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Denial of Service (DoS) and Brute-Force Protection

Recently it has become clear to me that, although the terms Denial of Service (DoS), Distributed Denial of Service (DDoS) and Brute-Force are used by many, people don't really understand them. This has caused confusion and problems on more than one project, so I thought I would write my thoughts on their similarities, differences and protection mechanisms.

A Denial of Service is anything that happens (usually on purpose, but not necessarily) that takes a service off line or makes it unavailable to legitimate users. This could range from a hacker exploiting a vulnerability and taking the service off line, to someone digging up a cable in the road. However, a Denial of Service could also be triggered by legitimate use of a service without any 'vulnerabilities'. Consider a service that performs operations on large sets of data that take a few seconds to complete. If I put in multiple requests for this service then I could tie it up and make it unresponsive for several minutes. Similarly, consider a website that has a page with a large video or flash animation on it. Again, relatively few requests for this resource could make the server slow and unresponsive. DoS is not just about hackers finding vulnerabilities.

Distributed Denial of Service, on the other hand, is a deliberate attempt by someone to deny service by performing large numbers of requests from a large number of hosts at once. Whilst it is relatively easy to spot a single host attempting a large number of requests and block them, it can be hard to pick up on many hosts making few requests and harder to block them. There are many solutions to combat DDoS by caching content and providing high bandwidth to large numbers of nodes, such as those available from the likes of Akamai. However, logic flaws or lengthy processing in the application can only really be fixed by the application developers.

Brute-Force, on the other hand, has nothing to do with DoS or making a service slow or unavailable. I was amazed that people didn't know this! Brute-Force is all about submitting a, usually, large number of requests to a service to obtain information that was not intended by the developer. An example would be having no account lockout after several incorrect login attempts. It would then be possible to try a whole dictionary or even every character combination to eventually find the password for a user. This is an example of Brute-Force, but there are many others, such as finding database versions, telephone numbers, transactions, parcel delivery addresses, etc. This can only really be stopped with application logic.
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