Detailed explanation of the solution to the failure of telnet to RedHat Linux

Reason for failure:

1. The telnet package is not installed, check whether the telnet package is installed:

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

means it is installed

2. The telnet package has been installed, but telnet-server has not been installed. Check whether the telnet-server package is installed:

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

means it has been installed

3.telnetConfiguration fileQuestion:

[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 
}

Change the value corresponding to disable to no or Comment this line and restart the xinetd daemon process: service xinetd restart.

4.Linux Firewall reason, check the firewall status:

[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

means it is not closed. If the firewall is closed, there is no need to open the /etc Add to the /sysconfig/iptables configuration file: -A RH-Lokkit-0-50-INPUT -p tcp -m tcp --dport 23 --syn -j ACCEPT.

Close the firewall: service iptables stop (Invalid after restart: the firewall starts automatically at boot)

Start the firewall: service iptables start

Restart the firewall: service iptables restart

Disable the firewall from automatically starting at boot: chkconfig iptables off

5. By default, Linux does not allow the root user to log in to the Linux host through telnet. If you want to allow the root user to log in, The following 3 methods can be used:

(1) Modify the /etc/pam.d/login configuration file

RedHat Linux’s restrictions on remote login In the /etc/pam.d/login file, just comment the restriction content.

[root@vm-rhel root]# cat /etc/pam.d/login#%PAM-1.0auth       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) Remove the /etc/securetty folder

Verification rules are set in the /etc/securetty file, which defines that the root user can only Record on the terminal of tty1-tty6, Delete or rename the file to avoid the verification rules and enable the root user to remotely log in to the Linux host through telnet.

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

(3) First log in as a normal user, and then switch to the root user

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

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