Tag: TCP Challenge ACK

This image explains the TCP Challenge ACK mechanism.

At the top, it shows a normal “TCP Connection Established” state. Below that, it illustrates two attack scenarios and the defense mechanism:

1. First scenario: An attacker sends a SYN packet with SEQ(attack) value to an already connected session. The server responds with a TCP Challenge ACK.
2. Second scenario: An attacker sends an RST packet with SEQ(attack) value. The server checks if the SEQ(attack) value is within the receive window size (RECV_WIN_SIZE):
   • If the value is inside the window (YES) – The session is reset.
   • If the value is outside the window (NO) – A TCP Challenge ACK is sent.

Additional information at the bottom includes:

• The Challenge ACK is generated in the format seed ACK = SEQ(attack)+@
• The net.ipv4.tcp_challenge_ack_limit setting indicates the limit number of TCP Challenge ACKs sent per second, which is used to block RST DDoS attacks.

Necessity and Effectiveness of TCP Challenge ACK:

TCP Challenge ACK is a critical mechanism for enhancing network security. Its necessity and effectiveness include:

• Preventing Connection Hijacking: Detects and blocks attempts by attackers trying to hijack legitimate TCP connections.
• Session Protection: Protects existing TCP sessions from RST/SYN packets with invalid sequence numbers.
• Attack Validation: Verifies the authenticity of packets through Challenge ACKs, preventing connection termination by malicious packets.
• DDoS Mitigation: Protects systems from RST flood attacks that maliciously terminate TCP connections.
• Defense Against Blind Attacks: Increases the difficulty of blind attacks by requiring attackers to correctly guess the exact sequence numbers for successful attacks.

With Claude