Network Denial of Service
Adversaries may perform Network Denial of Service (DoS) attacks to degrade or block the availability of targeted resources to users. Network DoS can be performed by exhausting the network bandwidth services rely on. Example resources include specific websites, email services, DNS, and web-based applications. Adversaries have been observed conducting network DoS attacks for political purposes and to support other malicious activities, including distraction, hacktivism, and extortion.
A Network DoS will occur when the bandwidth capacity of the network connection to a system is exhausted due to the volume of malicious traffic directed at the resource or the network connections and network devices the resource relies on. For example, an adversary may send 10Gbps of traffic to a server that is hosted by a network with a 1Gbps connection to the internet. This traffic can be generated by a single system or multiple systems spread across the internet, which is commonly referred to as a distributed DoS (DDoS). Many different methods to accomplish such network saturation have been observed, but most fall into two main categories: Direct Network Floods and Reflection Amplification.
To perform Network DoS attacks several aspects apply to multiple methods, including IP address spoofing, and botnets.
Adversaries may use the original IP address of an attacking system, or spoof the source IP address to make the attack traffic more difficult to trace back to the attacking system or to enable reflection. This can increase the difficulty defenders have in defending against the attack by reducing or eliminating the effectiveness of filtering by the source address on network defense devices.
Botnets are commonly used to conduct DDoS attacks against networks and services. Large botnets can generate a significant amount of traffic from systems spread across the global internet. Adversaries may have the resources to build out and control their own botnet infrastructure or may rent time on an existing botnet to conduct an attack. In some of the worst cases for DDoS, so many systems are used to generate the flood that each one only needs to send out a small amount of traffic to produce enough volume to saturate the target network. In such circumstances, distinguishing DDoS traffic from legitimate clients becomes exceedingly difficult. Botnets have been used in some of the most high-profile DDoS attacks, such as the 2012 series of incidents that targeted major US banks.
For DoS attacks targeting the hosting system directly, see Endpoint Denial of Service.
Direct Network Flood
Direct Network Floods are when one or more systems are used to send a high-volume of network packets towards the targeted service's network. Almost any network protocol may be used for Direct Network Floods. Stateless protocols such as UDP or ICMP are commonly used but stateful protocols such as TCP can be used as well.
Adversaries may amplify the volume of their attack traffic by using Reflection. This type of Network DoS takes advantage of a third-party server intermediary that hosts and will respond to a given spoofed source IP address. This third-party server is commonly termed a reflector. An adversary accomplishes a reflection attack by sending packets to reflectors with the spoofed address of the victim. Similar to Direct Network Floods, more than one system may be used to conduct the attack, or a botnet may be used. Likewise, one or more reflector may be used to focus traffic on the target.
Reflection attacks often take advantage of protocols with larger responses than requests in order to amplify their traffic, commonly known as a Reflection Amplification attack. Adversaries may be able to generate an increase in volume of attack traffic that is several orders of magnitude greater than the requests sent to the amplifiers. The extent of this increase will depending upon many variables, such as the protocol in question, the technique used, and the amplifying servers that actually produce the amplification in attack volume. Two prominent protocols that have enabled Reflection Amplification Floods are DNS and NTP, though the use of several others in the wild have been documented. In particular, the memcache protocol showed itself to be a powerful protocol, with amplification sizes up to 51,200 times the requesting packet.
|Filter Network Traffic||
When flood volumes exceed the capacity of the network connection being targeted, it is typically necessary to intercept the incoming traffic upstream to filter out the attack traffic from the legitimate traffic. Such defenses can be provided by the hosting Internet Service Provider (ISP) or by a 3rd party such as a Content Delivery Network (CDN) or providers specializing in DoS mitigations.
Depending on flood volume, on-premises filtering may be possible by blocking source addresses sourcing the attack, blocking ports that are being targeted, or blocking protocols being used for transport.
As immediate response may require rapid engagement of 3rd parties, analyze the risk associated to critical resources being affected by Network DoS attacks and create a disaster recovery plan/business continuity plan to respond to incidents. 
Detection of Network DoS can sometimes be achieved before the traffic volume is sufficient to cause impact to the availability of the service, but such response time typically requires very aggressive monitoring and responsiveness or services provided by an upstream network service provider. Typical network throughput monitoring tools such as netflow, SNMP, and custom scripts can be used to detect sudden increases in network or service utilization. Real-time, automated, and qualitative study of the network traffic can identify a sudden surge in one type of protocol can be used to detect an Network DoS event as it starts. Often, the lead time may be small and the indicator of an event availability of the network or service drops. The analysis tools mentioned can then be used to determine the type of DoS causing the outage and help with remediation.
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