telnet到RedHat Linux失敗--解決辦法

失敗原因：

1.telnet包未安裝，檢查telnet包是否安裝：  

[root@vm-rhel root]# rpm -qa telnet
 telnet-0.17-25

 表示已安裝

 2.telnet包已安裝，telnet-server未安裝，檢查telnet-server包是否安裝： 

[root@vm-rhel root]# rpm -qa telnet-server
 telnet-server-0.17-25

 表示已安裝

 3.telnet配置文件問題：

[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 
}

將disable對應的值修改為no或者註釋該行並重啟xinetd守護進程：service xinetd restart。

4.Linux防火牆原因，查看防火牆狀態：

[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

表示未關閉,如果防火牆已關閉，則不需要在/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。

關閉防火牆：service iptables stop (重啟後失效：防火牆開機自動啟動)

啟動防火牆：service iptables start

重啟防火牆：service iptables restart

禁止防火牆開機自動啟動：chkconfig iptables off

5.預設情況下Linux不允許root用戶以telnet方式登錄Linux主機，若要允許root用戶登錄可採用以下3中方法：

  （1）修改/etc/pam.d/login配置文件

         RedHat Linux對於遠程登錄的限制體現在/etc/pam.d/login文件中，把限制內容註釋即可。

[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文件夾

          驗證規則設置在/etc/securetty文件中，該文件定義了root用戶只能在tty1-tty6的終端上記錄，刪除該文件或將其改名即可避開驗證規則從而實現root用戶以telnet方式遠程登錄Linux主機。

[root@vm-rhel root]# mv /etc/securetty /etc/securetty.bak

  （3）先用普通用戶登錄，然後切換到root用戶

[bboss@vm-rhel bboss]$ su root
Password: 
[root@vm-rhel bboss]#