Denial-of-service attack

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A denial-of-service attack (also, DoS attack) is an attack on a computer system or network that causes a loss of service to users, typically the loss of network connectivity and services by consuming the bandwidth of the victim network or overloading the computational resources of the victim system.

1 Methods of attack
- 1.1 Effects of DoS
2 Distributed DoS attacks
3 Surviving distributed attacks
4 External links

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Methods of attack

A DoS attack can be perpetrated in a number of ways. There are three basic types of attack:

consumption of computational resources, such as bandwidth, disk space, or CPU time
disruption of configuration information, such as routing information
disruption of physical network components

A nuke attack sends a packet, usually ICMP, which is malformed or fragmented in an invalid way, triggering a bug in the operating system and crashing the targeted computer. This is known as the ping of death.

WinNuke is a similar kind of attack, exploiting the vulnerability in the NetBIOS handler in Windows 95. A string of out-of-band data is sent to TCP port 139 of the victim machine, causing it to lock up and display a Blue Screen of Death. This attack was very popular between the IRC-dwelling script kiddies, due to easy availability of a user-friendly click-and-crash WinNuke program.

Various DoS-causing exploits can cause server-running software to get confused and fill the disk space or consume all available memory or CPU time.

Other kinds of DoS rely primarily on brute force, flooding the target with an overwhelming flux of packets, oversaturating its connection bandwidth or depleting the target's system resources. Bandwidth-saturating floods rely on the attacker having higher bandwidth available than the victim; a common way of achieving this today is via Distributed Denial of Service, employing a botnet. Other floods may use specific packet types or connection requests to saturate finite resources by, for example, occupying the maximum number of open connections or filling the victim's disk space with logs.

An attacker with access to a victim computer can bring it to a crawl or even to a crash by using a fork bomb.

On IRC, IRC floods are a common electronic warfare weapon.

Ping flood is based on sending the victim an overwhelming number of ping packets, usually using the "ping -f" command. It is very simple to launch, and a T1 owner can easily defeat a dial-up user.

SYN flood sends a flood of TCP/SYN packets, often with a forged sender address. Each of these packets is handled like a connection request, causing the server to spawn a half-open connection, by sending back a TCP/SYN-ACK packet, and waiting for an TCP/ACK packet in response from the sender address. However, because the sender address is forged, the response never comes. These half-open connections saturate the number of available connections the server is able to make, keeping it from responding to legitimate requests until after the attack ends.

A smurf attack is one particular variant of a flooding DoS attack on the public Internet. It relies on mis-configured network devices that allow packets to be sent to all computer hosts on a particular network via the broadcast address of the network, rather than a specific machine. The network then serves as a smurf amplifier. In such an attack, the perpetrators will send large numbers of IP packets with a faked source address, that is set to the address of the intended victim. To combat Denial of Service attacks on the Internet, services like the Smurf Amplifier Registry have given network service providers the ability to identify misconfigured networks and to take appropriate action such as filtering.

A "banana attack" is another particular type of DoS. It involves redirecting outgoing messages from the client back onto the client, preventing outside access, as well as flooding the client with the sent packets.

Attempts to "flood" a network with bogus packets, thereby preventing legitimate network traffic, are the most common form of attack, often conducted by disrupting network connectivity with the use of multiple hosts in a distributed denial-of-service attack or DDoS. Specific means of attack include: a smurf attack, in which excessive ICMP requests are broadcast to an entire network; bogus HTTP requests on the World Wide Web; incorrectly formed packets; and random traffic. The source addresses of this traffic is usually spoofed in order to hide the true origin of the attack. Due to this and the many vectors of attack, there are no comprehensive rules that can be implemented on network hosts in order to protect against denial-of-service attacks, and it is a difficult feat to determine the source of the attack and the identity of the attacker. This is especially true with distributed attacks.

Attacks can be directed at any network device, including attacks on routing devices and Web, electronic mail, or Domain Name System servers.

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Effects of DoS

Denial of Service attacks can also lead to problems in the network 'branches' around the actual computer being attacked. For example, the bandwidth of a router between the Internet and a LAN may be consumed by a DoS, meaning not only will the intended computer be compromised, but the entire network will also be disrupted.

If the DoS is conducted on a sufficiently large scale, entire geographical swathes of Internet connectivity can also be compromised by incorrectly configured or flimsy network infrastructure equipment without the attacker's knowledge or intent. For this reason, most, if not all ISPs ban the practice.

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Distributed DoS attacks

In a distributed attack, the attacking computer hosts are often zombie computers with broadband connections to the Internet that have been compromised by viruses or Trojan horse programs that allow the perpetrator to remotely control the machine and direct the attack, often through a botnet/dosnet. With enough such slave hosts, the services of even the largest and most well-connected websites can be denied.

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Distributed reflected denial of service

A distributed reflected denial of service attack involves sending forged requests of some type to a very large number of computers that will reply to the requests. Using Internet protocol spoofing, the source address is set to that of the targeted victim, which means all the replies will go to (and flood) the target.

ICMP Echo Request attacks (described above) can be considered one form of reflected attack, as the flooding host(s) send Echo Requests to the broadcast addresses of mis-configured networks, thereby enticing a large number of hosts to send Echo Reply packets to the victim. Some early DDoS programs implemented a distributed form of this attack.

The first major attack involving DNS servers as reflectors occured in January, 2001. The attack was directed at the site Register.com, and was publicly discussed in a thread on the UNISOG mailing list. This attack, which forged requests for the MX records of AOL.com (to amplify the attack) lasted about a week before it could be traced back to all attacking hosts and shut off. It used a list of tens of thousands of DNS servers that was at least a year old (at the time of the attack.)

Vern Paxson published An Analysis of Using Reflectors for Distributed Denial-of-Service Attacks in June, 2001, describing the general problem of reflectors. He points out that many services can be exploited to act as reflectors, some harder to block than others.

In July 2002, the Honeynet Project Reverse Challenge was issued. The binary that was analyzed turned out to be yet another DDoS agent, which implemented several DNS related attacks, including an optimized form of a reflection attack.

The book "Internet Denial of Service: Attack and Defense Mechanisms" published in 2005, describes distributed reflected attacks on pages 19-20, 45, 51-52, and 297.

Randal Vaughn and Gadi Evron released an analysis of DNS Amplification Attacks (which use distributed reflection and amplification) on March 17, 2006, the same day that other news reports citing VeriSign as a source were published. These attacks involved a new mechanism that increased the amplification affect, and used a much larger list of DNS servers, than during the 2001/2002 time frame.

[Distributed reflector denial of service] attacks earlier this year used only about 6 percent of the more than 1 million name servers across the Internet to flood victim networks. Still, the attacks in some cases exceeded 8 gigabits per second, indicating a remarkably powerful electronic assault. [1]

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Unintentional/non-malicious DoS attacks

This describes a situation where a website ends up denied, not due to a deliberate attack by a single individual or group of individuals, but simply due to a sudden enormous spike in popularity. This can happen when an extremely popular website posts a prominent link to a second, less well-prepared site, for example, as part of a news story. The result is that a significant proportion of the primary site's regular users - potentially hundreds of thousands of people - click that link in the space of a few hours, having the same effect on the target website as a DDoS attack.

News sites and link sites - sites whose primary function is to provide links to interesting content elsewhere on the Internet - are most likely to cause this phenomenon. The canonical example is the Slashdot effect, though sites such as Digg and Fark have their own corresponding "effects".

Routers have also been known to create unintentional DoS attacks, as both D-Link and Netgear routers have created NTP vandalism by flooding NTP servers without respecting the restrictions of client types or geographical limitations.

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Well-known DDoS tools

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Surviving distributed attacks

There are steps that can be taken to mitigate the effects of a DDoS attack. As mentioned in the previous section, the first thing to start is the investigative process. One determines which core router (a router that handles Internet backbone traffic) is passing the packets to one's border router (a router that connects his or her network to the Internet). One would contact the owners of the core router, likely a telecom company or the internet service provider, and inform them of his or her problem. Ideally, there will be a process in place which can expedite one's requests for help. They, in turn, need to determine where the malicious traffic reaches their network and contact the source. By that point, it is out of one's hands.

Since it is not likely that the administrator will be able to quickly stop the DDoS flood, there are a few steps which might help mitigate the attack temporarily. If the target is a single machine, a simple IP address change can end the flood. The new address can be updated on internal DNS servers and given to a few crucial external users. This is especially useful for key servers (e.g. email or database) under attack on one's network.

There is a chance that some filtering techniques can help. If the attack is unsophisticated, there might be a specific signature to the traffic. A careful examination of captured packets sometimes reveals a trait on which you can base either router ACLs (access control lists) or firewall rules. Additionally, a large amount of traffic may originate from a specific provider or core router. If that is the case, one might consider temporarily blocking all traffic from that source, which should allow a portion of legitimate activity through. One would also be blocking "real" packets, or legitimate traffic, but this may be an unavoidable sacrifice. However, depending on the method of attack, this option may be unavailable to you if, for example, the participants' IP addresses are spoofed.

An alternative option, one which might be available to larger companies and networks, is to throw more hardware or bandwidth at the flood and wait it out. Again, it is not the best solution, nor the least expensive one. It may provide a temporary fix, nevertheless. A final method would be to simply disconnect the server from the network by physically pulling out the cable connecting the computer to the Internet (or disabling the hardware enabling this), which gives the SysAdmin a lot more time to work on the problem, but no service is then available for legitimate users. This method does not function on remotely-hosted servers such as virtual private servers which are then impossible to access for their administrators, so the problem is more difficult to fix.

The investigative process should begin immediately after the DoS attack begins. There will be multiple phone calls, call backs, emails, pages and faxes between the victim organization, one's provider and others involved. It is a time consuming process, so the process should begin immediately. It has taken some very large networks with plenty of resources several hours to halt a DDoS.

The easiest way to survive an attack is to have planned for the attack. Having a separate emergency block of IP addresses for critical servers with a separate route can be invaluable. A separate route (perhaps a DSL) is not that extravagant, and it can be used for load balancing or sharing under normal circumstances and switched to emergency mode in the event of an attack. Filtering is generally pretty ineffective, as the route to your filter will normally be swamped so only a trickle of traffic will survive.

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