How to start/stop/restart networking service under Linux operating systems after making changes to IP configuration from a shell prompt ?

Use the following commands as per your Linux distribution to start/stop/restart the networking service.

RedHat / CentOS / Fedora

To start networking service, enter

# /etc/init.d/network start

Or,

# service network start

To stop networking service, enter

# /etc/init.d/network stop

Or,

# service network stop

To restart networking service, enter

# /etc/init.d/network restart

Or,

# service network restart

Ubuntu / Debian

To start networking service, enter

# sudo /etc/init.d/networking start

Or,

# sudo service networking start

To stop networking service, enter

# sudo /etc/init.d/networking stop

Or,

# sudo service networking stop

To restart networking service, enter

# sudo /etc/init.d/networking restart

Or,

# service networking restart

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Using cron is via the crontab command which allows you to view or edit your crontab file, which is a per-user file containing entries describing commands to execute and the time(s) to execute them.

Display Root cronjob

# crontab -l

Display user cronjob

$ crontab -u user -l

Display daily cronjob

# cd /etc/cron.daily/
# cat * | more

Display hourly cronjob

# cd /etc/cron.hourly/
# cat * | more

Display weekly cronjob

# cd /etc/cron.weekly /
# cat * | more

Display monthly cronjob

# cd /etc/cron.monthly/
# cat * | more

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Under Linux operating systems use the dmidecode command to check system BIOS information. The output from the dmidecode command include the BIOS version, Bios release date, BIOS vendor, BIOS language and other BIOS characteristics available on the system.
Type the following command as root

# dmidecode --type bios

Output

SMBIOS 2.4 present.

Handle 0x0000, DMI type 0, 24 bytes
BIOS Information
        Vendor: Phoenix Technologies LTD
        Version: 6.00
        Release Date: 10/13/2009
        Address: 0xEA2E0
        Runtime Size: 89376 bytes
        ROM Size: 64 kB
        Characteristics:
                ISA is supported
                PCI is supported
                PC Card (PCMCIA) is supported
                PNP is supported
                APM is supported
                BIOS is upgradeable
                BIOS shadowing is allowed
                ESCD support is available
                USB legacy is supported
                Smart battery is supported
                BIOS boot specification is supported
                Targeted content distribution is supported
        BIOS Revision: 4.6
        Firmware Revision: 0.0

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What is OpenSSH ?

OpenSSH is a FREE version of the SSH connectivity tools that technical users of the Internet rely on. Users of telnet, rlogin, and ftp may not realize that their password is transmitted across the Internet unencrypted, but it is. OpenSSH encrypts all traffic (including passwords) to effectively eliminate eavesdropping, connection hijacking, and other attacks. Additionally, OpenSSH provides secure tunneling capabilities and several authentication methods, and supports all SSH protocol versions.

Install OpenSSH under CentOS Linux

To install the server and client type the following command:

[root@lifelinux ~]# yum install openssh-server

Start the service:

[root@lifelinux ~]# service sshd start

Set sshd service to automatically start at boot time, type the following command

[root@lifelinux ~]# chkconfig sshd on

Check & Make sure port 22 is opened:

[root@lifelinux ~]# netstat -tulpn | grep :22

Sample outputs:

tcp        0      0 :::22                       :::*                        LISTEN      1703/sshd

OpenSSH Server Configuration

Now you need to edit the /etc/ssh/sshd_config file using vi or any other editor and you need to change the following Directives or add the following directives for OpenSSH server configuration these are only some of basic directives if you want to know available directives check here
To change SSH listening port, example change port to 2222

Port 2222

To disable root logins, edit or add as follows:

PermitRootLogin no

Allow only SSH protocol 2

Protocol 2

Allow only user1 and user2 over SSH:

AllowUsers user1 user2

Firewall Settings

Open /etc/sysconfig/iptables file, enter:

[root@lifelinux ~]# vi /etc/sysconfig/iptables

Add the following line

-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT

Save and close the file. Restart the firewall:

[root@lifelinux ~]# /etc/init.d/iptables restart

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