Denial of Service (DoS) attacks

A Denial of Service attack (DoS) is an attempt at preventing users from accessing part of a network, notably an internet server. This is usually temporary but may be a very damaging act or a big breach of security. It doesn’t just affect networks; an individual can also be a target for such an attack.
The attacker may be able to prevent a user from:
• accessing their emails
• accessing websites/web pages
• accessing online services (such as banking).
One method of attack is to flood the network with useless traffic. How does this cause the problem?
When a user types in or clicks on a URL of a website (using their web browser), a request is sent to the internet server which contains the website or web page.
Obviously, the server can only handle a finite number of requests. So if it becomes overloaded by an attacker sending out thousands of requests, it won’t be able to service the user’s legitimate request. This is effectively a denial of service. This can happen to a user’s email account, for example, by an attacker sending out
many spam messages to their email account. Internet Service Providers (ISPs) only allow a specific data quota for each user. Consequently, if the attacker sends out very large or many hundreds of emails to the user’s account, it will quickly get clogged up and the user won’t be able to receive legitimate emails.
An individual user or a website can guard against these attacks to some degree by:
• using an up-to-date malware/virus checker
• setting up a firewall to restrict traffic to and from the internet server or user’s computer
• applying email filters to manage or filter out unwanted traffic or spam emails.  
Signs that a user can look out for to see if they are a victim of one of these attacks include:
• slow network performance (opening files or accessing certain websites)
• unavailability or inability to access particular websites
• large amounts of spam mail reaching the user’s email account.