dos attack: ack scan

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23-10-2024

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The Ack Scan: A Stealthy Approach to Denial-of-Service Attacks

Denial-of-Service (DoS) attacks aim to disrupt the normal functioning of a network or a service by overwhelming it with traffic. One of the many techniques used in DoS attacks is the Ack scan, a stealthy method that tries to avoid detection by security systems. This article explores the Ack scan, its mechanics, and how it can be used to launch DoS attacks.

What is an Ack scan?

An Ack scan is a type of network scan that exploits the TCP/IP protocol's handshake mechanism. It involves sending only the acknowledgement (ACK) packet in a TCP connection sequence, bypassing the initial SYN (synchronization) and FIN (finish) packets.

Why is the Ack scan stealthy?

"ACK scans, a stealthy form of port scanning, are often used to identify open ports on target hosts while avoiding detection by intrusion detection systems (IDSs). They work by sending only acknowledgment (ACK) packets to the target ports." - Network Security: Principles and Practices, 5th Edition by William Stallings (2017)

Traditional port scans send SYN packets, which trigger alerts on IDSs designed to detect such activity. However, an Ack scan only sends ACK packets, which are expected in a normal TCP communication and are less likely to be flagged as malicious.

How does an Ack scan work in a DoS attack?

"In a DoS attack using an Ack scan, the attacker sends a large number of ACK packets to the target host. The target host, expecting a corresponding SYN packet, will try to establish a connection with the attacker. However, the attacker never sends a SYN packet, leaving the target host in a state of limbo. This can cause the target host's resources to become overwhelmed, leading to a Denial-of-Service attack." - Computer Networks and Internets by Douglas Comer (2013)

The attacker floods the target with ACK packets, forcing it to allocate resources for nonexistent connections. This creates a backlog, ultimately leading to service denial for legitimate users.

Example: A Denial-of-Service attack with an Ack scan

Imagine a server hosting a website. When a user requests a webpage, a TCP connection is established between the user's computer and the server. This involves sending SYN, ACK, and FIN packets.

In an Ack scan DoS attack, an attacker sends numerous ACK packets to the server, pretending to be legitimate users. The server, expecting corresponding SYN packets, will begin allocating resources for these connections. Since the attacker doesn't send SYN packets, the server never receives the complete handshake and is stuck in a state of waiting.

Eventually, the server's resources are depleted due to the backlog of incomplete connections, leading to a denial of service for legitimate users trying to access the website.

Mitigating Ack scan DoS attacks

"Defense against ACK scan DoS attacks involves implementing intrusion detection systems (IDSs) that can detect anomalous traffic patterns, including the large volume of ACK packets. Furthermore, network firewalls can be configured to block or limit ACK packets from unknown or suspicious sources." - Computer Security: Principles and Practice by William Stallings (2017)

Here are some key mitigation strategies:

• Intrusion Detection Systems (IDSs): IDSs can be configured to monitor network traffic for unusual patterns like a sudden surge in ACK packets from unknown sources.
• Firewall rules: Configuring firewalls to block or limit ACK packets from untrusted networks can effectively mitigate Ack scan attacks.
• Rate limiting: Limiting the number of ACK packets allowed from each source can prevent attackers from overwhelming the target server.
• TCP SYN cookie: This technique protects against SYN flood attacks, a related DoS attack, and can also help mitigate Ack scan attacks.

Conclusion:

The Ack scan is a subtle and effective method for launching DoS attacks. By exploiting the TCP handshake mechanism, attackers can overwhelm target servers with ACK packets, causing service denial for legitimate users. Understanding the mechanics of the Ack scan and implementing appropriate security measures is crucial for protecting systems against this type of attack.