在centos5开启telnet服务并验证

1.安装telnet服务

[root@localhost ~]# yum install telnet

2.检查是否成功安装

[root@localhost ~]# rpm -qa | grep telnet
telnet-0.17-.el5
telnet-server-0.17-.el5                                      #####有显示就是正确的

3.修改文件开启服务

[root@localhost home]# vim /etc/xinetd.d/telnet
# default: on
# description: The telnet server serves telnet sessions; it uses \
#       unencrypted username/password pairs for authentication.
service telnet
{
        flags           = REUSE
        socket_type     = stream
        wait            = no
        user            = root
        server          = /usr/sbin/in.telnetd
        log_on_failure  += USERID
        disable         = no                              #####是指禁止远方telnet，改为no就是启动
}

[root@localhost xinetd.d]# service  xinetd  restart
停止 xinetd：                                              [确定]
启动 xinetd：                                              [确定]

4.停止iptables、seliunx（可以在iptables中开启telnet的23端口，后面有介绍）

5.测试能否能用root账户telnet（若没配置一般是不行的）

6.修改配置使root登陆

当我们失败后，linux是会记录下失败记录作为日志在/var/log/secure

Oct  :: localhost login: pam_securetty(remote:auth): access denied: tty 'pts/1' is not secure !
Oct  :: localhost login: FAILED LOGIN  FROM 192.168.165.1 FOR root, Authentication failure

可以看到没有pts/1所以被拒绝了

我们可以在修改添加一个虚拟线程

[root@localhost xinetd.d]# vi /etc/securetty 

console
vc/
vc/
vc/
vc/
vc/
vc/
vc/
vc/
vc/
vc/
vc/
tty1
tty2
tty3
tty4
tty5
tty6
tty7
tty8
tty9
tty10
tty11
pts/

再次测试

Xshell:\> telnet 192.168.165.136

Connecting to 192.168.165.136:...
Connection established.
To escape to local shell, press 'Ctrl+Alt+]'.
CentOS release  (Final)
Kernel 2.6.-.el5 on an i686
login: root
Password:
Last login: Wed Oct  :: from 192.168.165.1
[root@localhost ~]# 

ps：不建议直接用root登陆，因为telnet是明文传输。建议用一个普通用户登录然后su到root用户权限

7.在有防火墙的情况下配置telnet

修改防火墙配置，添加一条开发telnet的23号端口

[root@localhost ~]# vi /etc/sysconfig/iptables

# Firewall configuration written by system-config-securitylevel
# Manual customization of this file is not recommended.
*filter
:INPUT ACCEPT [:]
:FORWARD ACCEPT [:]
:OUTPUT ACCEPT [:]
:RH-Firewall--INPUT - [:]
-A INPUT -j RH-Firewall--INPUT
-A FORWARD -j RH-Firewall--INPUT
-A RH-Firewall--INPUT -i lo -j ACCEPT
-A RH-Firewall--INPUT -p icmp --icmp-type any -j ACCEPT
-A RH-Firewall--INPUT -p  -j ACCEPT
-A RH-Firewall--INPUT -p  -j ACCEPT
-A RH-Firewall--INPUT -m state --state NEW -m tcp -p tcp --dport  -j ACCEPT                   ######开放23号端口
-A RH-Firewall--INPUT -p udp --dport  -d 224.0.0.251 -j ACCEPT
-A RH-Firewall--INPUT -p udp -m udp --dport  -j ACCEPT
-A RH-Firewall--INPUT -p tcp -m tcp --dport  -j ACCEPT
-A RH-Firewall--INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A RH-Firewall--INPUT -m state --state NEW -m tcp -p tcp --dport  -j ACCEPT
-A RH-Firewall--INPUT -j REJECT --reject-with icmp-host-prohibited
COMMIT
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