DoS and DDoS Attacks

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A denial-of-service (DoS) or distributed denial-of-service (DDoS) attack is an attempt to make a machine or network resource unavailable to its intended users. A DoS attack generally consists of efforts to temporarily or indefinitely interrupt or suspend services of a host connected to the Internet.

As clarification, distributed denial-of-service attacks are sent by two or more people, or bots, and denial-of-service attacks are sent by one person or system. As of 2014, the frequency of recognized DDoS attacks had reached an average rate of 28 per hour.

Perpetrators of DoS attacks typically target sites or services hosted on high-profile web servers such as banks, credit card payment gateways, and even root nameservers. Denial-of-service threats are also common in business, and are sometimes responsible for website attacks.

This technique has now seen extensive use in certain games, used by server owners, or disgruntled competitors on games, such as popular Minecraft multiplayer worlds, known as servers. Increasingly, DoS attacks have also been used as a form of resistance. Richard Stallman has stated that DoS is a form of ‘Internet Street Protests’. The term is generally used relating to computer networks, but is not limited to this field; for example, it is also used in reference to CPU resource management.

One common method of attack involves saturating the target mechanism with external communications requests, so much so that it cannot respond to legitimate traffic, or responds so slowly as to be rendered essentially unavailable. Such attacks usually lead to a server overload. In general terms, DoS attacks are implemented by either forcing the targeted computer(s) to reset, or consuming its resources so that it can no longer provide its intended service or obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.

Denial-of-service attacks are considered violations of the Internet Architecture Board’s Internet proper use policy, and also violate the acceptable use policies of virtually all Internet service providers. They also commonly constitute violations of the laws of individual nations.

The first demonstrated DDoS attack was introduced by well known hacker Khan C. Smith during a 1998 illegal Defcon event and later exposed for its use Botnet mechanisms during a lawsuit filed by Earthlink which claims has caused billions in economic damages.

Symptoms

The United States Computer Emergency Readiness Team (US-CERT) defines symptoms of denial-of-service attacks to include:

• Unusually slow network performance (opening files or accessing web sites)
• Unavailability of a particular web site
• Inability to access any web site
• Dramatic increase in the number of spam emails received—(this type of DoS attack is considered an e-mail bomb)
• Disconnection of a wireless or wired internet connection
• Long term denial of access to the web or any internet services

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 an attack, compromising not only the intended computer, but also the entire network or other computers on the LAN.

If the attack is conducted on a sufficiently large scale, entire geographical regions of Internet connectivity can be compromised without the attacker’s knowledge or intent by incorrectly configured or flimsy network infrastructure equipment.

Methods of attack

A denial-of-service attack is characterized by an explicit attempt by attackers to prevent legitimate users of a service from using that service. There are two general forms of DoS attacks: those that crash services and those that flood services. A DoS attack can be perpetrated in a number of ways. Attacks can fundamentally be classified into five families:

• Consumption of computational resources, such as bandwidth, memory, disk space, or processor time.
• Disruption of configuration information, such as routing information.
• Disruption of state information, such as unsolicited resetting of TCP sessions.
• Disruption of physical network components.
• Obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.

A DoS attack may include execution of malware intended to:

• Max out the processor’s usage, preventing any work from occurring.
• Trigger errors in the microcode of the machine.
• Trigger errors in the sequencing of instructions, so as to force the computer into an unstable state or lock-up.
• Exploit errors in the operating system, causing resource starvation and/or thrashing, i.e. to use up all available facilities so no real work can be accomplished or it can crash the system itself
• Crash the operating system itself.

In most cases DoS attacks involve forging of IP sender addresses (IP address spoofing) so that the location of the attacking machines cannot easily be identified and to prevent filtering of the packets based on the source address.

Internet Control Message Protocol (ICMP) flood – A smurf attack is one particular variant of a flooding DDoS attack on the public Internet. It relies on misconfigured 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 the source address faked to appear to be the address of the victim. The network’s bandwidth is quickly used up, preventing legitimate packets from getting through to their destination. 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.

Ping flood is based on sending the victim an overwhelming number of ping packets, usually using the “ping” command from Unix-like hosts (the -t flag on Windows systems is much less capable of overwhelming a target, also the -l (size) flag does not allow sent packet size greater than 65500 in Windows). It is very simple to launch, the primary requirement being access to greater bandwidth than the victim.

Ping of death is based on sending the victim a malformed ping packet, which might lead to a system crash.

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

Teardrop attacks – A teardrop attack involves sending mangled IP fragments with overlapping, over-sized payloads to the target machine. This can crash various operating systems because of a bug in their TCP/IP fragmentation re-assembly code. Windows 3.1x, Windows 95 and Windows NT operating systems, as well as versions of Linux prior to versions 2.0.32 and 2.1.63 are vulnerable to this attack.

Around September 2009, a vulnerability in Windows Vista was referred to as a “teardrop attack”, but the attack targeted SMB2 which is a higher layer than the TCP packets that teardrop used.

Peer-to-peer attacks – Attackers have found a way to exploit a number of bugs in peer-to-peer servers to initiate DDoS attacks. The most aggressive of these peer-to-peer-DDoS attacks exploits DC++. Peer-to-peer attacks are different from regular botnet-based attacks. With peer-to-peer there is no botnet and the attacker does not have to communicate with the clients it subverts. Instead, the attacker acts as a “puppet master,” instructing clients of large peer-to-peer file sharing hubs to disconnect from their peer-to-peer network and to connect to the victim’s website instead. As a result, several thousand computers may aggressively try to connect to a target website. While a typical web server can handle a few hundred connections per second before performance begins to degrade, most web servers fail almost instantly under five or six thousand connections per second. With a moderately large peer-to-peer attack, a site could potentially be hit with up to 750,000 connections in short order. The targeted web server will be plugged up by the incoming connections.

While peer-to-peer attacks are easy to identify with signatures, the large number of IP addresses that need to be blocked (often over 250,000 during the course of a large-scale attack) means that this type of attack can overwhelm mitigation defenses. Even if a mitigation device can keep blocking IP addresses, there are other problems to consider. For instance, there is a brief moment where the connection is opened on the server side before the signature itself comes through. Only once the connection is opened to the server can the identifying signature be sent and detected, and the connection torn down. Even tearing down connections takes server resources and can harm the server.

This method of attack can be prevented by specifying in the peer-to-peer protocol which ports are allowed or not. If port 80 is not allowed, the possibilities for attack on websites can be very limited.

Permanent denial-of-service attacks – A permanent denial-of-service (PDoS), also known loosely as phlashing, is an attack that damages a system so badly that it requires replacement or reinstallation of hardware. Unlike the distributed denial-of-service attack, a PDoS attack exploits security flaws which allow remote administration on the management interfaces of the victim’s hardware, such as routers, printers, or other networking hardware. The attacker uses these vulnerabilities to replace a device’s firmware with a modified, corrupt, or defective firmware image—a process which when done legitimately is known as flashing. This therefore “bricks” the device, rendering it unusable for its original purpose until it can be repaired or replaced.

The PDoS is a pure hardware targeted attack which can be much faster and requires fewer resources than using a botnet in a DDoS attack. Because of these features, and the potential and high probability of security exploits on Network Enabled Embedded Devices (NEEDs), this technique has come to the attention of numerous hacker communities.

PhlashDance is a tool created by Rich Smith (an employee of Hewlett-Packard’s Systems Security Lab) used to detect and demonstrate PDoS vulnerabilities at the 2008 EUSecWest Applied Security Conference in London.

Application-level floods – Various DoS-causing exploits such as buffer overflow 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.

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.

An attacker with shell-level access to a victim’s computer may slow it until it is unusable or crash it by using a fork bomb. A kind of application-level DoS attack is XDoS (or XML DoS) which can be controlled by modern web application firewalls (WAFs).

Nuke – A Nuke is an old denial-of-service attack against computer networks consisting of fragmented or otherwise invalid ICMP packets sent to the target, achieved by using a modified ping utility to repeatedly send this corrupt data, thus slowing down the affected computer until it comes to a complete stop.

A specific example of a nuke attack that gained some prominence is the WinNuke, which exploited the vulnerability in the NetBIOS handler in Windows 95. A string of out-of-band data was sent to TCP port 139 of the victim’s machine, causing it to lock up and display a Blue Screen of Death (BSOD).

HTTP POST DDOS attack – First discovered in 2009, the HTTP POST attack sends a complete, legitimate HTTP POST header, which includes a ‘Content-Length’ field to specify the size of the message body to follow. However, the attacker then proceeds to send the actual message body at an extremely slow rate (e.g. 1 byte/110 seconds). Due to the entire message being correct and complete, the target server will attempt to obey the ‘Content-Length’ field in the header, and wait for the entire body of the message to be transmitted, hence slowing it down. Further combined with the fact that Apache will, by default, accept requests up to 2GB in size, this attack can be particularly powerful. HTTP POST attacks are difficult to differentiate from legitimate connections, and are therefore able to bypass some protection systems. OWASP, an open source web application security project, has released a testing tool to test the security of servers against this type of attack.

R-U-Dead-Yet? (RUDY) – This attack targets web applications by starvation of available sessions on the web server. Much like Slowloris, RUDY keeps sessions at halt using never-ending POST transmissions and sending an arbitrarily large content-length header value.

Slow Read attack – Slow Read attack sends legitimate application layer requests but reads responses very slowly, thus trying to exhaust the server’s connection pool. Slow reading is achieved by advertising a very small number for the TCP Receive Window size and at the same time by emptying clients’ TCP receive buffer slowly. That naturally ensures a very low data flow rate.

Distributed attack – A distributed denial-of-service (DDoS) attack occurs when multiple systems flood the bandwidth or resources of a targeted system, usually one or more web servers. Such an attack is often the result of multiple compromised systems (for example a botnet) flooding the targeted system with traffic. When a server is overloaded with connections, new connections can no longer be accepted. The major advantages to an attacker of using a distributed denial-of-service attack are that multiple machines can generate more attack traffic than one machine, multiple attack machines are harder to turn off than one attack machine, and that the behavior of each attack machine can be stealthier, making it harder to track and shut down. These attacker advantages cause challenges for defense mechanisms. For example, merely purchasing more incoming bandwidth than the current volume of the attack might not help, because the attacker might be able to simply add more attack machines. This after all will end up completely crashing a website for periods of time.

Malware can carry DDoS attack mechanisms; one of the better-known examples of this was MyDoom. Its DoS mechanism was triggered on a specific date and time. This type of DDoS involved hardcoding the target IP address prior to release of the malware and no further interaction was necessary to launch the attack.

A system may also be compromised with a trojan, allowing the attacker to download a zombie agent, or the trojan may contain one. Attackers can also break into systems using automated tools that exploit flaws in programs that listen for connections from remote hosts. This scenario primarily concerns systems acting as servers on the web. Stacheldraht is a classic example of a DDoS tool. It utilizes a layered structure where the attacker uses a client program to connect to handlers, which are compromised systems that issue commands to the zombie agents, which in turn facilitate the DDoS attack. Agents are compromised via the handlers by the attacker, using automated routines to exploit vulnerabilities in programs that accept remote connections running on the targeted remote hosts. Each handler can control up to a thousand agents. In some cases a machine may become part of a DDoS attack with the owner’s consent, for example, in Operation Payback, organized by the group Anonymous.

These collections of systems compromisers are known as botnets. DDoS tools like Stacheldraht still use classic DoS attack methods centered on IP spoofing and amplification like smurf attacks and fraggle attacks (these are also known as bandwidth consumption attacks). SYN floods (also known as resource starvation attacks) may also be used. Newer tools can use DNS servers for DoS purposes. Unlike MyDoom’s DDoS mechanism, botnets can be turned against any IP address. Script kiddies use them to deny the availability of well known websites to legitimate users. More sophisticated attackers use DDoS tools for the purposes of extortion – even against their business rivals.

Simple attacks such as SYN floods may appear with a wide range of source IP addresses, giving the appearance of a well distributed DoS. These flood attacks do not require completion of the TCP three way handshake and attempt to exhaust the destination SYN queue or the server bandwidth. Because the source IP addresses can be trivially spoofed, an attack could come from a limited set of sources, or may even originate from a single host. Stack enhancements such as syn cookies may be effective mitigation against SYN queue flooding, however complete bandwidth exhaustion may require involvement.

If an attacker mounts an attack from a single host it would be classified as a DoS attack. In fact, any attack against availability would be classed as a denial-of-service attack. On the other hand, if an attacker uses many systems to simultaneously launch attacks against a remote host, this would be classified as a DDoS attack.

UK’s GCHQ has tools built for DDOS, named PREDATORS FACE and ROLLING THUNDER.

Telephony denial-of-service – Voice over IP has made abusive origination of large numbers of telephone voice calls inexpensive and readily automated while permitting call origins to be misrepresented through caller ID spoofing.

According to the US Federal Bureau of Investigation, telephony denial-of-service (TDoS) has appeared as part of various fraudulent schemes:

• A scammer contacts the victim’s banker or broker, impersonating the victim to request a funds transfer. The banker’s attempt to contact the victim for verification of the transfer fails as the victim’s telephone lines are being flooded with thousands of bogus calls, rendering the victim unreachable.
• A scammer contacts consumers with a bogus claim to collect an outstanding payday loan for thousands of dollars. When the consumer objects, the scammer retaliates by flooding the victim’s employer with thousands of automated calls. In some cases, displayed caller ID is spoofed to impersonate police or law enforcement agencies.
• A scammer contacts consumers with a bogus debt collection demand and threatens to send police; when the victim balks, the scammer floods local police numbers with calls on which caller ID is spoofed to display the victims number. Police soon arrive at the victim’s residence attempting to find the origin of the calls.

Telephony denial-of-service can exist even without Internet telephony. In the 2002 New Hampshire Senate election phone jamming scandal, telemarketers were used to flood political opponents with spurious calls to jam phone banks on election day. Widespread publication of a number can also flood it with enough calls to render it unusable, as happened with multiple +1-area code-867-5309 subscribers inundated by hundreds of misdialed calls daily in response to a popular song 867-5309/Jenny.

TDoS differs from other telephone harassment (such as prank calls and obscene phone calls) by the number of calls originated; by occupying lines continuously with repeated automated calls, the victim is prevented from making or receiving both routine and emergency telephone calls.

Related exploits include SMS flooding attacks and black fax or fax loop transmission.

Unintentional denial-of-service – 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. A VIPDoS is the same, but specifically when the link was posted by a celebrity.

When Michael Jackson died in 2009, websites such as Google and Twitter slowed down or even crashed. Many sites’ servers thought the requests were from a virus or spyware trying to cause a denial-of-service attack, warning users that their queries looked like “automated requests from a computer virus or spyware application”.

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 when receiving traffic from Slashdot. Sites such as Reddit, Digg, the Drudge Report, Fark, Something Awful, and the webcomic Penny Arcade have their own corresponding “effects”, known as “the Reddit hug of death”, “the Digg effect”, being “drudged”, “farking”, “goonrushing” and “wanging”; respectively.

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.

Similar unintentional denials-of-service can also occur via other media, e.g. when a URL is mentioned on television. If a server is being indexed by Google or another search engine during peak periods of activity, or does not have a lot of available bandwidth while being indexed, it can also experience the effects of a DoS attack.

Legal action has been taken in at least one such case. In 2006, Universal Tube & Rollform Equipment Corporation sued YouTube: massive numbers of would-be youtube.com users accidentally typed the tube company’s URL, utube.com. As a result, the tube company ended up having to spend large amounts of money on upgrading their bandwidth. The company appears to have taken advantage of the situation, with utube.com now containing ads for advertisement revenue.

In March 2014, after Malaysia Airlines Flight 370 went missing, DigitalGlobe launched a crowdsourcing service on which users could help search for the missing jet in satellite images. The response overwhelmed the company’s servers.

An unintentional denial-of-service may also result from a prescheduled event created by the website itself. This could be caused when a server provides some service at a specific time. This might be a university website setting the grades to be available where it will result in many more login requests at that time than any other.

Denial-of-service Level II – The goal of DoS L2 (possibly DDoS) attack is to cause a launching of a defense mechanism which blocks the network segment from which the attack originated. In case of distributed attack or IP header modification (that depends on the kind of security behavior) it will fully block the attacked network from the Internet, but without system crash.

Advanced Persistent DoS (APDoS) – An APDoS is more likely to be perpetrated by an advanced persistent threat (APT): actors who are well resourced, exceptionally skilled and have access to substantial commercial grade computer resources and capacity. APDoS attacks represent a clear and emerging threat needing specialised monitoring and incident response services and the defensive capabilities of specialised DDoS mitigation service providers. This type of attack involves massive network layer DDoS attacks through to focused application layer (HTTP) floods, followed by repeated (at varying intervals) SQLI and XSS attacks. Typically, the perpetrators can simultaneously use from 2 to 5 attack vectors involving up to several tens of millions of requests per second, often accompanied by large SYN floods that can not only attack the victim but also any service provider implementing any sort of managed DDoS mitigation capability. These attacks can persist for several weeks- the longest continuous period noted so far lasted 38 days. This APDoS attack involved approximately 50+ petabits (51,000+ terabits) of malicious traffic. Attackers in this scenario may (or often will) tactically switch between several targets to create a diversion to evade defensive DDoS countermeasures but all the while eventually concentrating the main thrust of the attack onto a single victim. In this scenario, threat actors with continuous access to several very powerful network resources are capable of sustaining a prolonged campaign generating enormous levels of un-amplified DDoS traffic.

APDoS attacks are characterised by:

• Advanced reconnaissance (pre-attack OSINT and extensive decoyed scanning crafted to evade detection over long periods)
• Tactical execution (attack with a primary and secondary victims but focus is on Primary)
• Explicit motivation (a calculated end game/goal target)
• Large computing capacity (access to substantial computer power and network bandwidth resources)
• Simultaneous multi-threaded ISO layer attacks (sophisticated tools operating at layers 3 through 7)
• Persistence over extended periods (utilising all the above into a concerted, well managed attack across a range of targets)

Performing DoS-attacks

A wide array of programs are used to launch DoS-attacks. Most of these programs are completely focused on performing DoS-attacks, while others are also true packet injectors, able to perform other tasks as well. Such tools are intended for benign use, but they can also be used in launching attacks on victim networks. Freely available automated tools like HOIC and LOIC are intended for DOS attack..

Countermeasure

Defensive responses to denial-of-service attacks typically involves the use of a combination of attack detection, traffic classification and response tools, aiming to block traffic that they identify as illegitimate and allow traffic that they identify as legitimate. A list of prevention and response tools is provided below:

Firewalls – Firewalls can be set up to have simple rules such to allow or deny protocols, ports or IP addresses. In the case of a simple attack coming from a small number of unusual IP addresses for instance, one could put up a simple rule to drop (deny) all incoming traffic from those attackers.

More complex attacks will however be hard to block with simple rules: for example, if there is an ongoing attack on port 80 (web service), it is not possible to drop all incoming traffic on this port because doing so will prevent the server from serving legitimate traffic. Additionally, firewalls may be too deep in the network hierarchy. Routers may be affected before the traffic gets to the firewall. Nonetheless, firewalls can effectively prevent users from launching simple flooding type attacks from machines behind the firewall.

Some stateful firewalls, like OpenBSD’s pf(4) packet filter, can act as a proxy for connections: the handshake is validated (with the client) instead of simply forwarding the packet to the destination. It is available for other BSDs as well. In that context, it is called “synproxy”.

Switches – Most switches have some rate-limiting and ACL capability. Some switches provide automatic and/or system-wide rate limiting, traffic shaping, delayed binding (TCP splicing), deep packet inspection and Bogon filtering (bogus IP filtering) to detect and remediate denial-of-service attacks through automatic rate filtering and WAN Link failover and balancing.

These schemes will work as long as the DoS attacks can be prevented by using them. For example SYN flood can be prevented using delayed binding or TCP splicing. Similarly content based DoS may be prevented using deep packet inspection. Attacks originating from dark addresses or going to dark addresses can be prevented using bogon filtering. Automatic rate filtering can work as long as set rate-thresholds have been set correctly and granularly. Wan-link failover will work as long as both links have DoS/DDoS prevention mechanism.

Routers – Similar to switches, routers have some rate-limiting and ACL capability. They, too, are manually set. Most routers can be easily overwhelmed under a DoS attack. Cisco IOS has optional features that can reduce the impact of flooding.

Application front end hardware – Application front end hardware is intelligent hardware placed on the network before traffic reaches the servers. It can be used on networks in conjunction with routers and switches. Application front end hardware analyzes data packets as they enter the system, and then identifies them as priority, regular, or dangerous. There are more than 25 bandwidth management vendors.

IPS based prevention – Intrusion-prevention systems (IPS) are effective if the attacks have signatures associated with them. However, the trend among the attacks is to have legitimate content but bad intent. Intrusion-prevention systems which work on content recognition cannot block behavior-based DoS attacks.

An ASIC based IPS may detect and block denial-of-service attacks because they have the processing power and the granularity to analyze the attacks and act like a circuit breaker in an automated way.

A rate-based IPS (RBIPS) must analyze traffic granularly and continuously monitor the traffic pattern and determine if there is traffic anomaly. It must let the legitimate traffic flow while blocking the DoS attack traffic.

DDS based defense – More focused on the problem than IPS, a DoS Defense System (DDS) can block connection-based DoS attacks and those with legitimate content but bad intent. A DDS can also address both protocol attacks (such as Teardrop and Ping of death) and rate-based attacks (such as ICMP floods and SYN floods).

Blackholing and sinkholing – With blackholing, all the traffic to the attacked DNS or IP address is sent to a “black hole” (null interface or a non-existent server). To be more efficient and avoid affecting network connectivity, it can be managed by the ISP.

Sinkholing routes traffic to a valid IP address which analyzes traffic and rejects bad packets. Sinkholing is not efficient for most severe attacks.

Clean pipes – All traffic is passed through a “cleaning center” or a “scrubbing center” via various methods such as proxies, tunnels or even direct circuits, which separates “bad” traffic (DDoS and also other common internet attacks) and only sends good traffic beyond to the server. The provider needs central connectivity to the Internet to manage this kind of service unless they happen to be located within the same facility as the “cleaning center” or “scrubbing center”.

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