Configuring CentOS Remote Access using SSH

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Remote Access to the CentOS DesktopDisplaying CentOS Applications Remotely (X11 Forwarding)

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Secure Shell (SSH) is a TCP/IP service that provides a secure mechanism for remotely logging into one system over either a local network or the internet from another system. SSH also provides the ability to transfer files between remote systems. When a user logs into a remote system using SSH, they receive a command prompt allowing them to enter commands on the remote system as if they were sitting at the remote system and had opened a terminal session.

In this chapter we will cover the steps necessary to configure a CentOS system to accept SSH connections. This involves installing the SSH server on the local systems and configuring the firewall to allow SSH connections.


Installing SSH on an CentOS System

In order for a system to accept SSH connections the system must first be running the SSH server. By default, CentOS installs the SSH server so it is not usually necessary to install it. To check if it is installed and running use the following command:

/sbin/service sshd status

This will generate output similar to the following if the service is installed and running:

sshd (pid 9488 9486 1850) is running...

If the service is not installed it can be installed as follows:

su -
yum install openssh-server

The installation process will download the SSH server, install it and start the service running in the background. You may now attempt to connect from a remote system (see below for details of how to do this). If you receive a "connection refused" message when you try to connect it you may need to configure the firewall to allow SSH connections to be established to this system. If the service is installed, but not running, it can be started from the command-line:

/sbin/service sshd start

The service may also be started by using the GUI Service Configuration tool accessed via the System -> Administration -> Services menu option. Once the tool has loaded, simply scroll down the list of services until you reach sshd. Select it and click on the Start button located in the toolbar. If you would like the service to start automatically each time the system is started, set the check box next to the service and click on the toolbar Save button.

Configuring the CentOS Firewall to Allow SSH Connections

If you are using a firewall to protect your system you may need to allow SSH connections before you will be able to connect from a remote system. If you are using the basic firewall configuration (see Basic CentOS Firewall Configuration) you can allow SSH connections using the Security Level and Firewall Configuration tool. This tool is launched from the System -> Administration -> Security Level and Firewall menu option as appears as follows:

Opening the ssh port on a CentOS firewall

If the box next to SSH is not already checked, check it and click Apply to allow ssh traffic to pass through the firewall.

Using SSH on CentOS

SSH can be used to log into your system from a remote system. It is also possible to test that the SSH server is running and accessible from the local machine. SSH connections are established using the ssh client utility.

To connect from your local machine back to itself use the following command:

ssh -l username ipaddresss

Where username is the name of the user you wish to log in as and ipaddress is the IP address of your system. You can also substitute the hostname of the system in place of the IP address. If you do not know the IP address run the ifconfig command in a terminal window. This will output information similar to:

eth0      Link encap:Ethernet  HWaddr 00:13:72:0B:14:57
          inet addr:  Bcast:  Mask:
          inet6 addr: fe80::213:72ff:fe0b:1457/64 Scope:Link
          RX packets:4261067 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4409081 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100
          RX bytes:2068332349 (1.9 GiB)  TX bytes:2408187471 (2.2 GiB)
          Base address:0xcce0 Memory:fe3e0000-fe400000

In the above output the IP address is shown as inet addr:, in this case

To connect from a remote system perform the same steps above using either the IP address or host name of the remote host to which you wish to connect. Enter your password when prompted and you will find yourself logged into the remote system.

Copying files using SSH

The SSH service provides a mechanism for securely copying files to and from a remote system. Copying is performed using the scp utility. To copy a file to a directory on a remote system, execute the following command:

scp myfile.txt [email protected]:/home/demo

Where myfile.txt is the name of the file to be uploaded to the remote system, username is the name of user account to be used to log into the remote system, is replaced by the real IP address or hostname of the system and /home/demo represents the directory into which the file should be copied.

The above file could similarly be copied from the remote system to the local system as follows:

scp [email protected]:/home/demo/myfile.txt .

The above command will copy the remote file to the current directory on the local system.

Disabling the SSH Server

Having configured the system to run the SSH server we can now look at how to disable it. As we mentioned previously the SSH server runs in the background as a service. In order to disable SSH we need to turn off the SSH service. This can be achieved using the Services tool or from the command line. To disable the sshd service from the command line:

su -
/sbin/service sshd stop

To launch the services tool click on the desktop System menu and select Services from the Administration sub-menu. If the tool is not listed in the menu it will first need to be installed. This can be achieved from a terminal window as follows:

su –
yum install system-config-services

Once the tool installed and launched, enter your password when prompted to do so. The Services tool will appear containing a list of all available services. Scroll down to find the sshd entry as shown below:

Enabling SSH Service on CentOS

Select the sshd entry and click on the Stop button. The SSH server is now stopped. If you wish to prevent the service from starting automatically next time the system starts, unset the check box and click on the toolbar Save button before exiting the tool.

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Kindle/ePub/PDF edition contains 39 chapters and over 240 pages.

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Remote Access to the CentOS DesktopDisplaying CentOS Applications Remotely (X11 Forwarding)