I'm in the design process for a Java web app that I will probably end up deploying to Google App Engine (GAE). The nice thing about GAE is that I really don't have to worry about fortifying my app from the dreaded DDoS attack - I just specify a "billing ceiling", and if my traffic peaks up to this ceiling (DDoS or otherwise), GAE will just shut my app down. In other words, GAE will essentially scale to any amount until you simply can't afford to keep the app running any longer.

So I'm trying to plan a contingency whereby, if I do hit this billing ceiling and GAE shuts my app down, my web app domain DNS settings "fail over" to another, non-GAE IP address. Some initial research has shown that certain CDNs like CloudFlare offer services for this exact situation. Basically, I just keep my DNS settings with them, and they provide an API I can hit to automate a failover procedure. Thus, if I detect that I'm at 99% my billing ceiling for my GAE app, I can hit this CloudFlare API, and CloudFlare will dynamically change my DNS settings to point away from the GAE servers to some other IP address.

My initial contingency would be to failover to a "read-only" (static content only) version of my web app hosted somewhere else, maybe by GoDaddy or Rackspace.

But then it suddenly dawned on me: if DDoS attacks target the domain name, what difference does it make if I rollover from my GAE IP address to my (say) GoDaddy IP address? In essence, the failover wouldn't do anything other than allows the DDoS attackers to bring down my backup/GoDaddy site!

In other words, DDoS attackers coordinate an attack on my web app, hosted by GAE, at www.blah-whatever.com, which is really an IP address of They cause my traffic to spike to 98% my billing ceiling, and my custom monitor triggers a CloudFlare failover from to The DDoS attackers don't care! They're still launching an attack against www.blah-whatever.com! The DDoS attack continues!

So I ask: what protection do CDNs like CloudFlare offer so that - when you need to fail over to another DNS - you aren't at risk for the same, continued DDoS attack? If such protection exists, are there any technical restrictions (e.g. read-only, etc.) that are placed on the failover site? If not, what good are they?! Thanks in advance!

  • Great Q! Lots can be learned from this :-) Commented Jul 26, 2012 at 9:03

2 Answers 2


They don't protect against DDoS attacks when in this configuration. A CDN doesn't "protect" against a DDoS attack -- they just mitigate its effects by having lots of hardware and bandwidth to throw at the problem. When the CDN changes the DNS settings to point directly at your server, the CDN is no longer handling requests for your website -- the clients never see the IP of the CDN, so the CDN can no longer offer you protection.

As far as "what good are they" -- DDoS attacks are not the point of using a CDN. The point of using a CDN is to decrease the latency between when someone requests a large chunk of data from one of your web servers and that person getting the data, by shortening the geographical distance between the server and the client. It's a perf optimization you can make; but it's really not designed to provide security from DDoS.

  • Thanks @Billy ONeal (+1) - so to summarize: I would want my "DDoS Failover" to actually redirect requests to the CDN servers, so they could throw enough hardware/bandwidth at the problem to keep the site up and running; although this isn't the primary function of a CDN. Is this more or less right? If so, a quick followup question: if I went this route and had my failover redirect to the CDN, would my web app be able to continue functioning like normal or do CDN's only serve back static content (i.e., my web app would become "read only", etc.)? Thanks again!
    – herpylderp
    Commented Jul 26, 2012 at 3:00
  • @herpylderp: Well, that depends on the nature of the site. CDNs only handle completely static content. If your server does "interesting things" then the CDN isn't going to help you. You usually don't get to run code on the CDN's servers. For instance, on the stack exchange sites, the images for each of the sites are hosted on sstatic.com, a CDN, but the main site is hosted in StackExchange's own datacenters. Commented Jul 26, 2012 at 3:19
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    CDN's typically charge based on volume, so you're just moving the billing cost from one vendor to another. AFAIK, DDoS mitigation usually involves automatic temporary blocking of IP ranges. Commented Jul 26, 2012 at 12:14
  • Thanks @Joeri Sebrechts (+1) - is there any difference between an "IP range" and an "IP subnet" or are they the same? I ask because GAE allows you to block IP subnets and am hoping this is what you're talking about.
    – herpylderp
    Commented Jul 26, 2012 at 12:35

I work for Incapsula, a Cloud Security company that also provides CDN based acceleration services (like CF).

I want to say that while (as correctly stated by @Billy ONeal) CDN by itself provides no DDoS protection, a Cloud based Proxy Network is a VERY effective DDoS mitigation tool.

And so, in case of DDoS on Cloud CDN, it is not the "CDN" but "Cloud" that protects you by taking in all the extra traffic generated by DDoS, while still allowing access to your site from different POPs around the world.

Also, because this a front-gate proxy solution, this technology can be used to mitigate level 3-4 network DDoS attacks (i.e. SYN Floods) which use spoofed IPs to send numerous SYN requests to your servers.

In this case a proxy will not establish a connection until an ACK response is received, thus preventing the SYN flood from happening.

There are also other ways you can use Cloud for website security (i.e. Bad Bot Blocking, Cloud-based WAF) and some of these can be also used for DDoS mitigation or prevention (stopping scanner bots is a good example for the later) but the main thing to understand here is that this is all based not on CDN but on Cloud technology.

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    Wow - thanks @Igal Zeifman (+1) - great answer! A few followup questions for you: (1) When you say "proxy network" or "front gate proxy", I assume you mean that the cloud is providing servers that act as middlemen between clients and my app servers, yes? If not can you please explain? And (2) does CloudFlare and/or Incapsula provide functionality for these other services (stopping/blocking bots, WAF, etc.)? Thanks again!
    – herpylderp
    Commented Jul 26, 2012 at 15:01
  • Also, by "POP" I assume you mean "Points of presence", yes?
    – herpylderp
    Commented Jul 26, 2012 at 15:02
  • Hi, Thanks. Much appreciated. To answer you questions: Front Gate Proxy: the term "proxy" implies on an intermediary relationship between the said network and your site. Meaning that the network will "sit" in-front of your site (hence "front gate") as a first line of defense, basically receiving all traffic 1st and filtering out all the "bad stuff", in our case by using Bad Bot blocking rules and vectors, WAF and so on. In case of DDoS this network will also help balance the extra traffic, thus preventing DDoS related issues. (i.e. crashes) POP = Points of Presence. You are 100% correct. Commented Jul 29, 2012 at 7:15

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