Advanced DDoS Protection Service

Denial-of-service (DoS) attacks are on the rise and have evolved into complex and overwhelming security challenges for organizations large and small. Although DoS attacks are not a recent phenomenon, the methods and resources available to conduct and mask such attacks have dramatically evolved to include distributed (DDoS) and, more recently, distributed reflector (DRDoS) attacks—attacks that simply cannot be addressed by traditional on-premise solutions.

CloudFlare’s advanced DDoS protection, provisioned as a service at the network edge, matches the sophistication and scale of such threats, and can be used to mitigate DDoS attacks of all forms and sizes including those that target the UDP and ICMP protocols, as well as SYN/ACK, DNS amplification and Layer 7 attacks. This document explains the anatomy of each attack method and how the CloudFlare network is designed to protect your web presence from such threats.

Below you will find detailed information on these attacks and how the CloudFlare network protects against them:

Layer 3/4 attacks

Most DDoS attacks target the transport and network layers of a communications system. These layers are represented as layers 3 and 4 of the OSI model. The so called “transport” layer of the network stack specifies the protocol (e.g., TCP or UDP) by which two hosts on a network communicate with one another. Attacks directed at layers 3 and 4 are designed to flood a network interface with attack traffic in order to overwhelm its resources and deny it the ability to respond to legitimate traffic. More specifically, attacks of this nature aim to saturate the capacity of a network switch, or overwhelm a server’s network card or its CPU’s ability to handle attack traffic.

Layer 3 and 4 attacks are difficult—if not impossible—to mitigate with an on-premise solution. If an attacker can send more traffic than a network link can handle, no amount of additional hardware resources will help to mitigate such an attack. For example, if you have a router with a 10Gbps port and an attacker sends you 11Gbps of attack traffic, no amount of intelligent software or hardware will allow you to stop the attack if the network link is completely saturated.

Very large layer 3/4 attacks nearly always originate from a number of sources. These many sources each send attack traffic to a single Internet location creating a tidal wave that overwhelms a target’s resources. In this sense, the attack is distributed. The sources of attack traffic can be a group of individuals working together, a botnet of compromised PCs, a botnet of compromised servers, misconfigured DNS resolvers or even home Internet routers with weak passwords.

Because an attacker launching a layer 3/4 attack doesn’t care about receiving a response to the requests they send, the packets that make up the attack do not have to be accurate or correctly formatted. Attackers will regularly spoof all information in the attack packets, including the source IP, making it look as if the attack is coming from a virtually infinite number of sources. As packet data can be fully randomized, even techniques such as upstream IP filtering become virtually useless.

With CloudFlare, all attack traffic that would otherwise directly hit your server infrastructure is automatically routed to CloudFlare’s global Anycast network of datacenters. Once attack traffic is shifted, we are able to leverage the significant global capacity of our network, as well as racks-upon-racks of server infrastructure, to absorb the floods of attack traffic at our network edge. This means that CloudFlare is able to prevent even a single packet of attack traffic from a traditional layer 3/4 attack from ever reaching a site protected by CloudFlare.

DNS amplification attacks

DNS amplification attacks, one form of DRDoS, are on the rise and have become the largest source of Layer 3/4 DDoS attacks. CloudFlare routinely mitigates attacks that exceed 100Gpbs, and recently protected a customer from an attack that exceeded 300Gbps—an attack the New York Times deemed the “largest publicly announced DDoS attack in the history of the Internet.”

In a DNS reflection attack the attacker sends a request for a large DNS zone file—with the source IP address spoofed as the IP address of the intended victim—to a large number of open DNS resolvers. The resolvers then respond to the request, sending the large DNS zone answer to the IP address of the intended victim. The attackers’ requests themselves are only a fraction of the size of the responses, allowing the attacker to amplify their attack to many times the size of the bandwidth resources they themselves control.

DNS Reflection Attack Without CloudFlare

An attacker gathers resources, like botnets or unsecured DNS recursors, and imitates the target’s IP address. The resources then send a flood of replies to the target, knocking it offline.

A DNS reflection attack without CloudFlare

DNS Reflection Attack With CloudFlare

An attacker gathers resources, like botnets or unsecured DNS recursors, and imitates the target’s IP address. The resources then send a flood of replies to the target, but they are blocked regionally by CloudFlare’s data centers. Legitimate traffic can still access the web property.

A DNS reflection attack with CloudFlare

There are two criterion for an amplification attack: 1.) a query can be sent with a spoofed source address (e.g., via a protocol like ICMP or UDP that does not require a handshake); and 2.) the response to the query is significantly larger than the query itself. DNS is a core, ubiquitous Internet platform that meets these criteria, and therefore has become the largest source of amplification attacks.

DNS queries are typically transmitted over UDP, meaning that, like ICMP queries used in a SMURF attack (described below), they are fire-and-forget. As a result, the source attribute of a DNS query can be spoofed and the receiver has no way of determining its veracity before responding. DNS is also capable of generating a much larger response than query. For example, you can send the following (tiny) query (where x.x.x.x is the IP of an open DNS resolver):

dig ANY @x.x.x.x +edns=0

And get back the following gigantic response:

; <<>> DiG 9.7.3 <<>> ANY @x.x.x.x
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 5147
;; flags: qr rd ra; QUERY: 1, ANSWER: 27, AUTHORITY: 4, ADDITIONAL: 5
;; ANSWER SECTION: 4084 IN SOA 2012102700 7200 3600 24796800 3600 4084 IN A 4084 IN MX 10 4084 IN MX 10 4084 IN TXT "v=spf1 a mx ip4: ip4: ip6:2001:04F8::0/32 ip6:2001:500:60::65/128 ~all" 4084 IN TXT "$Id:,v 1.1724 2012-10-23 00:36:09 bind Exp $" 4084 IN AAAA 2001:4f8:0:2::d 4084 IN NAPTR 20 0 "S" "SIP+D2U" "" 484 IN NSEC A NS SOA MX TXT AAAA NAPTR RRSIG NSEC DNSKEY SPF 4084 IN DNSKEY 256 3 5 BQEAAAAB2F1v2HWzCCE9vNsKfk0K8vd4EBwizNT9KO6WYXj0oxEL4eOJ aXbax/BzPFx+3qO8B8pu8E/JjkWH0oaYz4guUyTVmT5Eelg44Vb1kssy q8W27oQ+9qNiP8Jv6zdOj0uCB/N0fxfVL3371xbednFqoECfSFDZa6Hw jU1qzveSsW0= 4084 IN DNSKEY 257 3 5 BEAAAAOhHQDBrhQbtphgq2wQUpEQ5t4DtUHxoMVFu2hWLDMvoOMRXjGr hhCeFvAZih7yJHf8ZGfW6hd38hXG/xylYCO6Krpbdojwx8YMXLA5/kA+ u50WIL8ZR1R6KTbsYVMf/Qx5RiNbPClw+vT+U8eXEJmO20jIS1ULgqy3 47cBB1zMnnz/4LJpA0da9CbKj3A254T515sNIMcwsB8/2+2E63/zZrQz Bkj0BrN/9Bexjpiks3jRhZatEsXn3dTy47R09Uix5WcJt+xzqZ7+ysyL KOOedS39Z7SDmsn2eA0FKtQpwA6LXeG2w+jxmw3oA8lVUgEf/rzeC/bB yBNsO70aEFTd 4084 IN SPF "v=spf1 a mx ip4: ip4: ip6:2001:04F8::0/32 ip6:2001:500:60::65/128 ~all" 484 IN RRSIG NS 5 2 7200 20121125230752 20121026230752 4442 oFeNy69Pn+/JnnltGPUZQnYzo1YGglMhS/SZKnlgyMbz+tT2r/2v+X1j AkUl9GRW9JAZU+x0oEj5oNAkRiQqK+D6DC+PGdM2/JHa0X41LnMIE2NX UHDAKMmbqk529fUy3MvA/ZwR9FXurcfYQ5fnpEEaawNS0bKxomw48dcp Aco= 484 IN RRSIG SOA 5 2 7200 20121125230752 20121026230752 4442 S+DLHzE/8WQbnSl70geMYoKvGlIuKARVlxmssce+MX6DO/J1xdK9xGac XCuAhRpTMKElKq2dIhKp8vnS2e+JTZLrGl4q/bnrrmhQ9eBS7IFmrQ6s 0cKEEyuijumOPlKCCN9QX7ds4siiTIrEOGhCaamEgRJqVxqCsg1dBUrR hKk= 484 IN RRSIG MX 5 2 7200 20121125230752 20121026230752 4442 VFqFWRPyulIT8VsIdXKMpMRJTYpdggoGgOjKJzKJs/6ZrxmbJtmAxgEu /rkwD6Q9JwsUCepNC74EYxzXFvDaNnKp/Qdmt2139h/xoZsw0JVA4Z+b zNQ3kNiDjdV6zl6ELtCVDqj3SiWDZhYB/CR9pNno1FAF2joIjYSwiwbS Lcw= 484 IN RRSIG TXT 5 2 7200 20121125230752 20121026230752 4442 Ojj8YCZf3jYL9eO8w4Tl9HjWKP3CKXQRFed8s9xeh5TR3KI3tQTKsSeI JRQaCXkADiRwHt0j7VaJ3xUHa5LCkzetcVgJNPmhovVa1w87Hz4DU6q9 k9bbshvbYtxOF8xny/FCiR5c6NVeLmvvu4xeOqSwIpoo2zvIEfFP9deR UhA= 484 IN RRSIG AAAA 5 2 7200 20121125230752 20121026230752 4442 hutAcro0NBMvKU/m+2lF8sgIYyIVWORTp/utIn8KsF1WOwwM2QMGa5C9 /rH/ZQBQgN46ZMmiEm4LxH6mtaKxMsBGZwgzUEdfsvVtr+fS5NUoA1rF wg92eBbInNdCvT0if8m1Sldx5/hSqKn8EAscKfg5BMQp5YDFsllsTauA 8Y4= 484 IN RRSIG NAPTR 5 2 7200 20121125230752 20121026230752 4442 ZD14qEHR7jVXn5uJUn6XR9Lvt5Pa7YTEW94hNAn9Lm3Tlnkg11AeZiOU 3woQ1pg+esCQepKCiBlplPLcag3LHlQ19OdACrHGUzzM+rnHY50Rn/H4 XQTqUWHBF2Cs0CvfqRxLvAl5AY6P2bb/iUQ6hV8Go0OFvmMEkJOnxPPw 5i4= 484 IN RRSIG NSEC 5 2 3600 20121125230752 20121026230752 4442 rY1hqZAryM045vv3bMY0wgJhxHJQofkXLeRLk20LaU1mVTyu7uair7jb MwDVCVhxF7gfRdgu8x7LPSvJKUl6sn731Y80CnGwszXBp6tVpgw6oOcr Pi0rsnzC6lIarXLwNBFmLZg2Aza6SSirzOPObnmK6PLQCdmaVAPrVJQs FHY= 484 IN RRSIG DNSKEY 5 2 7200 20121125230126 20121026230126 4442 i0S2MFqvHB3wOhv2IPozE/IQABM/eDDCV2D7dJ3AuOwi1A3sbYQ29XUd BK82+mxxsET2U6hv64crpbGTNJP3OsMxNOAFA0QYphoMnt0jg3OYg+AC L2j92kx8ZdEhxKiE6pm+cFVBHLLLmXGKLDaVnffLv1GQIl5YrIyy4jiw h0A= 484 IN RRSIG DNSKEY 5 2 7200 20121125230126 20121026230126 12892 j1kgWw+wFFw01E2z2kXq+biTG1rrnG1XoP17pIOToZHElgpy7F6kEgyj fN6e2C+gvXxOAABQ+qr76o+P+ZUHrLUEI0ewtC3v4HziMEl0Z2/NE0MH qAEdmEemezKn9O1EAOC7gZ4nU5psmuYlqxcCkUDbW0qhLd+u/8+d6L1S nlrD/vEi4R1SLl2bD5VBtaxczOz+2BEQLveUt/UusS1qhYcFjdCYbHqF JGQziTJv9ssbEDHT7COc05gG+A1Av5tNN5ag7QHWa0VE+Ux0nH7JUy0N ch1kVecPbXJVHRF97CEH5wCDEgcFKAyyhaXXh02fqBGfON8R5mIcgO/F DRdXjA== 484 IN RRSIG SPF 5 2 7200 20121125230752 20121026230752 4442 IB/bo9HPjr6aZqPRkzf9bXyK8TpBFj3HNQloqhrguMSBfcMfmJqHxKyD ZoLKZkQk9kPeztau6hj2YnyBoTd0zIVJ5fVSqJPuNqxwm2h9HMs140r3 9HmbnkO7Fe+Lu5AD0s6+E9qayi3wOOwunBgUkkFsC8BjiiGrRKcY8GhC kak= 484 IN RRSIG A 5 2 7200 20121125230752 20121026230752 4442 ViS+qg95DibkkZ5kbL8vCBpRUqI2/M9UwthPVCXl8ciglLftiMC9WUzq Ul3FBbri5CKD/YNXqyvjxyvmZfkQLDUmffjDB+ZGqBxSpG8j1fDwK6n1 hWbKf7QSe4LuJZyEgXFEkP16CmVyZCTITUh2TNDmRgsoxrvrOqOePWhp 8+E= 4084 IN NS 4084 IN NS 4084 IN NS 4084 IN NS
;; AUTHORITY SECTION: 4084 IN NS 4084 IN NS 4084 IN NS 4084 IN NS
;; ADDITIONAL SECTION: 484 IN A 484 IN AAAA 2001:500:60::65 484 IN A 484 IN AAAA 2001:4f8:0:2::2b 4084 IN SRV 0 1 5060
;; Query time: 176 msec
;; SERVER: x.x.x.x#53(x.x.x.x)
;; WHEN: Tue Oct 30 01:14:32 2012
;; MSG SIZE rcvd: 3223

That’s a 64 byte query that resulted in a 3,223 byte response. In other words, an attacker is able to achieve a 50x amplification over whatever traffic they can initiate to an open DNS resolver.

CloudFlare’s “Anycast” network was specifically designed to stop massive layer 3/4 attacks. By using Anycast, we are able to announce the same IP addresses from each of our 23 worldwide data centers. The network itself load balances requests to the nearest facility. Under normal circumstances this helps us ensure that your site’s visitors are automatically routed to the nearest data center on our network to ensure the best performance. When there is an attack, Anycast serves to effectively scatter and dilute attack traffic across our entire network of data centers. Because every data center announces the same IP address for any CloudFlare customer, traffic cannot be directed to any one location. Instead of the attack being many-to-one, it becomes many-to-many with no single point on the network a single point of failure.

SMURF attacks

One of the first amplification attacks was known as a SMURF attack. In a SMURF attack an attacker sends ICMP requests (i.e., ping requests) to a network’s broadcast address (i.e., X.X.X.255) announced from a router configured to relay ICMP to all devices behind the router. The attacker then spoofs the source of the ICMP request to be the IP address of the intended victim. Because ICMP does not include a handshake, the destination has no means of verifying if the source IP is legitimate. The router receives the request and passes it on to all the devices that sit behind it. Each of these devices then respond back to the ping. The attacker is able to amplify the attack by a multiple equal to the number of devices behind the router (i.e., if you have 5 devices behind the router then the attacker is able to amplify the attack 5x, see the diagram below).

Smurf attack diagram

SMURF attacks are largely a thing of the past. For the most part, network operators have configured their routers to disable the relay of ICMP requests sent to a network’s broadcast address.

ACK attacks

In order to understand an ACK attack, one must delve into the world of TCP. When a TCP connection is established there is a handshake. The server initiating the TCP session first sends a SYN (for synchronize) request to the receiving server. The receiving server responds with an ACK (for acknowledge). After that handshake, data can be exchanged.

In an ACK reflection attack, the attacker sends lots of SYN packets to servers with a spoofed source IP address pointing to the intended victim. The servers then respond to the victim’s IP with an ACK creating the attack.

Like DNS reflection attacks, ACK attacks disguise the source of the attack making it appear to come from legitimate servers. However, unlike a DNS reflection attack, there is no amplification factor: the bandwidth from the ACKs is symmetrical to the bandwidth the attacker has to generate the SYNs. The CloudFlare network is configured to drop unmatched ACKs, which mitigates these types of attacks.

Layer 7 attacks

A new breed of attacks target Layer 7 of the OSI model, the “application” layer. These attacks focus on specific characteristics of web applications that create bottlenecks. For example, the so-called Slow Read attack sends packets slowly across multiple connections. Because Apache opens a new thread for each connection, and since connections are maintained as long as there is traffic being sent, an attacker can overwhelm a web server by exhausting its thread pool relatively quickly.

CloudFlare has protections in place against many of these attacks, and in real world experiences we generally reduce HTTP attack traffic by 90%. For most attacks, and for most of our customers, this is enough to keep them online. However, the 10% of traffic that does get through traditional protections can still be overwhelming to customers with limited resources or in the face of very large attacks. In this case, CloudFlare offers a security setting called “I’m Under Attack” mode (IUAM).

IUAM is a security level you can set for your site when you’re under attack. When IUAM is turned on, CloudFlare will add an additional layer of protections to stop malicious HTTP traffic from being passed to your server. While a number of additional checks are performed in the background, an interstitial page is presented to your site’s visitors for 5 seconds while the checks are completed. Think of it as a challenge where the tests are automatic and visitors never need to fill in a CAPTCHA.

Web page shown to users when you're under attack

After verified as legitimate by the automated tests, visitors are able to browse your site unencumbered. Javascript and cookies are required for the tests, and to record the fact that the tests were correctly passed. The page which your visitors see when in IUAM can be fully customized to reflect your branding. I’m Under Attack mode does not block search engine crawlers or your existing CloudFlare whitelist.

Making DoS a thing of the past

As technology advances DoS attacks will only increase in complexity and magnitude. Traditional on-premise DoS solutions simply can not adapt to the wide range of new attack vectors, and are rendered completely ineffective for attacks that exceed an organization’s network capacity.

The CloudFlare network is designed to mitigate and keep pace with the changing threat landscape. CloudFlare, as an operator of one of the largest global networks on the Internet, is able to leverage its aggregate network capacity across 76 points of presence, and is able to learn from attacks against any individual customer to protect all customers on our network.