UDP, by design, is a connection-less protocol that does not validate source IP addresses. Unless the application-layer protocol uses countermeasures such as session initiation, it is very easy to forge the IP packet datagram to include an arbitrary source IP address [7]. When many UDP packets have their source IP address forged to a single address, the server responds to that victim, creating a reflected Denial of Service (DoS) Attack.
Recently, certain UDP protocols have been found to have particular responses to certain commands that are much larger than the initial request. Where before, attackers were limited linearly by the number of packets directly sent to the target to conduct a DoS attack, now a single packet can generate tens or hundreds of times the bandwidth in its response. This is called an amplification attack, and when combined with a reflective DoS attack on a large scale it makes it relatively easy to conduct DDoS attacks.
To measure the potential effect of an amplification attack, we use a metric called the bandwidth amplification factor (BAF). BAF can be calculated as the number of UDP payload bytes that an amplifier sends to answer a request, compared to the number of UDP payload bytes of the request [9] [10].
The list of known protocols, and their associated bandwidth amplification factors, is listed below. US-CERT would like to offer thanks to Christian Rossow for providing this information to us. For more information on bandwith amplificatication factors, please see Christian’s blog[1] and associated research paper[2].
| Protocol | Bandwidth Amplification Factor | Vulnerable Command |
|---|---|---|
| DNS | 28 to 54 | see: TA13-088A[3] [1] |
| NTP | 556.9 | see: TA14-013A[4] [2] |
| SNMPv2 | 6.3 | GetBulk request |
| NetBIOS | 3.8 | Name resolution |
| SSDP | 30.8 | SEARCH request |
| CharGEN | 358.8 | Character generation request |
| QOTD | 140.3 | Quote request |
| BitTorrent | 3.8 | File search |
| Kad | 16.3 | Peer list exchange |
| Quake Network Protocol | 63.9 | Server info exchange |
| Steam Protocol | 5.5 | Server info exchange |
References
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