Introduction #

In modern computer networks, TCP（Transmission Control Protocol）serves as the foundational protocol for internet communication and plays an indispensable role. TCP is not only responsible for reliable data transfer but also ensures network connection stability and efficiency through complex handshakes and flow control mechanisms. It is a connection-oriented, reliable, byte-stream-based transport layer protocol that ensures the reliable transfer of data between various applications.

However, in the design and implementation of the TCP protocol, there is a mechanism that, although important, is often overlooked—TCP Reset (TCP Reset). In certain situations, TCP sends a “reset” (RST) segment to terminate the currently ongoing data transmission, which is commonly referred to as TCP Reset. This mechanism can have significant negative impacts on network security and system stability in some cases. This article will delve into the hazards and implications of TCP Reset and provide a detailed explanation of how to address this issue.

This technique can be applied in benevolent firewalls but can also be used for internet censorship or attacks, where malicious actors interrupt TCP connections as a form of “man-in-the-middle” attack. The Great Firewall (GFW) automatically executes TCP Reset attacks to block websites that do not comply with local laws and regulations.

Basic Concepts #

Background #

When two computers need to establish a bidirectional virtual connection, TCP is used together with IP (UDP is a connectionless IP protocol). The TCP software on both machines will communicate by exchanging streams of packets (e.g., a computer with a browser and a web server). Using a TCP connection provides an easy way for computers to exchange data items that are too large for single packets, such as video clips, email attachments, or music files. Although some web pages are small enough for single packets, they are still sent through TCP connections for convenience.

TCP Reset #

TCP Reset is an error handling mechanism in the TCP protocol used to quickly restore communication states when network connections encounter abnormalities. When a TCP connection breaks or encounters severe errors (e.g., damaged packets, timeouts), the sender sends a TCP segment with the RST (Reset) flag to the receiver, indicating that the connection should be reset.

The design of the TCP Reset mechanism aims to quickly clean up invalid connections and ensure that network resources are not unnecessarily occupied. However, in practical applications, due to misconfigurations, malicious attacks, or protocol vulnerabilities, TCP Resets can lead to serious security issues and performance risks.

According to its original design, it is a useful tool. A common application is when one computer (Computer A) crashes during a TCP connection. The other end’s computer (Computer B) continues sending TCP packets because it doesn’t know Computer A has crashed. After restarting Computer A, it will receive data packets from the old pre-crash connection. Since Computer A lacks the context of these packets and doesn’t know how to handle them, it can send a TCP Reset to Computer B. This reset informs Computer B that the connection is no longer active. The user on Computer B can now try other connections or take alternative actions.

TCP Reset Spoofing #

In the above scenario, the TCP reset is sent by one of the computers acting as an endpoint in the connection. However, during operation, a third computer can monitor the TCP packets on the connection and then send a “spoofed” packet containing a TCP reset to one or both endpoints. The headers in the spoofed packet must falsely indicate that it is coming from an endpoint rather than the spoofer. This information includes the endpoint’s IP address and port number. Each field in the IP and TCP header must be set to convincing spoof values for a successful pseudo-reset, fooling the endpoint into closing the TCP connection. A correctly formatted spoofed TCP reset can be an extremely effective way of interrupting any TCP connection that the spoofer is monitoring.

How TCP Resets Are Generated #

A TCP reset (RST) is a mechanism used to terminate a TCP connection or reject an abnormal connection request. During normal TCP communication, both parties establish the connection through a three-way handshake and close it through a four-way teardown. However, in some cases, a host on the network may send a TCP packet with the RST flag set, which results in the current connection being reset.

Here are some of the main reasons why TCP resets occur:

1.Abnormal Connection Termination #

• During normal communication, if one side suddenly powers off, crashes, or for other reasons can no longer maintain the connection, it sends an RST packet to notify the other side to terminate the connection. In this case, the RST packet is legitimate and used to clean up the abnormal state.

2.Network Configuration Errors #

• During the configuration of network devices such as routers and firewalls, if the rules are set incorrectly, some legitimate TCP traffic may be mistakenly marked as illegal, triggering an RST reset. For example, some ACL (access control lists) or NAT (network address translation) configuration errors can cause connections to be terminated unintentionally.

3.Interruption by Intermediate Devices #

• In complex network environments, certain intermediate devices such as firewalls and load balancers may send RST packets due to improper handling. Especially when dealing with malformed TCP packets (such as reassembly failures), these devices may consider the connection abnormal and proactively send RSTs to reset the connection.

4.Security Threats #

• Malicious attackers can initiate denial of service (DoS) attacks or distributed denial of service (DDoS) attacks by sending spoofed RST packets. By sending a large number of RST segments to target servers, attackers can prevent legitimate clients from communicating with the server normally, leading to service interruptions.

5.Protocol Implementation Issues #

• Bugs in some operating systems or application’s TCP/IP protocol stack may cause unnecessary RST packets to be sent under certain conditions. For example, some older versions of operating systems may trigger unintended RST resets when handling certain abnormal traffic.

6.Network Failures #

• Hardware issues: Physical damage or configuration errors in devices such as switches and routers.
• Bandwidth constraints: High traffic demands can lead to network congestion, resulting in TCP resets.

7.Security Attacks #

• SYN Flood attacks: Attackers send a large number of spoofed connection requests (SYN packets), causing server resources to be exhausted and unable to respond normally to valid connection requests.
• Malicious RST segments: Hackers disrupt normal communication processes by sending RST segments to target devices.

8.Application Errors #

• Timeout issues: Applications fail to properly handle TCP retransmission mechanisms, leading to long waits for unanswered data packets being automatically abandoned and triggering RSTs.
• Resource exhaustion: For example, memory leaks or suspended threads may cause applications to malfunction and trigger TCP resets.

9.OS Configuration #

• Firewall rule settings: Incorrect firewall rules may mistakenly block legitimate packets as malicious attacks.
• Network interface issues: Problems such as faulty or incompatible network card drivers.

TCP Reset and Its Hazards and Impacts #

In 2016, during a large-scale DDoS attack initiated by the Mirai botnet, attackers flooded target servers with massive RST packets and other malformed data packets by infecting numerous Internet of Things (IoT) devices such as cameras and routers. This attack not only caused server crashes due to overload but also led to widespread internet service disruptions.

1. Risk to Service Availability #

The TCP reset mechanism can inadvertently terminate legitimate connections, thereby affecting the availability of services. For example, under high concurrency scenarios, certain network devices or applications may mistakenly send RST packets, forcing many ongoing connections to be abruptly terminated, leading to service interruptions.

2. Unauthorized Access Risks #

In some cases, attackers can exploit the TCP reset mechanism to launch malicious attacks, such as session hijacking (Session Hijacking). By sending forged RST packets, attackers can forcibly terminate a user’s legitimate connection and take control of the session. This type of attack is more common in systems with weak authentication mechanisms.

3. Resource Consumption #

A large number of abnormal TCP reset requests can overburden network devices and servers. For example, attackers can simulate a DDoS attack by sending a flood of RST packets, causing the target server or network device to become overwhelmed until service disruption occurs.

4. Risk to Data Integrity #

In certain scenarios, TCP resets can disrupt unfinished data transfers, thereby affecting data integrity and consistency. This is particularly important in network communications, especially in sensitive fields such as finance and healthcare.

5. Interruption of Data Communication #

The most direct impact of a TCP reset is the interruption of ongoing data communication. Whether it’s a normal connection closure or an accidental network failure, this can result in terminated data packets that need to be retransmitted or the establishment of new connections from scratch.

6. Resource Wastage #

When a TCP reset occurs, both clients and servers must spend additional time attempting to restore broken connections or establish new ones. This repetitive process not only wastes network bandwidth but also consumes system resources such as CPU and memory.

7. Security Risks #

While TCP resets are typically used to prevent malicious users from exploiting unauthorized access, they can also be abused by hackers to launch denial-of-service (DoS) attacks or distributed denial-of-service (DDoS) attacks. By sending a large number of RST packets to the target device, attackers can block legitimate communication requests and cause service interruptions.

8. Impact on User Experience #

For applications that rely on TCP for data transmission, frequent reconnection processes can lead to poor user experiences. For instance, in online gaming or video calling scenarios, unstable connections directly affect the quality of the user experience.

How to Address TCP Resets #

To effectively mitigate the risks posed by TCP resets, we need to adopt comprehensive measures from multiple perspectives including network configuration, protocol implementation, and security protection.

1. Software-Level Optimization #

• Error Handling Mechanism: Implement robust error handling strategies in applications to identify and respond to situations that may trigger TCP resets. For example, automatically re-attempting connections or data transmission upon detecting network anomalies.
• Timeout and Retries Logic: Set reasonable timeout intervals and configure appropriate retry counts to minimize unnecessary reconnect attempts while ensuring performance is not degraded by excessive waiting.

2. Hardware and Network Configuration #

• Optimize TCP Parameter Settings: Adjust TCP-related parameters (e.g., maximum retransmissions, keep-alive timers) based on application requirements to enhance connection stability and efficiency.
• Use Reliable Network Service Providers: Select network service providers with good stability to reduce the frequency of TCP resets caused by network issues.

3. DDoS Attack Defense #

• Deploy Firewalls and IDS/IPS: Use security devices like firewalls or intrusion detection/prevention systems to monitor and block malicious RST packets targeting servers.
• Implement High-Availability Architecture: Use load balancing techniques to distribute requests across multiple backend servers, reducing the impact of single points of failure and improving overall fault tolerance.

4. User Education and Communication #

• Guidance for Potential Human Errors: Provide appropriate operation guidance and prompts for actions that may cause TCP resets (e.g., connecting to multiple networks simultaneously).
• Direct Communication with Users: Explain issues and measures taken when necessary, fostering trust and maintaining good service relationships.

5. Strengthening Network Device Security Configuration #

• Filter Unnecessary RST Packets: Use firewalls or ACLs to restrict external RST packets. For instance, block RST packets from untrusted source addresses.
• Enable Strict Traffic Monitoring: Deploy network traffic analysis tools (e.g., Nexus, Tcpdump) for real-time monitoring of abnormal RST packet traffic and timely alerts.

6. Optimize TCP Protocol Implementation #

• Configure Reasonable TCP Parameters: Adjust OS or application-level TCP parameters (e.g., SO_RCVBUF, SO_SNDBUF sizes) to avoid unexpected resets due to buffer overflow.
• Use Reliable Authentication Mechanisms: Enable SSL/TLS encryption for critical communication and implement mutual TLS (Mutual TLS) for enhanced security.

7. Enhance Application Security #

• Strict Session State Management: Ensure accurate tracking of TCP connection states at the application level, including verifying RST packet legitimacy during handling.
• Limit Unnecessary Port Listening: Avoid exposing unnecessary network service ports to reduce potential attack surfaces.

8. Deploy Professional Security Protection Tools #

• Deploy Traffic Cleaning Devices: Implement professional security devices (e.g., IPS, IDS) to detect and filter abnormal traffic, including malicious RST packets.
• Enable Behavior Analysis Technology: Use machine learning algorithms to analyze network traffic patterns and identify unusual TCP reset activities for timely response.

9. Establish a Rapid Response Mechanism #

• Develop an Emergency Plan: Predefine contingency plans for potential TCP reset attacks. For example, isolate affected network segments or services upon detecting abnormal RST packets.
• Enhance Log Analysis Capabilities: Through detailed logging and analysis, quickly identify the root cause of issues and assess the impact scope.

Case Analysis and Practice #

To better understand the dangers of TCP reset attacks and their mitigation strategies, we can consider a few real-world cases:

A TCP Reset Attack on an E-commerce Platform #

During a promotional event, the platform experienced a large-scale RST packet attack, resulting in numerous user connections being forcibly terminated. Post-event analysis revealed that the attackers exploited unpatched network device vulnerabilities to send a high volume of forged RST packets. Ultimately, by updating firewall rules and deploying traffic cleaning devices, the platform successfully thwarted the attack.

An Unauthorized Access Incident on a Financial System #

During a system maintenance session, the security team discovered unauthorized external IP addresses sending large volumes of RST packets in an attempt to hijack legitimate user sessions. By analyzing logs, it was confirmed that this was part of an advanced persistent threat (APT) against the system. Following this, the team strengthened mutual authentication mechanisms and restricted external device access to internal services.

Conclusion #

TCP reset is an important network protocol mechanism that helps us quickly recover connections in normal circumstances. However, its misuse or abuse can lead to severe security risks and performance issues. To address these challenges, we need to take comprehensive measures from multiple dimensions, including network configuration, protocol implementation, and application security.

As cybersecurity threats continue to evolve, we require more intelligent and automated defense mechanisms, such as AI-based traffic analysis and real-time behavior detection technologies. Only through ongoing technological innovation and enhanced security awareness can we better protect our network systems from TCP reset attacks.