How to respond to a network distributed denial‐of‐service (DDoS) incident.
- DDoS attacks often take the form of flooding the network with unwanted traffic; some attacks focus on overwhelming resources of a specific system.
- It will be very difficult to defend against the attack without specialized equipment or your ISP’s help.
- Often, too many people participate during incident response; limit the number of people on the team.
- DDoS incidents may span days. Consider how your team will handle a prolonged attack. Humans get tired.
- Understand your equipment’s capabilities in mitigating a DDoS attack. Many under‐appreciate the capabilities of their devices, or overestimate their performance.
Prepare for a Future Incident
- If you do not prepare for a DDoS incident in advance, you will waste precious time during the attack.
- Contact your ISP to understand the paid and free DDoS mitigation it offers and what process you should follow.
- Create a whitelist of the source IPs and protocols you must allow if prioritizing traffic during an attack. Include your big customers, critical partners, etc.
- Confirm DNS time‐to‐live (TTL) settings for the systems that might be attacked. Lower the TTLs, if necessary, to facilitate DNS redirection if the original IPs get attacked.
- Establish contacts for your ISP, law enforcement, IDS, firewall, systems, and network teams.
- Document your IT infrastructure details, including business owners, IP addresses and circuit IDs; prepare a network topology diagram and an asset inventory.
- Understand business implications (e.g., money lost) of likely DDoS attack scenarios.
- If the risk of a DDoS attack is high, consider purchasing specialized DDoS mitigation products or services.
- Collaborate with your BCP/DR planning team, to understand their perspective on DDoS incidents.
- Harden the configuration of network, OS, and application components that may be targeted by DDoS.
- Baseline your current infrastructure’s performance, so you can identify the attack faster and more accurately.
Analyze the Attack
- Understand the logical flow of the DDoS attack and identify the infrastructure components affected by it.
- Review the load and logs of servers, routers, firewalls, applications, and other affected infrastructure.
- Identify what aspects of the DDoS traffic differentiate it from benign traffic (e.g., specific source IPs, destination ports, URLs, TCP flags, etc.).
- If possible, use a network analyzer (e.g. tcpdump, ntop, Aguri, MRTG, a NetFlow tool) to review the traffic.
- Contact your ISP and internal teams to learn about their visibility into the attack, and to ask for help.
- If contacting the ISP, be specific about the traffic you’d like to control (e.g., blackhole what networks blocks? rate‐limit what source IPs?)
- Find out whether the company received an extortion demand as a precursor to the attack.
- If possible, create a NIDS signature to focus to differentiate between benign and malicious traffic.
- Notify your company’s executive and legal teams; upon their direction, consider involving law enforcement.
Mitigate the Attack’s Effects
- While it is very difficult to fully block DDoS attacks, you may be able to mitigate their effects.
- Attempt to throttle or block DDoS traffic as close to the network’s “cloud” as possible via a router, firewall, load balancer, specialized device, etc.
- Terminate unwanted connections or processes on servers and routers and tune their TCP/IP settings.
- If possible, switch to alternate sites or networks using DNS or another mechanism. Blackhole DDoS traffic targeting the original IPs.
- If the bottle neck is a particular a feature of an application, temporarily disable that feature.
- If possible, add servers or network bandwidth to handle the DDoS load. (This is an arms race, though.)
- If possible, route traffic through a traffic‐scrubbing service or product via DNS or routing changes.
- If adjusting defenses, make one change at a time, so you know the cause of the changes you may observe.
- Configure egress filters to block the traffic your systems may send in response to DDoS traffic, to avoid adding unnecessary packets to the network.
Wrap‐Up the Incident and Adjust
- Consider what preparation steps you could have taken to respond to the incident faster or more effectively.
- If necessary, adjust assumptions that affected the decisions made during DDoS incident preparation.
- Assess the effectiveness of your DDoS response process, involving people and communications.
- Consider what relationships inside and outside your organizations could help you with future incidents.
Key DDoS Incident Response Steps
- Preparation: Establish contacts, define procedures, and gather tools to save time during an attack.
- Analysis: Detect the incident, determine its scope, and involve the appropriate parties.
- Mitigation: Mitigate the attack’s effects on the targeted environment.
- Wrap‐up: Document the incident’s details, discuss lessons learned, and adjust plans and defenses.
Additional DDoS Response References
- Denial‐of‐Service Attack‐Detection Techniques http://www.computer.org/portal/site/dsonline…
- A Summary of DoS/DDoS Prevention, etc. Techniques http://sans.org/reading_room/whitepapers/intrusion/1212.php
- Network Protocols and Tools Cheat Sheets http://packetlife.net/cheatsheets/
This articleincorporates insights from Daniel Fairchild, Chris Lemieux, Peter McLaughlin, Jose Nazario, Donald Smith, Jim Tuttle, and Lenny Zeltser.