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Thursday, November 14, 2013

Installing and Configuring OpenSSH Server In Linux - in the Command Line

Here's a good Article about OpenSSH and showing how to install it in the Command Line. Here's an Excerpt from the Article below...


How to Install and Configure OpenSSH Server In Linux

Skipping on down...

Why Use OpenSSH And Over Telnet Or Ftp?

The most important reason why should use OpenSSH tools over ftp and telnet is that all communications and user credentials using OpenSSH are encrypted, they are also protected from man in the middle attacks. If a third party tries to intercept your connection, OpenSSH detects it and informs you about that.

What Are Some Of The OpenSSH Features?

  1. Secure Communication
  2. Strong Encryption (3DES, Blowfish, AES, Arcfour)
  3. X11 Forwarding (encrypt X Window System traffic)
  4. Port Forwarding (encrypted channels for legacy protocols)
  5. Strong Authentication (Public Key, One-Time Password and Kerberos Authentication)
  6. Agent Forwarding (Single-Sign-On)
  7. Interoperability (Compliance with SSH 1.3, 1.5, and 2.0 protocol Standards)
  8. SFTP client and server support in both SSH1 and SSH2 protocols.
  9. Kerberos and AFS Ticket Passing
  10. Data Compression

Installation of OpenSSH in Linux

To install OpenSSH, open a terminal and run the following commands with superuser permissions.

On Ubuntu/Debian/Linux Mint

$ sudo apt-get install openssh-server openssh-client

On RHEL/Centos/Fedora

Type the following yum command to install openssh client and server.

# yum -y install openssh-server openssh-clients

Configuration of OpenSSH

It’s time to configure our OpenSSH behaviour through the ssh config file, but before editing the /etc/ssh/sshd_config file we need to backup a copy of it, so in case we make any mistake we have the original copy.

Open a terminal and run the following command to make a copy of the original sshd configuration file.

$ sudo cp /etc/ssh/sshd_config  /etc/ssh/sshd_config.original_copy

As you can see from the command I typed, I added the original_copy suffix, so every time I see this file I know it is an original copy of the sshd config file.

How Do I Connect to OpenSSH

Before we go further, we need to verify if our openssh server is working or not. How to do that? You can try to connect to the openssh server from your localhost through your openssh client or do a portscan with nmap, but I like to use a small tool called netcat, also known as the TCP/IP Swiss army knife. I love working with this amazing tool on my machine, so let me show it to you.

# nc -v -z 22

Referring to the netcat results, the ssh service is running on port 22 on my machine. Very good! What if we want to use another port, instead of 22? We can do that by editing the sshd configuration file.

Set your OpenSSH to listen on TCP port 13 instead of the default TCP port 22. Open the sshd_config file with your favourite text editor and change the port directive to 13.

# What ports, IPs and protocols we listen for  Port 13

Restart OpenSSH server so the changes in config file can take place by typing the following command and run netcat to verify if the port you set for listening is open or not.

$ sudo /etc/init.d/ssh restart

Should we verify is our openssh server is listening on port 13, or not?. This verification is necessary, so I am calling my lovely tool netcat to help me do the job.

# nc -v -z 13

Do you like to make your openssh server display a nice login banner? You can do it by modifying the content of /etc/ file and adding the following line inside the sshd configuration file.

Banner /etc/



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