Install SSH Server on Linux (Debian, Ubuntu, Fedora, CentOS, RedHat)

Debian / Ubuntu

sudo apt-get install -y openssh-server

RPM based Distros, Fedora / CentOS / RedHat

sudo dnf install -y openssh-server

or use yum

sudo yum install -y openssh-server

Start ssh service

sudo systemctl start sshd

By default the SSH service should start when the system starts, but if not try the following command to enable the service on boot up.

Debian / Ubuntu

systemctl enable ssh

Fedora, CentOS, RedHat

systemctl enable ssh

Change SSH port

Not necessary, but it is a good idea to change the default ssh port.  To change the port edit the sshd file.

vi /etc/ssh/sshd_config

Allow SSH access from a specific host using hosts.allow and hosts.deny on Linux

This is just a quick write on the hosts.allow and deny files.  You can lookup “spawn” and/or “twist” for some advanced usage.


So to limit an IP address, or a IP range access to SSH, do the following

Deny all incoming request for SSH

Edit the “hosts.deny” file

vi /etc/hosts.deny

add the following line

sshd : ALL

Now edit “hosts.allow” and allow the client IP, or IP range to access SSH

vi /etc/hosts.allow

add the following line to allow a single IP

sshd :

If you want to allow the whole subnet, then replace the above line with this one

sshd : 192.168.1.

hosts.allow overrides hosts.deny.  So you deny everything and then allow exceptions.

How to Install ZenPack on Zenoss 5.1

To install a ZenPack on Zenoss 5.1 and later, you are going to need to stop the zenoss services, restart a couple services that are needed to install the ZenPack, install the ZenPack, and then restart the Zenoss service.

Stop Zenoss.Core :

serviced service stop 

Make sure the service is stopped by running the following command.  When it reports back the Zenoss is Stopped, then continue.

 if ( serviced service status Zenoss.Core | awk '{print $3}' | grep -q Stopped) ; then echo "Zenoss is not running!" ; else echo "Zenoss is not Stopped!" ; fi

Or you can run this command, just make sure it says it is stopped.

serviced service status zenoss.core

Create a Snapshot

serviced service snapshot Zenoss.core

Start the following three services

serviced service start Infrastructure

serviced service start Infrastructure
serviced service start zeneventserver
serviced service start Zope

Change directory to tmp, make sure you put the zenpack in /tmp

cd /tmp

Install the Zenpack

serviced service run zope zenpack-manager install *.egg


serviced service run zope zenpack-manager install ZenPacks.zenoss.MikroTik-1.1.1.egg

Restart the Zenoss Service

serviced service restart zenoss.core

Log into Zenoss and check it.

How To change the Hostname in CentOS 7

Check current hostname with the hostname command.


Change hostname with hostnamectl

hostnamectl set-hostname newhostname



Changing hostname from “localhost” to “newhost”.

[root@localhost ~]# hostname
[root@localhost ~]# hostnamectl set-hostname newhost
[root@localhost ~]# hostname
[root@localhost ~]# 



How To Enable/Disable SELinux

Disable SELinux

You can manually edit the SELinux config file in /etc/selinux/config and change the variable SELINUX=enforcing to disabled

vi /etc/selinux/config

or you can use this little command

sed -i.bak -e 's/^SELINUX=.*/SELINUX=disabled/g' /etc/selinux/config

Enable SELinux

You can enable SELinux manually the same as above but set SELINUX=disabled to SELINUX=enforcing

vi /etc/selinux/config


sed -i.bak -e 's/^SELINUX=.*/SELINUX=enforcing/g' /etc/selinux/config

How to View Installed Programs in linux

Debian Based Distros

The below command should work for Debain, Ubuntu, and the Raspberry Pi Raspbian.

dpkg --get-selections


RPM Based Distros

The following works on Fedora, CentOS, ReHat.

rpm -qa


The above commands return all of the packages installed on a system.  If you want to look for a specific program or package you can use grep to filter the results.

rpm -qa | grep program


dpkg --get-selections | grep program

How to set a static ip address on CentOS, Fedora, or Red Hat Linux

Open up the following file with your favorite text editor. Change eth0 to the interface you need, like “wlan0” or “eth1”.

 vi /etc/sysconfig/network-scripts/ifcfg-eth0

The file should look something like the following.


To set the static IP address change “BOOTPROTO=dhcp” to “BOOTPROTO=static” and add the following to the end of the file. If you want/need the interface to come up when the computer boots up then be sure to change “ONBOOT=no” to “ONBOOT=yes”.


So your file should now look like this.


Save the file and restart networking.

service network restart

Finally, check you IP address with ifconfig.

root@localhost ~]# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 0A:2G:F3:56:66:4B  
          inet addr:  Bcast:  Mask:


How to Open a Port on Redhat, CentOS or Fedora Linux

By default iptables firewall stores its configuration at /etc/sysconfig/iptables file. You need to edit this file and add rules to open port number.

Open port 80

To open a different port just enter the port number.

Open flle /etc/sysconfig/iptables:

# vi /etc/sysconfig/iptables

Append rule as follows:

A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT

Save and close the file.  Restart iptables:

# /etc/init.d/iptables restart


#service iptables restart


Verify that port is open

Run the following command:

netstat -tulpn | less

Make sure iptables is allowing the port you enabled:

iptables -L -n

If you need more info you can refer to the man page:

man iptables