Blocking TCP: ACK=0 vs SYN Packets

TL;DR

Both blocking inbound TCP segments with ACK=0 and SYN=1 flags are methods to prevent unwanted connections, but they target different stages of the connection process. Blocking SYN packets stops new connections from *starting*, while blocking ACK=0 packets usually deals with established connections being abused or attempting malicious activity. They aren’t interchangeable; you need both for comprehensive cyber security.

Understanding TCP Handshake & Flags

Before we dive into the blocking methods, let’s quickly recap how a TCP connection is made:

1. SYN (Synchronize): The client sends this to initiate a connection.
2. SYN-ACK (Synchronize-Acknowledge): The server responds with this, acknowledging the SYN and requesting its own synchronization.
3. ACK (Acknowledgement): The client acknowledges the SYN-ACK, completing the handshake.

These flags are crucial for understanding how each blocking method works.

1. Blocking Inbound TCP Segments with SYN=1

This is a common technique to prevent unwanted connection attempts. It’s often used as part of a firewall rule or intrusion prevention system (IPS).

• What it does: Drops any incoming TCP packet that has the SYN flag set.
• Why it works: Since the handshake *starts* with the SYN packet, blocking these prevents new connections from being established in the first place. It’s a proactive measure.
• Example (iptables):

  sudo iptables -A INPUT -p tcp --syn -j DROP

2. Blocking Inbound TCP Segments with ACK=0

Blocking packets with the ACK flag set to 0 is a bit more nuanced.

• What it does: Drops incoming TCP packets where the acknowledgement number field is zero.
• Why it works: The ACK flag confirms receipt of data. Packets with ACK=0 are often associated with:
  
  • Initial SYN probes (sometimes): Although less common, a poorly crafted initial probe might have ACK=0.
  • TCP Reset attacks: Malicious actors can send packets with ACK=0 to disrupt established connections.
  • Established connection abuse: An attacker attempting to hijack or interfere with an existing session.
• Example (iptables):

  sudo iptables -A INPUT -p tcp --tcp-flags SYN,ACK ACK -j DROP

3. How are they the same?

• Both prevent unwanted traffic: Both methods aim to stop potentially harmful packets from reaching your system.
• Both used in cyber security: They’re both valuable tools for building a secure network environment.

4. How are they different?

1. Connection Stage: SYN=1 blocks *new* connections; ACK=0 typically deals with existing or attempted malicious interactions.
2. Attack Vectors: SYN flooding attacks are prevented by blocking SYN packets. Reset attacks and connection hijacking attempts are often mitigated by blocking ACK=0 packets.
3. False Positives: Blocking only SYN can sometimes be less disruptive, as it doesn’t affect established connections. Blocking ACK=0 requires more careful configuration to avoid legitimate traffic issues.

5. Practical Considerations

• Firewall Rules: Implement these rules within your firewall (iptables, pfSense, etc.).
• Monitoring: Regularly monitor logs for dropped packets to ensure you aren’t blocking legitimate traffic.
• Stateful Firewalls: Modern stateful firewalls often handle SYN flood protection automatically, reducing the need for manual rule creation. However, understanding these flags is still important for troubleshooting and advanced configuration.