How to configure highly available remote logins (such as SSH and Telnet) on Linux

How to configure high-availability remote login (such as SSH and Telnet) on Linux

Abstract: Remote login is one of the most commonly used functions in Linux systems, which allows us to connect to remote hosts through the network and operate. In practical applications, in order to ensure system availability and security, the remote login service needs to be configured in high availability mode. This article will introduce how to configure highly available remote login on Linux, including SSH and Telnet, and provide relevant code examples.

1. SSH configuration

SSH (Secure Shell) is a remote login protocol based on encryption technology. Secure remote login can be achieved through SSH in an unsecured network. The following are the steps to configure SSH high availability:

1. Install and configure SSH server
   In Linux systems, the commonly used software package for SSH servers is OpenSSH. First, we need to ensure that the OpenSSH package is installed on the host machine.

$ sudo apt-get install openssh-server # Ubuntu/Debian
$ sudo yum install openssh-server # CentOS/RHEL

After the installation is completed, the SSH server will automatically Starts and listens on the default port 22.

2. Configure SSH communication key
   In order to achieve high availability, we can use an SSH communication key pair for authentication instead of using the traditional username/password.

First, we need to generate an SSH key pair:

$ ssh-keygen -t rsa

Then, copy the public key to all remote hosts:

$ ssh-copy-id remote_host

3. Configuring SSH load balancing
   In order to achieve high availability of SSH, we can use load balancing to distribute remote login requests to multiple on a host. Commonly used load balancing software include HAProxy and Nginx.

Taking HAProxy as an example, first install HAProxy:

$ sudo apt-get install haproxy # Ubuntu/Debian
$ sudo yum install haproxy # CentOS/RHEL

Then, edit the HAProxy configuration file:

$ sudo vi /etc/haproxy/haproxy.cfg

Add the following configuration block in the file:

frontend ssh

bind *:22
mode tcp
default_backend ssh_servers

backend ssh_servers

mode tcp
balance roundrobin
server ssh1 192.168.1.101:22 check
server ssh2 192.168.1.102:22 check
server ssh3 192.168.1.103:22 check

Save and exit the file, then restart the HAProxy service:

$ sudo systemctl restart haproxy

Now you can use HAProxy to provide IP address to connect to the SSH server.

2. Telnet configuration

Telnet is a protocol used for remote login, but it does not have the encryption function like SSH, so it is not suitable for use in insecure networks. The following are the steps to configure Telnet high availability:

1. Install and configure Telnet server
   In Linux systems, the commonly used software package for Telnet servers is telnetd. First, we need to ensure that the telnetd package has been installed on the host (example: CentOS/RHEL):

$ sudo yum install telnet-server

After the installation is complete, the Telnet server will Starts automatically and listens on the default port 23.

2. Configuring Telnet Load Balancing
   Similar to SSH, we can use load balancing to achieve high availability of Telnet. In Linux, we can use iptables and netfilter to achieve load balancing.

First, edit the iptables configuration file:

$ sudo vi /etc/sysctl.conf

Add the following configuration at the end of the file:

net.ipv4.ip_forward = 1

Save and exit the file, then load the configuration file:

$ sudo sysctl -p

Next, execute the following command to configure load balancing ：

$ sudo iptables -t nat -A PREROUTING -p tcp --dport 23 -j DNAT --to-destination 9c3735a17da6c34f1799512b11b0e54c:23
$ sudo iptables -t nat -A POSTROUTING - j MASQUERADE

Where, 9c3735a17da6c34f1799512b11b0e54c is the IP address of the target Telnet server.

3. Configuring Service Discovery
   In order to achieve high availability, we need to find a way to distribute client requests to multiple Telnet servers. Commonly used service discovery software include ZooKeeper and Consul.

Taking ZooKeeper as an example, first install ZooKeeper:

$ sudo apt-get install zookeeperd # Ubuntu/Debian
$ sudo yum install zookeeper # CentOS/RHEL

Then, edit the ZooKeeper configuration file:

$ sudo vi /etc/zookeeper/conf/zoo.cfg

Add the following configuration to the file:

server. 1=zoo1:2888:3888
server.2=zoo2:2888:3888
server.3=zoo3:2888:3888

where zoo1, zoo2, and zoo3 are the host names of the ZooKeeper server .

Save and exit the file, then restart the ZooKeeper service:

$ sudo service zookeeper restart

Now you can connect to the Telnet server using the IP address provided by ZooKeeper.

Conclusion
By configuring a highly available remote login service, we can ensure the availability and security of the system. This article describes how to configure high availability of SSH and Telnet on Linux and provides relevant code examples. I hope this article can be helpful to you, thank you for reading!

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