In the ever-evolving landscape of cybersecurity, Linux system administrators and security professionals face constant challenges in protecting their systems from various threats. While advanced security tools have their place, the power of basic Linux command line tools for forensic analysis should not be underestimated. This guide will walk you through practical approaches to detect and respond to suspicious activities using simple, built-in Linux commands.

The 1000:1 Rule in Linux Forensics

One crucial concept in Linux forensics is the "1000:1 rule." This principle highlights an important asymmetry in cybersecurity:

• Defenders must be aware of thousands of potential attack vectors.
• Attackers only need to find one successful method to compromise a system.
• Once a system is compromised, defenders need to find just one piece of evidence to uncover the breach.

This rule emphasizes the importance of thorough, systematic examination of your Linux systems using readily available tools.

Prioritizing Common Attacks in Linux Security

While Zero Day exploits often grab headlines, they are rare and typically expensive to deploy. Instead, focus your efforts on detecting and preventing common attacks, which are:

1. More prevalent
2. Equally damaging
3. Often provide attackers with deniability

By addressing these common threats, you significantly improve your overall security posture.

Key Areas of Focus in Linux Forensics

When investigating a Linux system for potential security breaches, concentrate on these three main areas:
1. Suspicious Directories
Directories form the backbone of the Linux file system. Look out for:

• Directory Mimicry: Folders attempting to masquerade as legitimate system directories
• Hidden Directories: Concealed folders, often using special characters or unusual naming conventions
• Unusual Attributes: Directories with abnormal permissions, creation dates, or those differing from a clean system image

2. Suspicious Files
Files often contain telltale signs of system compromise. Be alert for:

• Tampered Logs: Especially audit logs that have been erased or reduced to zero bytes
• Misleading File Types: Files with extensions that don't match their actual content
• Unusual File Locations: Legitimate system files appearing in unexpected places or modified binaries

3. Suspicious Processes
Running processes can reveal ongoing malicious activities. Watch for:

• Deceptive Process Names: Malicious processes often mimic names of legitimate system processes
• Unexpected Open Ports: Unfamiliar open ports or outbound connections
• Deleted but Active Processes: Processes that continue running despite being deleted from disk

Essential Linux Commands for Forensic Analysis

Leverage these powerful, built-in Linux commands for effective forensic investigation:

1. ls - List files and directories
   • Usage: ls -la /path/to/directory
   • Purpose: Identify hidden or suspicious files and folders
2. ps - Display running processes
   • Usage: ps aux or ps -ef
   • Purpose: Spot unusual or potentially malicious processes
3. netstat - Show network connections and listening ports
   • Usage: netstat -tuln
   • Purpose: Detect unexpected network activity or open ports
4. strings - Extract readable text from binary files
   • Usage: strings /path/to/suspicious/file
   • Purpose: Analyze binaries for suspicious content or hidden information
5. last - View recent login activity
   • Usage: last
   • Purpose: Identify unauthorized access attempts or successful logins

Implementing a Regular Security Check Routine

To maintain a secure Linux environment, implement these best practices:

1. Establish a baseline of your system's normal state
2. Regularly scan for new or modified files in sensitive directories
3. Monitor running processes and compare against known-good configurations
4. Review network connections and open ports periodically
5. Analyze system logs for unusual activities or login attempts

Effective Linux forensics doesn't always require advanced or expensive tools. By leveraging basic Linux commands and adopting a systematic approach to system analysis, you can uncover significant security issues and maintain a robust defense against common threats.
Remember, the 1000:1 rule works in your favor as a defender. Even the most sophisticated attacker is likely to leave traces that can be detected through careful observation and the use of these simple yet powerful Linux commands.

Stay vigilant, keep your Linux systems updated, and regularly perform these basic checks to ensure the ongoing security of your Linux environment.
Additional Resources

• Linux Security Basics
• NIST Guide to Computer Security Log Management
• The Art of Command Line
• Sherlock Linux

​
By mastering these fundamental Linux forensics techniques, you'll be well-equipped to detect and respond to potential security threats, safeguarding your systems and data from compromise.

Linux Mnemomic: 
List of directories at the root level and a mnemonic to remember them.
bin, boot, dev, etc, home, lib, mnt, media, sbin, usr, var​

"Binny’s boot doesn’t even have leather material; might sell used version"

Let's understand each of the Linux root-level directory functions:
bin: User Binary (bin) contains common commands like cd, ls, and ps etc used by all the systems.
boot: Contains Bootloader related information
dev: Device Files contains specials files to represent the attached devices tot he system such as USB etc. Also contain the VM's

etc.: contains configuration and system scripts such as the start/stop of each program. No binaries in this directory. A couple of key files in etc is 

• passwd: This file contains information about all users accessing the machine, including their names, the path to their home directory, and all programs that start running when they log in.
• shadow: This file contains the authentication info of the user, including username and salted password
• profile:  Files and commands that start up at the login or startup and can be set at the Global Level.
• networks:  Contains the names of the network that the system is located on.
• hosts:  The file contains the machine IP address if it's connected to the network
• cron: contains cron scripts
• init.d: Malware uses it to hide and initiate execution upon the system startup

home: Contains Data files and config files of a paticular user
lib: contains software libraries and kernel modules required by /bin and /sbin
mnt:: Temporary Mounting point for the removable external and remote file system 
media: Mounted and Unmount information about each media like CD -Rom
sbin: System Administrator Binaries (sbin) contain root only binaries such is ifconfig, fdisk, \  
usr: Contains binaries that are used by a User
var: Contains Security and Application Logs. syslog, varlog, faillog, also contain /var/tmp

Note: Remember Linux Directories are case-sensitive