Alert (TA14-017A)

DETECTION

Detection of DRDoS attacks is not easy because of their use of large, trusted servers that provide UDP services. Network operators of these exploitable services may apply traditional DoS mitigation techniques. To detect a DRDOS attack, watch out for abnormally large responses to a particular IP address, which may indicate that an attacker is using the service.

If you are a victim of DRDoS attack, there are a few things you can do to detect such activity and respond:

1. Detect and alert large UDP packets to higher order ports.
2. Detect and alert on any non-stateful UDP packets. (A simple snort example is below. You will need to customize this approach to your environment with whitelist and known services.)

   Simple Snort rule example for stateless UDP check

   var HOME_NET [10.10.10.20] preprocessor stream5_global: track_ip yes, track_tcp yes,track_udp yes,track_icmp no,max_tcp 262144, max_udp 131072 preprocessor stream5_ip: timeout 180 preprocessor stream5_tcp: policy first, use_static_footprint_sizes preprocessor stream5_udp: timeout 180, ignore_any_rules alert udp HOME_NET 1024: -> any any (msg:"UDP Session start"; flowbits:set,logged_in; flowbits:noalert; sid: 1001;) alert udp any any -> HOME_NET 1024: (msg:"UDP Stateless"; flowbits:isnotset,logged_in; sid: 1002)

3. If you are an upstream provider maintain updated contacts and methods with downstream customers to send alerts by network.

In general, network and server administrators for Internet service providers (ISPs) should use the following best practices to avoid becoming amplifier nodes:

1. Detect spoofed packets using network flow. (In order to validate before blocking it, read more in the Mitigation section on blocking spoofed traffic.)
2. Monitor for an unusual number of requests to UDP services at risk using network flow or other summarized network data.
3. Use network flow to detect service anomalies (bytes-per-packet, packets-per-second anomalies).

MITIGATION

If you are a victim of DRDoS attack there are a few things you can do to mitigate this attack:

1. Use stateful UDP inspection to reduce impact on critical services on your border firewall or border router (like reflexive ACL [11] )
2. Using Border Gateway Protocol (BGP), create a Remotely Triggered Blackhole, preferably in coordination with your upstream provider or ISP. [12]
3. Maintain a list of primary upstream provider emergency contacts to coordinate response to the attack. If you are an upstream provider, conduct mitigation in coordination with your downstream customers.

In general, network and server administrators for Internet service providers (ISPs) should use the following as best practices to avoid becoming amplifier nodes:

1. Keep your software and configuration up to date to deny or limit abuse (e.g., DNS response rate limit [13] [14] [15])
2. Disable and remove unwanted services or deny access to local services over the Internet.
3. Enable network-based rate-limiting to legitimate services you provide over the Internet using UDP-based protocols (e.g., quality of service (QoS) on switching and routing devices)
4. Work with Customer Provider Edge manufactures for secure configuration and software [16]

As a service provider, to avoid any misuse of Internet resources

1. Block spoofed packets by using ingress filtering (the Spoofer Project [17] and IETF BCP 38 and BCP 84 guidelines [18])
2. Use traffic shaping on UDP service requests to ensure repeated access to over-the-internet resources is not abusive.[19] [20]