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Install and configure OpenSSH-server on Linux

Secure shell (SSH) is a network protocol providing shell services on a remote machine via a secure channel. OpenSSH is an open-source implementation of the ssh protocol, allowing encrypted communication over a network via a suite of software.
OpenSSH is developed by the Open BSD group and it is released under Simplified BSD License.

OpenSSH features

The following is a list of OpenSSH features:

  • Open Source Project
  • Free Licensing
  • Strong Encryption (3DES, Blowfish, AES, Arcfour)
  • X11 Forwarding (encrypt X Window System traffic)
  • Port Forwarding (encrypted channels for legacy protocols)
  • Strong Authentication (Public Key, One-Time Password and Kerberos Authentication)
  • Agent Forwarding (Single-Sign-On)
  • Interoperability (Compliance with SSH 1.3, 1.5, and 2.0 protocol Standards)
  • SFTP client and server support in both SSH1 and SSH2 protocols.
  • Kerberos and AFS Ticket Passing
  • Data Compression

More about OpenSSH features read here.

OpenSSH installation

Install OpenSSH on RHEL/CentOS 5.x/6.x

To install OpenSSH and auto-start OpenSSH server on CentOS or RHEL type following command:

# yum -y install openssh-server openssh-clients
# service sshd start
# hkconfig sshd on

Install OpenSSH on Fedora 15/16/17/18/19

To install OpenSSH and auto-start OpenSSH server on Fedora 15/16/17/18/19 type following command:

# sudo yum -y install openssh-server openssh-clients
# sudo service sshd start
# sudo systemctl enable sshd.service

Install OpenSSH on Debian/Ubuntu based distribution

To install OpenSSH and auto-start OpenSSH server on Debian/Ubuntu based distribution, type following command:

# sudo apt-get install openssh-server openssh-client
# sudo update-rc.d ssh defaults

How to login to OpenSSH

To login to your computer from a Unix-like machine, go to a command-line and type:

# ssh username@ipaddress
# ssh username@hostname

For example:

# ssh rasho@192.168.0.2
OpenSSH
OpenSSH

Configure OpenSSH server

If you would like to configure OpenSSH server edit /etc/ssh/sshd_config OpenSSH configuration file.
There are a couple of OpenSSH options you may be interested in:
Port 22
By default, sshd listens on port 22 to listen for incoming ssh connections. By changing the default ssh port, you may be able to avert various automated attacks from hackers.
PermitRootLogin no
The PermitRootLogin option tells whether root can log in to the system via ssh.
AllowUsers user1 user2
Using the AllowUsers option, you can selectively disable ssh service for particular Linux users. You can specify multiple users separated by space.
Once you have modified /etc/ssh/sshd_config, make sure to restart ssh service.
To restart OpenSSH on Ubuntu or Debian:

# sudo /etc/init.d/ssh restart

To restart OpenSSH on CentOS, Fedora or RHEL:

# service sshd restart

Read also: Disable or Enable SSH Root Login and Limit SSH Access in Linux

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