Bandwidth based DOS
Single DOSDos Sending Huge amount of data to a server with lower bandwidth from a client with higher bandwidth which ends up saturating the server pipe and queue up future requests, new requests will have to wait or perhaps denied service. Example, the attacker have 100mb/s bandwidth (upload) the server has 10Mb/s download. If the attacker starts sending 100 mb worth of data to the server, it will take it 1 second to leave the pipe. However, The server can only download 10 mb each second for processing because thats its bandwidth, so it needs 10 seconds to completely download that 100mb and process. In this 10 seconds the server is fully busy serving just 1 client. Other requests will not be able to even reach the server, they might get queued and they may never be executed. Thus denied service. It is important to know that the server must have an end point that actually accept such large data. Like upload file with no limit. Another example, is UDP where there is no connection.
Distributed DOSDdos this previous scenario is less likely since servers usually has much more bandwidth than a single computer. A common attack is to do a DOS in distributed manner. Assume a server with 1 Gb and client with 10 mb/s no matter how much data the client can send it can only send 10mb per second, and the server can go through them real quick. Example, the client sends 1GB, it will leave the client’s pipe into 100 (10mb) means the client will take 100 seconds just to upload all the data because it can only sends 10 mb each seconds. And the server is processing it so fast it each second and it will still have enough bandwidth to process other requests(1000-10). But imagine 100 users with 10 mb connection each, all coordinate to send 1 Gb worth of data to the server at the same time (critical that its in the same time) 100x10 each second they can send 1 Gb in total to the server, the server can only process 1 GB per second so the server will not be able to process any other requests because its bandwidth is saturated processing this 1 GB from different place. Make it 200 users and you just clogged the pipe.
Max connections based DOSAnother type of denial of service attack is by somehow force the server to reach its max connections. The web server usually sets a maximum number of tcp connections so that it doesn’t run out of memory. an attacker can perform a DOS attack to force the server to reach its max connection. once it does, it wont accept any more connections thus deny service of future requests. However it is not easy, web servers have good preventive measures to minimize unnecessary tcp connections. So you cannot just establish a connection and ghost the server. This isn’t your ex boyfriend. Server has good timeouts for connections that are idle, terminated or potentially harmful. However one possible attack is to establish a connection but send the data slowly so when the server tries to timeout it immediately reset the timeout and keep the connection alive! Assuming the max tcp connection is 200, Run your script 200 times and you just created 200 connections to the server so no new connection can connect.
Servers do preventive measures to prevent alot of connections from the same client. In this case you wont be able to execute the 200 slow connections,
Do this multiple times from different machines you reached the maximum.
Vulnerability based DOSAnother way to deny service is to destroy the server all together. If an attacker knows a zero day vulnerability, like say a buffer stack overflow that can be executed on an input that completely overflow the stack buffer and overwrite piece of memory to either terminate the process all together or even worse execute malicious code to gain control over the server.