Welcome back, aspiring forensic investigators.
Linux is everywhere today. It runs web servers, powers many smartphones, and can even be found inside the infotainment systems of cars. A few reasons for its wide use are that Linux is open source, available in many different distributions, and can be tailored to run on both powerful servers and tiny embedded devices. It is lightweight, modular, and allows administrators to install only the pieces they need. Those qualities make Linux a core part of many organizations and of our daily digital lives. Attackers favour Linux as well. Besides being a common platform for their tools, many Linux hosts suffer from weak monitoring. Compromised machines are frequently used for reverse proxies, persistence, reconnaissance and other tasks, which increases the need for forensic attention. Linux itself is not inherently complex, but it can hide activity in many small places. In later articles we will dive deeper into what you can find on a Linux host during an investigation. Our goal across the series is to build a compact, reliable cheat sheet you can return to while handling an incident. The same approach applies to Windows investigations as well.
Today we will cover the basics of Linux forensics. For many incidents this level of detail will be enough to begin an investigation and perform initial response actions. Let’s start.
OS & Accounts
OS Release Information
The first thing to check is the distribution and release information. Different Linux distributions use different defaults, package managers and filesystem layouts. Knowing which one you are examining helps you predict where evidence or configuration will live.
bash> cat /etc/os-release
Common distributions and their typical uses include Debian and Ubuntu, which are widely used on servers and desktops. They are stable and well documented. RHEL and CentOS are mainly in enterprise environments with long-term support. Fedora offers cutting-edge features, Arch is rolling releases for experienced users, Alpine is very small and popular in containers. Security builds such as Kali or Parrot have pentesting toolsets. Kali contains many offensive tools that hackers use and is also useful for incident response in some cases.
Hostname
Record the system’s hostname early and keep a running list of hostnames you encounter. Hostnames help you map an asset to network records, correlate logs across systems, identify which machine was involved in an event, and reduce ambiguity when combining evidence from several sources.
bash> cat /etc/hostname
bash> hostname
Timezone
Timezone information gives a useful hint about the likely operating hours of the device and can help align timestamps with other systems. You can read the configured timezone with:
bash> cat /etc/timezone
User List
User accounts are central to persistence and lateral movement. Local accounts are recorded in /etc/passwd (account metadata and login shell) and /etc/shadow (hashed passwords and aging information). A malicious actor who wants persistent access may add an account or modify these files. To inspect the user list in a readable form, use:
bash> cat /etc/passwd | column -t -s :
You can also list users who are allowed interactive shells by filtering the shell field:
bash> cat /etc/passwd | grep -i 'ash'
Groups
Groups control access to shared resources. Group membership can reveal privilege escalation or lateral access. Group definitions are stored in /etc/group. View them with:
bash> cat /etc/group
Sudoers List
Users who can use sudo can escalate privileges. The main configuration file is /etc/sudoers, but configuration snippets may also exist under /etc/sudoers.d. Review both locations:
bash> ls -l /etc/sudoers.d/
bash> sudo cat /etc/sudoers
Login Information
The /var/log directory holds login-related records. Two important binary files are wtmp and btmp. The first one records successful logins and logouts over time, while btmp records failed login attempts. These are binary files and must be inspected with tools such as last (for wtmp) and lastb (for btmp), for example:
bash> sudo last -f /var/log/wtmp
bash> sudo lastb -f /var/log/btmp
System Configuration
Network Configuration
Network interface configuration can be stored in different places depending on the distribution and the network manager in use. On Debian-based systems you may see /etc/network/interfaces. For a quick look at configured interfaces, examine:
bash> cat /etc/network/interfaces
bash> ip a show
Active Network Connections
On a live system, active connections reveal current communications and can suggest where an attacker is connecting to or from. Traditional tools include netstat:
bash> netstat -natp
A modern alternative is ss -tulnp, which provides similar details and is usually available on newer systems.
Running Processes
Enumerating processes shows what is currently executing on the host and helps spot unexpected or malicious processes. Use ps for a snapshot or interactive tools for live inspection:
bash> ps aux
If available, top or htop give interactive views of CPU/memory and process trees.
DNS Information
DNS configuration is important because attackers sometimes alter name resolution to intercept or redirect traffic. Simple local overrides live in /etc/hosts. DNS server configuration is usually in /etc/resolv.conf. Often attackers might perform DNS poisoning or tampering to redirect victims to malicious services. Check the relevant files:
bash> cat /etc/hosts
bash> cat /etc/resolv.conf
Persistence Methods
There are many common persistence techniques on Linux. Examine scheduled tasks, services, user startup files and systemd units carefully.
Cron Jobs
Cron is often used for legitimate scheduled tasks, but attackers commonly use it for persistence because it’s simple and reliable. System-wide cron entries live in /etc/crontab, and individual service-style cron jobs can be placed under /etc/cron.d/. User crontabs are stored under /var/spool/cron/crontabs on many distributions. Listing system cron entries might look like:
bash> cat /etc/crontab
bash> ls /etc/cron.d/
bash> ls /var/spool/cron/crontabs
Many malicious actors prefer cron because it does not require deep system knowledge. A simple entry that runs a script periodically is often enough.
Services
Services or daemons start automatically and run in the background. Modern distributions use systemd units which are typically found under /etc/systemd/system or /lib/systemd/system, while older SysV-style scripts live in /etc/init.d/. A quick check of service scripts and unit files can reveal backdoors or unexpected startup items:
bash> ls /etc/init.d/
bash> systemctl list-unit-files --type=service
bash> ls /etc/systemd/system
.Bashrc and Shell Startup Files
Per-user shell startup files such as ~/.bashrc, ~/.profile, or ~/.bash_profile can be modified to execute commands when an interactive shell starts. Attackers sometimes add small one-liners that re-establish connections or drop a backdoor when a user logs in. The downside for attackers is that these files only execute for interactive shells. Services and non-interactive processes will not source them, so they are not a universal persistence method. Still, review each user’s shell startup files:
bash> cat ~/.bashrc
bash> cat ~/.profile
Evidence of Execution
Linux can offer attackers a lot of stealth, as logging can be disabled, rotated, or manipulated. When the system’s logging is intact, many useful artifacts remain. When it is not, you must rely on other sources such as filesystem timestamps, process state, and memory captures.
Bash History
Most shells record commands to a history file such as ~/.bash_history. This file can show what commands were used interactively by a user, but it is not a guaranteed record, as users or attackers can clear it, change HISTFILE, or disable history entirely. Collect each user’s history (including root) where available:
bash> cat ~/.bash_history
Tmux and other terminal multiplexers themselves normally don’t provide a persistent command log. Commands executed in a tmux session run in normal shell processes. Whether those commands are saved depends on the tmux configurations.
Commands Executed With Sudo
When a user runs commands with sudo, those events are typically logged in the authentication logs. You can grep for recorded COMMAND entries to see what privileged commands were executed:
bash> cat /var/log/auth.log* | grep -i COMMAND | less
Accessed Files With Vim
The Vim editor stores some local history and marks in a file named .viminfo in the user’s home directory. That file can include command-line history, search patterns and other useful traces of editing activity:
bash> cat ~/.viminfo
Log Files
Syslog
If the system logging service (for example, rsyslog or journald) is enabled and not tampered with, the files under /var/log are often the richest source of chronological evidence. The system log (syslog) records messages from many subsystems and services. Because syslog can become large, systems rotate older logs into files such as syslog.1, syslog.2.gz, and so on. Use shell wildcards and standard text tools to search through rotated logs efficiently:
bash> cat /var/log/syslog* | head
When reading syslog entries you will typically see a timestamp, the host name, the process producing the entry and a message. Look for unusual service failures, unexpected cron jobs running, or log entries from unknown processes.
Authentication Logs
Authentication activity, such as successful and failed logins, sudo attempts, SSH connections and PAM events are usually recorded in an authentication log such as /var/log/auth.log. Because these files can be large, use tools like grep, tail and less to focus on the relevant lines. For example, to find successful logins you run this:
bash> cat /var/log/auth.log | grep -ai accepted
Other Log Files
Many services keep their own logs under /var/log. Web servers, file-sharing services, mail daemons and other third-party software will have dedicated directories there. For example, Apache and Samba typically create subdirectories where you can inspect access and error logs:
bash> ls /var/log
bash> ls /var/log/apache2/
bash> ls /var/log/samba/
Conclusion
A steady, methodical sweep of the locations described above will give you a strong start in most Linux investigations. You start by verifying the OS, recording host metadata, enumerating users and groups, then you move to examining scheduled tasks and services, collecting relevant logs and history files. Always preserve evidence carefully and collect copies of volatile data when possible. In future articles we will expand on file system forensics, memory analysis and tools that make formal evidence collection and analysis easier.
Source: HackersArise
Source Link: https://hackers-arise.com/digital-forensics-an-introduction-to-basic-linux-forensics/