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telnet 到 RedHat Linux 失败原因：
1.telnet 包未安装，检查 telnet 包是否安装：
[root@vm-rhel root]# rpm -qa telnet
telnet-0.17-25
表示已安装
2.telnet 包已安装，telnet-server 未安装，检查 telnet-server 包是否安装：
[root@vm-rhel root]# rpm -qa telnet-server
telnet-server-0.17-25
表示已安装
3.telnet 配置文件问题：
[root@vm-rhel root]# cat /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 = yes
}
将 disable 对应的值修改为 no 或者注释该行并重启 xinetd 守护进程：service xinetd restart。
4.Linux 防火墙原因，查看防火墙状态：
[root@vm-rhel root]# service iptables status
Table: filter
Chain INPUT (policy ACCEPT)
target prot opt source destination
RH-Lokkit-0-50-INPUT all — anywhere anywhere
Chain FORWARD (policy ACCEPT)
target prot opt source destination
RH-Lokkit-0-50-INPUT all — anywhere anywhere
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
Chain RH-Lokkit-0-50-INPUT (2 references)
target prot opt source destination
ACCEPT udp — 192.168.1.1 anywhere udp spt:domain dpts:1025:65535
ACCEPT tcp — anywhere anywhere tcp dpt:smtp flags:SYN,RST,ACK/SYN
ACCEPT tcp — anywhere anywhere tcp dpt:http flags:SYN,RST,ACK/SYN
ACCEPT tcp — anywhere anywhere tcp dpt:ftp flags:SYN,RST,ACK/SYN
ACCEPT tcp — anywhere anywhere tcp dpt:ssh flags:SYN,RST,ACK/SYN
ACCEPT tcp — anywhere anywhere tcp dpt:telnet flags:SYN,RST,ACK/SYN
ACCEPT udp — anywhere anywhere udp spts:bootps:bootpc dpts:bootps:bootpc
ACCEPT udp — anywhere anywhere udp spts:bootps:bootpc dpts:bootps:bootpc
ACCEPT all — anywhere anywhere
ACCEPT all — anywhere anywhere
REJECT tcp — anywhere anywhere tcp dpts:0:1023 flags:SYN,RST,ACK/SYN reject-with icmp-port-unreachable
REJECT tcp — anywhere anywhere tcp dpt:nfs flags:SYN,RST,ACK/SYN reject-with icmp-port-unreachable
REJECT udp — anywhere anywhere udp dpts:0:1023 reject-with icmp-port-unreachable
REJECT udp — anywhere anywhere udp dpt:nfs reject-with icmp-port-unreachable
REJECT tcp — anywhere anywhere tcp dpts:x11:6009 flags:SYN,RST,ACK/SYN reject-with icmp-port-unreachable
REJECT tcp — anywhere anywhere tcp dpt:xfs flags:SYN,RST,ACK/SYN reject-with icmp-port-unreachable
表示未关闭, 如果防火墙已关闭，则不需要在 /etc/sysconfig/iptables 配置文件中添加：-A RH-Lokkit-0-50-INPUT -p tcp -m tcp –dport 23 –syn -j ACCEPT。
关闭防火墙：service iptables stop (重启后失效：防火墙开机自动启动)
启动防火墙：service iptables start
重启防火墙：service iptables restart
禁止防火墙开机自动启动：chkconfig iptables off
5. 默认情况下 Linux 不允许 root 用户以 telnet 方式登录 Linux 主机，若要允许 root 用户登录可采用以下 3 中方法：
（1）修改 /etc/pam.d/login 配置文件
RedHat Linux 对于远程登录的限制体现在 /etc/pam.d/login 文件中，把限制内容注释即可。
[root@vm-rhel root]# cat /etc/pam.d/login
#%PAM-1.0
auth required pam_securetty.so
auth required pam_stack.so service=system-auth
#auth required pam_nologin.so
account required pam_stack.so service=system-auth
password required pam_stack.so service=system-auth
session required pam_stack.so service=system-auth
session optional pam_console.so
（2）移除 /etc/securetty 文件夹
验证规则设置在 /etc/securetty 文件中，该文件定义了 root 用户只能在 tty1-tty6 的终端上记录，删除该文件或将其改名即可避开验证规则从而实现 root 用户以 telnet 方式远程登录 Linux 主机。
[root@vm-rhel root]# mv /etc/securetty /etc/securetty.bak
（3）先用普通用户登录，然后切换到 root 用户
[bboss@vm-rhel bboss]$ su root
Password:
[root@vm-rhel bboss]#
本文永久更新链接地址 ：http://www.linuxidc.com/Linux/2017-06/144497.htm
