CentOS5.8上安装telnet服务
当你需要升级openssh的时候，为了避免中途升级失败，导致远程不上，先将telnet开启。
1.查看系统版本及是否安装telnet
[root@centos32 comodo]# cat /etc/issue
CentOS release 5.8 (Final)
Kernel \r on an \m
[root@centos32 comodo]# rpm -qa|grep telnet
telnet-0.17-39.el5
其中CentOS系统默认已经安装了telnet，但是telnet-server软件包需要自行安装
2.安装telnet-server
1].可以下载rpm包安装（下载地址:http://rpmfind.net/linux/rpm2html/search.php?query=telnet-server）
2].可以#yum install telnet-server
安装完成后检查安装是否成功：
[root@centos32 comodo]# rpm -qa|grep telnet
telnet-0.17-39.el5
telnet-server-0.17-39.el5
3.启动telnet服务
方法1：图形桌面System->Administration->Services->"On Demand Services"菜单页选中telnet并保存
方法2：图形桌面Applications->Accessories->Terminal中输入#/usr/sbin/ntsysv回车选中telnet并保存
方法3：修改/etc/xinetd.d/telnet将disable = yes修改为no, 可以使用命令#sed -i 's/disable = yes/disable = no/' /etc/xinetd.d/telnet
方法4：修改启动项#/sbin/chkconfig telnet on
方法3和方法4需要重启telnet服务,运行命令#/etc/rc.d/init.d/xinetd restart
4.测试服务
[root@centos32 comodo]# telnet localhost
Trying 127.0.0.1...
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
CentOS release 5.8 (Final)
Kernel 2.6.18-308.16.1.el5 on an i686
login: comodo
Password:
Last login:
[comodo@centos32 ~]$ exit(此处可知telnet已经使用用户comodo成功登录)
logout
Connection closed by foreign host.
[root@centos32 comodo]#
5.允许root登录telnet（默认情况下，telnet禁止root用户登录）
方法1.修改/etc/pam.d/remote注释掉auth required pam_securetty.so
[root@centos32 comodo]# vim /etc/pam.d/remote
#%PAM-1.0
#auth required pam_securetty.so
方法2.先查看root登录使用的伪终端号，然后把这个伪终端加到/etc/securetty中
step 1:打开一个终端输入
[root@centos32 comodo]# telnet localhost
Trying 127.0.0.1...
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
CentOS release 5.8 (Final)
Kernel 2.6.18-308.16.1.el5 on an i686
login: root
Password:
Login incorrect
login: Login timed out after 60 seconds
Connection closed by foreign host.
step 2:打开一个终端输入
[root@centos32 comodo]#tail /var/log/secure
centos32 login: pam_securetty(remote:auth): access denied: tty 'pts/7' is not secure !
centos32 login: FAILED LOGIN 1 FROM centos32 FOR root, Authentication failure
step 3:把pts/7加入到/etc/securetty后，telnet即可使用root账户登录
[root@centos32 comodo]# vim /etc/securetty
pts/7
当你需要升级openssh的时候，为了避免中途升级失败，导致远程不上，先将telnet开启。