**Note: The content in this article is only for educational purposes and understanding of cybersecurity concepts. It should enable people and organizations to have a better grip on threats and know how to protect themselves against them. Please use this information responsibly.**
In the world of cybersecurity, sniffing has a big impact on network troubleshooting and malicious activities. Passive sniffing watches network traffic, active sniffing deliberately interferes with network operations to capture traffic that is not intended for the attacker’s device. This article will dig deep into active sniffing and its usual methods and give you a hands-on guide on how to do active sniffing in a safe, ethical setting.
What is Active Sniffing?
Active sniffing is a way attackers mess with networks to grab and watch data they shouldn’t see. This trick works well on switched networks, which send packets just to the right places. Attackers can send fake packets or pretend to be other devices on the network. This lets them redirect traffic through their own device, giving them a chance to intercept data.
Key Differences Between Passive and Active Sniffing
| Passive Sniffing | Active Sniffing |
| Listens to broadcast traffic. | Actively interferes with or manipulates network traffic. |
| Works on hubs (non-switched networks) | Effective on switched networks |
| Harder to detect | More detectable due to network changes |
How Active Sniffing Works
To get around the traffic-filtering systems of a switched network active sniffing employs different tactics to redirect or copy traffic through the attacker’s device. This involves:
- Injecting false packets to manipulate network behavior.
- Sending spoofed information to confuse network devices.
- Redirecting or duplicating traffic to capture data not originally directed to the attacker.
The following are the most common methods of active sniffing:
Common Techniques of Active Sniffing
1. ARP Spoofing
What It Is: ARP (Address Resolution Protocol) Spoofing, or ARP Poisoning, tricks network devices into associating the attacker’s MAC address with another device’s IP address.
How It Works: The attacker sends fake ARP messages to the network, falsely associating their MAC address with a legitimate device’s IP. This redirection forces data meant for that device to pass through the attacker’s machine.
Effect: Allows interception of traffic, often between a user and the network gateway, enabling potential data theft or modification.
2. MAC Flooding
What It Is: MAC flooding overloads a network switch’s MAC address table with a large number of fake MAC addresses.
How It Works: When the switch’s MAC table is full, it defaults to broadcasting all incoming packets to every device, similar to how a hub functions.
Effect: The attacker can then capture traffic that would otherwise be directed only to specific devices, as the switch’s capacity to filter traffic is compromised.
3. DNS Spoofing
What It Is: DNS spoofing (or DNS poisoning) redirects traffic by falsifying DNS responses, sending users to malicious websites instead of legitimate destinations.
How It Works: By inserting false DNS information into the DNS cache of a user’s device or network, the attacker can resolve a domain to a a different IP address. This is often achieved through rogue DNS servers or by intercepting DNS responses.
Effect: Users trying to access trusted websites may unknowingly visit malicious sites instead, exposing them to phishing or malware.
The Impact of Active Sniffing
Active sniffing has a significant impact on network security, as it enables attackers to intercept sensitive information, redirect traffic, and even manipulate data. However, unlike passive sniffing, active sniffing is more detectable because it alters normal network behavior, potentially triggering alerts for network administrators. The impact can be critical, including:
- Interception of private communications
- Redirection to malicious sites for phishing attacks
- Potential data manipulation
Because of these dangers, network admins who play by the rules use active sniffing. They use it in controlled security checks to look for vulnerabilities.
Performing Active Sniffing Using ARP Spoofing
Requirements:
- A computer running Kali Linux or a similar penetration-testing environment.
- ARP spoofing tool (such as `arpspoof` or `ettercap`).
Step-by-Step Guide to ARP Spoofing
How ARP works?
- (A) knows (B)’s IP address but wants its MAC address.
- (A) Will 1st check for (B)’s MAC address in its ARP table. If not found, then: -
- (A) will send an ARP request as a broadcast.
- All systems in the Local Area Network (LAN) receive the message, check destination IP and drop the packet if it doesn’t belong to them.
- The intended device (B) then sends out its MAC address as a unicast reply to the sender (A)
- (A) updates its ARP table (also called ARP cache).
MITM (Man in The Middle) attack is performed on Address Resolution Protocol (ARP) by ARP Spoofing or ARP Poisoning (used interchangeably). In this, the attacker sends out many ARP requests to both targets. This would change the data of the ARP cache. By this, the attacker impersonates the location of a specific IP address another user is trying to reach. This would lead to the target sending its data to the attacker.
Now, let us demonstrate this using Ettercap GUI.
- Login to your Kali Linux machine and make sure your target system is logged in (we are using a Debian virtual machine in this case).
- Open Ettercap GUI in Kali Linux System.
- Select sniffing at startup and the appropriate primary interface (eth0 in this case) and click on the tick to accept.
- Click on menu -> Hosts -> Hosts list
Sometimes, this may not show scan results immediately. So, to get the scan results we can click the “Search icon”.
- This will show the target IP address (in this case, the Debian system that is on the same Local Area Network) and the default gateway. We will add the target system to target 1 and the default gateway to target 2. This is because we have to sniff the data that the Debian system will send to the default gateway using our Kali Linux machine. Since we are using a virtualized environment (VMware) here, the default gateway for all the virtual machines will be the same. The default gateway is the entry point and exit point in this virtual machine subnet. So, it acts as a routing device that knows how to pass the traffic between different subnets and networks.
192.168.150.132 is the IP address of Debian target VM and 192.168.150.2 is its default gateway.
- Now select MITM menu -> ARP Poisoning
- Select “Sniff remote connections”. This sends many ARP requests to the targets.
- We have selected the URL http://testphp.vulnweb.com on our target machine (i.e. Debian system). We go to sign up and enter the username and password.
9. The Ettercap GUI in our attacker machine now captures all the details. It shows the website visited, the protocol used, the port number IP of the server, and the username and password entered.
Note that as the website is using HTTP protocol, the username and password are captured in plain text since no encryption or hashing happens in HTTP protocol. Hence to bypass this, most websites nowadays use the HTTPS protocol.
Detection and Prevention
Due to the disruption caused by active sniffing, network security systems may detect and alert administrators to ARP spoofing attempts. Standard preventive measures include:
- Static ARP entries to prevent spoofing on crucial network devices.
- Port security features to limit MAC address flooding.
- Intrusion Detection Systems (IDS) that monitor for abnormal ARP traffic.
Conclusion
Active sniffing is a highly effective but easily detectable method of intercepting and examining network traffic on switched networks. While there certainly are legitimate uses for active sniffing in security testing, all unauthorized active sniffing is illegal and, therefore, unethical. This chapter’s hands-on tutorial illustrates how one of the most widely used techniques – ARP spoofing – can be carried out in a controlled lab environment, explains how attackers intercept traffic, and describes how such attacks can be detected and mitigated.
Active sniffing requires strong network security measures, which include intrusion of ARP protection, DNS security, and proactive monitoring, among others, to prevent data piracy.
