What is SYN Flood Attack: Signs, Risks & Prevention

Distributed denial-of-service (DDoS) attacks remain one of the most common and disruptive cyber threats today. In 2024, industry telemetry shows that large-scale DDoS events continue to occur frequently and that peak attack volumes in recent years have reached hundreds of gigabits per second. Among DDoS techniques, the SYN flood (sometimes called SYN flooding) is a long‑standing TCP-layer attack that exploits how the TCP three‑way handshake works. Because it targets a fundamental part of TCP connection setup, SYN floods can quickly exhaust server or firewall resources and degrade or entirely stop legitimate traffic. This article explains how SYN floods work, the signs of an attack, why they are dangerous, real‑world examples, and best practices to protect networks and services.

What is a SYN flood attack?

A SYN flood is a DDoS attack targeting the TCP connection handshake. 

1. In regular TCP communication, a client sends a SYN (synchronize) packet to a server.
2. The server replies with SYN‑ACK, and the client finishes with an ACK, establishing a connection.
3.  A SYN flood abuses this by sending large volumes of SYN packets and never completing the handshake (either by sending no ACK or spoofing the source address). 
4. The targeted server allocates state (memory, entries in connection tables, TCP control blocks) for each half‑open connection and, if overwhelmed, will run out of resources to handle legitimate connection attempts, resulting in degraded service or complete denial of service.

How SYN flood attack work?

When a TCP server receives a SYN, it typically allocates some kernel memory and data structures while it waits for the final ACK. 

• In a SYN flood, the attacker floods the server with SYN packets at rates higher than the server can complete handshakes for. 
• If source IP addresses are spoofed or attackers never send the final ACK, the server’s backlog of half‑open connections grows until either the backlog queue fills or system resources (CPU, memory, connection table slots) are exhausted. 
• At that point, new legitimate SYNs are dropped or delayed.

Variations and amplification

There are several variations:

• Spoofed SYN floods: spoofing source IPs forces the server to hold state while responses go to bogus addresses, so no ACKs arrive.
  
• Low‑rate or pulsed SYN floods: bursts timed to evade simplistic rate limits, requiring more nuanced detection.
  
• Volumetric hybrid attacks: attackers combine SYN floods with other vectors (UDP floods, HTTP GET floods, amplification) to simultaneously overwhelm different parts of the stack.

Signs of a SYN flood attack on your network

Detecting a SYN flood quickly is critical. Common indicators include:

• Sudden spike in incoming SYN packets (often visible in raw packet counters or network-monitoring tools).
  
• There are a high number of half‑open or SYN‑RECEIVED TCP connections on servers (tools such as netstat, ss, or the server’s TCP stack metrics will show elevated counts).
  
• Elevated CPU or memory usage on network devices and application servers while legitimate request completion rates fall.
  
• Increased TCP retransmissions and timeouts (clients retry because their connections never complete).
  
• Legitimate connections may fail or time out while traffic volume may appear normal on aggregate metrics (because the attack is targeting connection state rather than bandwidth).
  
• Network monitoring or IDS/IPS alerts indicating unusual TCP flags or abnormal SYN/ACK ratios.
  

Why are SYN floods dangerous?

The following are the main reasons SYN floods are dangerous.

1. Exploits a core part of TCP

SYN floods target connection management itself, not a higher‑level application. This means they can affect any TCP‑based service (web, SSH, email, databases) and often require remedial OS or network edge changes.

2. Low cost, high effect

Small packet sizes and modest bandwidth can force servers to waste significant memory and CPU. Attackers can use large numbers of compromised devices (botnets) or IP spoofing to scale and obscure the attack sources.

3. Hard to distinguish from legitimate bursts

Because SYN packets are normal during legitimate traffic spikes, distinguishing an attack from a sudden surge in legitimate connection attempts can be nontrivial. This makes automatic mitigation risky unless carefully tuned.

4. Can be used as part of multi-vector attacks

SYN floods are frequently used with other attack types to saturate multiple layers of the target (TCP state tables, application threads, bandwidth), complicating defense.

SYN flood attack examples

Below are representative examples that show how SYN floods manifest and how they are used:

1. The small web server is overwhelmed

A small e‑commerce site with a single web server begins receiving thousands of SYN packets per second from spoofed IP addresses. The server’s TCP backlog fills, and it cannot accept new connections from real customers; orders fail during checkout. The mitigation required modifying kernel parameters (to enable SYN cookies) and applying rate limiting at the network edge.

2. IoT botnet DDoS

An IoT botnet issues SYN floods from many devices distributed worldwide. The target is a DNS provider using TCP for zone transfers and TCP‑based management. Because the attack sources are numerous and legitimate‑looking, the provider deploys scrubbing at a DDoS mitigation service to strip malicious SYNs and pass only legitimate connections.

3.  Pulsing attack to bypass simple rate limiting

An attacker sends SYN bursts in short pulses timed to slip under coarse rate limits, forcing defenders to implement more sophisticated behavior analysis (tracking half‑open counts per source range and using adaptive thresholds).

Best practices to prevent SYN flood attacks

Defending against SYN floods requires layered controls: host‑level hardening, network filtering, and mitigation services.

1. Host / OS level hardening

Enable TCP SYN cookies: modern TCP stacks can respond to SYNs using a cookie instead of allocating full state until the handshake completes. On Linux:

        sysctl -w net.ipv4.tcp_syncookies=1

• Tune TCP backlog and timeouts: increase somaxconn/backlog appropriately and reduce the time kernel holds half‑open connections when safe.
  
• Connection tracking limits: tune conntrack and file-descriptor limits so servers degrade gracefully.
  

2. Firewall and edge filtering

• Rate limiting on SYN packets: drop or throttle excessive SYNs per source IP or per IP block using firewall rules (iptables, nftables) or network ACLs.
  
• Per-source connection limits: use connlimit modules to limit new concurrent connections from a single IP or subnet.
  
• SYN proxying / SYN cookies at the edge: Some firewalls and load balancers can terminate and validate handshakes before forwarding valid connections to backends.
  
• Drop obviously spoofed traffic: block private or unroutable source addresses on public-facing interfaces.
  

Example iptables rule (basic rate limit):

# limit new TCP connections (SYNs) to 20 per second with burst of 40

iptables -A INPUT -p tcp –syn -m limit –limit 20/second –limit-burst 40 -j ACCEPT

iptables -A INPUT -p tcp –syn -j DROP

(Adjust values to your legitimate traffic profile; overly aggressive limits can block genuine users.)

Example sysctl knobs (Linux examples):

sysctl -w net.ipv4.tcp_syncookies=1

sysctl -w net.ipv4.tcp_max_syn_backlog=4096

sysctl -w net.ipv4.tcp_synack_retries=2

3. Network and cloud provider protections

• Cloud DDoS protections/scrubbing services: Many cloud providers and specialist DDoS services will absorb and scrub attack traffic before it reaches your network.
  
• Anycast and load balancing: distributing service endpoints geographically helps spread load and makes single targets less effective.
  
• Blackholing vs. selective filtering: Avoid blunt blackholing of traffic unless the victim service is already unusable. Prefer scrubbing and selective filtering.
  

4. Monitoring, detection, and response

• Baselines and alerting: Monitor SYN rates, half‑open connection counts, and SYN/ACK ratios, and set alerts for anomalies.
  
• Runbooks and incident response: have documented steps to apply mitigations (enable cookies, adjust firewall rules, engage provider scrubbing).
  
• Periodic testing and red‑team exercises: validate that defenses activate and that legitimate traffic is preserved.

SYN flood attack vs other DDoS attacks

These are some major differences among others and DDOS attacks.

Immediate steps when you detect a SYN flood

1. Confirm: check SYN packet rates, half‑open connection counts, and server logs.
   
2. Enable SYN cookies and adjust backlog/timeouts if not already set.
   
3. Apply targeted rate limits or per‑source limits on the edge firewall.
   
4. Consider diverting traffic to a scrubbing service or CDN (if available).
   
5. Block obvious bad source ranges (careful: avoid blocking whole IP blocks with legitimate users).
   
6. Engage ISP or DDoS mitigation provider if the attack exceeds local capacity.
   
7. Document and preserve logs for analysis and possible law enforcement.

Conclusion

SYN flood attacks remain a practical and effective DDoS vector because they exploit the core TCP handshake and force servers to hold connection state, often with little bandwidth from the attacker. The defensive approach is layered: harden hosts (SYN cookies, kernel tuning), filter and rate limit at the edge (SYN proxying, per‑source limits), and enlist network‑level protections (scrubbing services, CDN/anycast distribution) when attacks scale beyond local mitigation capabilities. Continuous monitoring, baseline network behavior, and a tested incident response playbook are essential because distinguishing legitimate traffic surges from attacks can be difficult under pressure.

FAQs

Here are some frequently asked questions.