Networks and Hosts Design and Diagnosis

# 4.3 Linux Host Security

### Remove unnecessary software

Unnecessary software occupies disk space and could introduce security flaws. To remove unnecessary software enter one of the following commands:

– yum list installed or dnf list installed to see installed RPM packages on the computer.

– apt

    – apt autoremove automatically removes unused packages

    – apt list list all installed packages

– dpkg get-selections to see installed Debian packages on the computer.

Use one of the following commands to uninstall unnecessary packages.

– yum erase packagename

– dnf remove packagename

– apt remove packagename

– rpm -e packagename

– dpkg -r packagename

### Check for unnecessary network services  

Unnecessary network services waste computer resources and increase the system’s attack service. To remove unnecessary network services:

1. Find all installed services and determine which are not needed:  DNS, SNMP, DHCP and others.

    – systemctl –type=service –state=active

2. Use the man command and the Internet to research services you don’t recognize.

    – If the service is not needed, determine if it is a dependency for another service.

3. Disable the service by using the following command:

    – systemctl disable servicename

4. Use one of the following commands to immediately stop the script:

    – systemctl stop servicename

5. Use one of the following commands to remove the script package entirely.

    – yum erase packagename

    – dnf remove packagename

    – apt remove packagename

    – rpm -e packagename

    – dpkg -r packagename

### Use NMAP

1. Install the nmap utility if it is not already installed.

    – yum install nmap

    – dnf install nmap

    – apt -i nmap

2. Use both of the following commands to scan for open ports:

    – nmap -sT ipaddress|fqdn scans for TCP ports

    – nmap -sU ipaddress|fqdn scans for UDP ports

3. Disabled any services not needed:

    – systemctl disable servicename

    – systemctl stop servicename

## Use netstat

***netstat***

-a lists both listening and non-listening sockets.

-l (lowercase ‘L’) lists listening sockets.

-s displays statistics for each protocol.

-i displays a table of all network interfaces.