Last Updated: 2014-05-08 13:47:08 UTC
by Johannes Ullrich (Version: 1)
It started with DNS: Simple short DNS queries are easily spoofed and the replies can be much larger then the request, leading to an amplification of the attack by orders of magnitude. Next came NTP. Same game, different actors: NTP's "monlist" feature allows for small requests (again: UDP, so trivially spoofed) and large responses.
Today, we received a packet capture from a reader showing yet another reflective DDoS mode: SNMP. The "reflector" in this case stands nicely in line for our "Internet of Things" theme. It was a video conferencing system. Firewalling these systems is often not practical as video conferencing systems do use a variety of UDP ports for video and audio streams which are dynamically negotiated. As a result, they are often left "wide open" . And of course, these systems not only let attackers spy on your meeting rooms, but the also make great SNMP reflectors as it turns out.
Here is an anonymized traffic capture (target anonymized with 192.0.2.1):
22.214.171.124 -> 192.0.2.1 SNMP 87 getBulkRequest 126.96.36.199.188.8.131.52.1.1
192.0.2.1 -> 184.108.40.206 IPv4 1514 Fragmented IP protocol (proto=UDP 17, off=0, ID=1f48)
192.0.2.1 -> 220.127.116.11 IPv4 1514 Fragmented IP protocol (proto=UDP 17, off=1480, ID=1f48)
... [ additional fragments omitted ] ...
192.0.2.1 -> 18.104.22.168 IPv4 1514 Fragmented IP protocol (proto=UDP 17, off=54760, ID=1f48)
192.0.2.1 -> 22.214.171.124 IPv4 1514 Fragmented IP protocol (proto=UDP 17, off=56240, ID=1f48)
192.0.2.1 -> 126.96.36.199 SNMP 664 get-response [... large response payload ...]
That is right! A simple 87 byte "getBulkRequest" leads to about 60k Bytes of fragmented data being sent back. We can only assume that poor 188.8.131.52 is the victim of this DoS attack. The user reporting this saw about 5 MBit/sec of traffic.
Similar to what we have seen with other reflective attacks like this, the fragmentation of the traffic is likely going to make filtering even harder. Prolexic posted a white paper about some of the different DrDOS attacks, including SNMP attacks 
So what to do:
- SNMP should probably not traverse your perimeter. Stop it at the firewall. (and I don't think video conferencing systems need it)
- proper egress/ingress control is a good idea. But you already knew that.