connected to 172.26.152.68.
Kernel 3.10.0-862.14.4.el7.x86_64 on an x86_64
jzyue login: root
Password: 
Last login: Thu May 30 09:27:56 from 221.130.253.135
Welcome to Alibaba Cloud Elastic Compute Service !

二、升级openssh版本

1、查看openssh版本

[root@jzyue ~]# rpm -qa|grep openssh
openssh-clients-7.4p1-16.el7.x86_64
openssh-server-7.4p1-16.el7.x86_64
openssh-7.4p1-16.el7.x86_64

2、安装基础包

[root@jzyue ~]# yum install gcc gcc-c++ zlib zlib-devel openssl openssl-devel pam-devel -y

3、官网下载最新的源码包传到服务器

http://ftp.openbsd.org/pub/OpenBSD/OpenSSH/portable/

4、删除旧版本openssh

[root@jzyue scrips]# rpm -qa | grep openssh
openssh-clients-7.4p1-16.el7.x86_64
openssh-server-7.4p1-16.el7.x86_64
openssh-7.4p1-16.el7.x86_64
[root@jzyue scrips]# rpm -e `rpm -qa | grep openssh` --nodeps

普及一下rpm命令参数：
安装一个包 rpm -ivh
升级一个包 rpm -Uvh
卸载一个包 rpm -e
查询一个包 rpm -q
列出所有安装包 rpm -qa
列出包中的文件 rpm -ql

5、编译安装

[root@jzyue scrips]# ls
openssh-8.0p1.tar.gz
[root@jzyue scrips]# tar xf openssh-8.0p1.tar.gz 
[root@jzyue scrips]# ls
openssh-8.0p1  openssh-8.0p1.tar.gz
[root@jzyue openssh-8.0p1]# ./configure --prefix=/usr --sysconfdir=/etc/ssh --with-ssl --with-zlib --with-md5-passwords --with-pam
[root@jzyue openssh-8.0p1]# make && make install

6、备份配置文件

[root@jzyue openssh-8.0p1]# cp -a /etc/ssh /etc/ssh_bak
[root@jzyue openssh-8.0p1]# cp ssh_config /etc/ssh/
cp: overwrite ‘/etc/ssh/ssh_config’? y
[root@jzyue openssh-8.0p1]# cp sshd_config /etc/ssh/
cp: overwrite ‘/etc/ssh/sshd_config’? y
[root@jzyue openssh-8.0p1]# cp moduli /etc/ssh/
cp: overwrite ‘/etc/ssh/moduli’? y
[root@jzyue openssh-8.0p1]# cp contrib/redhat/sshd.init /etc/init.d/sshd

7、开启SSH服务，查看版本

[root@jzyue openssh-8.0p1]# /etc/init.d/sshd start
Reloading systemd:                                         [  OK  ]
Starting sshd (via systemctl):                             [  OK  ]
[root@jzyue openssh-8.0p1]# chkconfig sshd on
[root@jzyue openssh-8.0p1]# ssh -V
OpenSSH_8.0p1, OpenSSL 1.0.2k-fips  26 Jan 2017