Deploy, Configure, and Maintain Systems

1. Network Configuration

Introduction: Network configuration is the process of setting up and managing network interfaces to enable communication between systems. This involves assigning IP addresses, configuring network interfaces, and setting up routing. Proper network configuration is crucial for ensuring reliable and efficient network communication.

Fundamentals:

• Network Interfaces: Network interfaces are the hardware or virtual points of connection between a computer and a network. They can include Ethernet ports, wireless adapters, and loopback interfaces. Properly managing these interfaces ensures that the system can send and receive data over the network.

Commands:

• Check Network Interfaces:

ip a

Displays all network interfaces and their statuses, including IP addresses, MAC addresses, and other details.

• Bring an Interface Up or Down:

ip link set dev <interface> up

ip link set dev <interface> down

Activates (up) or deactivates (down) a network interface, enabling or disabling its ability to send and receive data.

• Assign an IP Address:

ip addr add <IP_address>/<subnet_mask> dev <interface>

Assigns a specific IP address and subnet mask to a network interface, configuring it for network communication.

• IP Addressing: Each device on a network must have a unique IP address to communicate with other devices. IP addresses can be assigned manually (static IP) or automatically (dynamic IP) via DHCP. The IP address, along with the subnet mask, defines the network and the host portion of the address.

Commands:

• Add a Default Gateway:

ip route add default via <gateway_IP>

Configures the default gateway for the system, specifying the router that should handle traffic destined for external networks.

• Routing: Routing determines the path that data takes to reach its destination. The default gateway is the router that forwards traffic from the local network to other networks, such as the internet. Proper routing configuration ensures that data packets are directed correctly.
• Configuration Files: Network settings are often made persistent by configuring network scripts and configuration files. These files define the settings for each network interface and ensure that the configurations are applied at boot time.

Example Configuration File:

• Location: /etc/sysconfig/network-scripts/ifcfg-<interface>

TYPE=Ethernet

BOOTPROTO=none

NAME=eth0

DEVICE=eth0

ONBOOT=yes

IPADDR=192.168.1.10

NETMASK=255.255.255.0

GATEWAY=192.168.1.1

DNS1=8.8.8.8

This file specifies the network configuration for the eth0 interface, including its type, IP address, subnet mask, gateway, and DNS server.

2. Network Time Protocol (NTP)

Introduction: Network Time Protocol (NTP) is used to synchronize the clocks of computers over a network. Accurate timekeeping is essential for system operations, including logging events, maintaining security, and coordinating tasks across multiple systems.

Fundamentals:

• Time Synchronization: Synchronizing system clocks across all network devices ensures consistency in time-stamped events, which is crucial for troubleshooting, security auditing, and data integrity.

Commands:

• Install NTP:

yum install ntp

Installs the NTP package, providing the necessary tools and services for time synchronization.

• NTP Servers: NTP servers provide the correct time to clients. Servers can be public (e.g., pool.ntp.org) or private within an organization’s network. Clients query these servers to adjust their system clocks.

Configuration File:

• Location: /etc/ntp.conf

server 0.centos.pool.ntp.org iburst

server 1.centos.pool.ntp.org iburst

server 2.centos.pool.ntp.org iburst

server 3.centos.pool.ntp.org iburst

Specifies the NTP servers to be used for time synchronization. The iburst option allows for quicker synchronization when the server is reachable.

Commands:

• Start and Enable NTP Service:

systemctl start ntpd

systemctl enable ntpd

Starts the NTP service and enables it to start automatically on boot, ensuring continuous time synchronization.

• Check NTP Status:

ntpq -p

Displays the status of NTP peers and synchronization, showing information about the NTP servers being used and their synchronization states.

3. Scheduling Tasks

Introduction: Scheduling tasks involves automating repetitive system administration tasks such as backups, updates, and maintenance. This is achieved using utilities like cron and at, which allow tasks to be scheduled at specific times or intervals.

Fundamentals:

• cron: cron is a time-based job scheduler that allows users to schedule jobs (commands or scripts) to run periodically at fixed times, dates, or intervals. The scheduling information is stored in crontab files.

Commands:

• Edit Crontab:

crontab -e

Opens the crontab file for editing, allowing users to schedule tasks by specifying the timing and the command to be executed.

Crontab File Example:

• Location: /etc/crontab or user-specific with crontab -e

* * * * * command_to_execute

This format represents a cron job to be executed every minute of every day.

• Example:

0 2 * * * /usr/local/bin/backup.sh

Schedules the backup.sh script to run daily at 2:00 AM.

• at: at is a utility that schedules a command to run once at a specific time in the future. Unlike cron, which is used for recurring tasks, at is used for one-time tasks.

Commands:

• Schedule a One-Time Task:

echo “/usr/local/bin/backup.sh” | at 2am

Schedules the backup.sh script to run once at 2:00 AM.

• List Scheduled at Jobs:

atq

Lists all scheduled at jobs, displaying their job IDs and scheduled times.

• Remove a Scheduled at Job:

atrm <job_id>

Removes a scheduled at job using its job ID, preventing it from being executed.

• Crontab Files: Crontab files define the schedule and the commands to be executed. Each line in a crontab file represents a task and includes fields for minute, hour, day of month, month, and day of week, followed by the command to be executed.
• Automation Benefits: Automating tasks reduces manual intervention, minimizes errors, and ensures tasks are executed consistently and on time.

4. Modifying the System Bootloader

Introduction: The bootloader is a crucial component that loads the operating system kernel during the boot process. GRUB2 (GRand Unified Bootloader version 2) is the default bootloader for many Linux distributions, including RHEL and CentOS. Modifying the bootloader configuration is essential for tasks such as changing boot parameters, troubleshooting, and managing multiple operating systems.

Fundamentals:

• GRUB2: GRUB2 is a versatile and powerful bootloader capable of loading a variety of operating systems. It provides a menu from which users can select the OS to boot.
• Configuration Files: The main configuration file for GRUB2 specifies various settings, including the default boot entry, timeout duration, and kernel parameters. Changes to the configuration file must be applied to the GRUB2 configuration.

Configuration File Example:

• Location: /etc/default/grub

GRUB_TIMEOUT=5

GRUB_DISTRIBUTOR=”$(sed ‘s, release .*$,,g’ /etc/system-release)”

GRUB_DEFAULT=saved

GRUB_DISABLE_SUBMENU=true

GRUB_TERMINAL_OUTPUT=”console”

GRUB_CMDLINE_LINUX=”rd.lvm.lv=centos/swap crashkernel=auto rhgb quiet”

GRUB_DISABLE_RECOVERY=”true”

Defines GRUB2 settings such as the timeout period before booting the default entry, kernel parameters, and display options.

Commands:

• Update GRUB2 Configuration:

grub2-mkconfig -o /boot/grub2/grub.cfg

Generates a new GRUB2 configuration file based on the settings in /etc/default/grub and other scripts in /etc/grub.d/.

• Set Default Boot Entry:

grub2-set-default <menu_entry>

Sets the default boot entry by specifying the menu entry to be used by default.

• View GRUB2 Menu Entries:

awk -F\’ ‘/menuentry / {print $2}’ /boot/grub2/grub.cfg

Lists the available GRUB2 menu entries, allowing users to identify the entry names.

• Boot Entries: Boot entries in GRUB2 define the available operating systems or kernels to boot. Each entry includes parameters such as the kernel to load and any specific options to pass to the kernel.
• Updating Configuration: After modifying the bootloader settings, the configuration must be updated to apply the changes. This ensures that the new settings are used during the next boot.

5. Updating and Installing Packages

Introduction: Package management is essential for maintaining the system by installing, updating, and removing software. YUM (Yellowdog Updater, Modified) is the package manager used in RHEL and CentOS. It simplifies the process of managing software packages, including resolving dependencies and ensuring that the system has the latest security patches and software updates.

Fundamentals:

• Package Installation: Installing new software packages ensures that the system has the necessary tools and applications to perform various tasks. YUM handles the installation process, including dependency resolution.

Commands:

• Install a Package:

yum install <package_name>

Installs the specified package, along with any required dependencies.

• Package Updating: Keeping software up to date is crucial for security and performance. Updates may include security patches, bug fixes, and new features. YUM allows for seamless updates of installed packages.

Commands:

• Update a Package:

yum update <package_name>

Updates the specified package to the latest available version.

• Package Removal: Removing unnecessary or obsolete packages helps maintain a clean system and frees up resources. YUM ensures that dependencies are managed correctly during the removal process.

Commands:

• Remove a Package:

yum remove <package_name>

Removes the specified package and any dependencies that are no longer needed.

• Repositories: YUM uses repositories, which are collections of packages. Configuring repositories correctly ensures access to the required software and updates.

Commands:

• List Installed Packages:

yum list installed

Displays all packages currently installed on the system.

• Search for Packages:

yum search <package_name>

Searches for packages in the configured repositories that match the specified name or description.

• Configuration Files: YUM configuration files define various settings such as repository locations, caching options, and behavior during package management operations.

Configuration File Example:

• Location: /etc/yum.repos.d/<repo_file>.repo

[base]

name=CentOS-$releasever – Base

baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/

enabled=1

gpgcheck=1

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

Configures a YUM repository, specifying its name, base URL, and GPG key for verifying package integrity.

6. Using LDAP for Authentication

Introduction: LDAP (Lightweight Directory Access Protocol) is used for centralized authentication, allowing users to log in to multiple systems with a single set of credentials. This enhances security and simplifies user management. LDAP integrates with other services like Kerberos for secure authentication.

Fundamentals:

• Centralized Authentication: Using LDAP for authentication centralizes user management, reducing the need to manage individual user accounts on each system. Users can authenticate against the LDAP directory, ensuring consistency and ease of management.

Commands:

• Install LDAP Client Packages:

yum install sssd oddjob-mkhomedir authconfig

Installs the necessary packages for integrating the system with an LDAP server and managing user home directories.

• LDAP Structure: LDAP directories are hierarchical structures of entries, where each entry represents a user, group, or resource. The entries are organized in a tree-like structure, with the root at the top.

Configuration Files:

• SSSD Configuration Example:
  • Location: /etc/sssd/sssd.conf

[sssd]

config_file_version = 2

services = nss, pam

domains = example.com

[domain/example.com]

id_provider = ldap

auth_provider = ldap

ldap_uri = ldap://ldap.example.com

ldap_search_base = dc=example,dc=com

• Set Correct Permissions:

chmod 600 /etc/sssd/sssd.conf

Configures SSSD to use LDAP for authentication and ensures the configuration file has the correct permissions for security.

Commands:

• Configure LDAP Authentication:

authconfig –enableldap –enableldapauth –ldapserver=<LDAP_server> –ldapbasedn=”dc=example,dc=com” –enablemkhomedir –update

Configures the system to use LDAP for authentication and sets up automatic home directory creation for LDAP users.

• Start and Enable SSSD:

systemctl start sssd

systemctl enable sssd

Starts the SSSD service and enables it to start automatically on boot, ensuring continuous LDAP authentication.

• Test LDAP Configuration:

getent passwd <username>

Retrieves the user information from the LDAP directory, verifying that LDAP authentication is working correctly.

• Integration with Other Services: LDAP often integrates with other services like Kerberos for secure authentication and SSSD (System Security Services Daemon) for managing access to remote directories.
• Security: LDAP can be configured to use secure connections (LDAPS) and integrate with Kerberos to provide robust and secure authentication mechanisms, ensuring the integrity and confidentiality of user credentials.

This comprehensive article provides the essential theoretical background, commands, and configurations for Deploy, Configure, and Maintain Systems, including descriptions of what each command does.