What is a CDN?

What is a CDN?

A content delivery network (CDN) refers to a geographically distributed group of servers which work together to provide fast delivery of Internet content.

A CDN allows for the quick transfer of assets needed for loading Internet content including HTML pages, javascript files, stylesheets, images, and videos. The popularity of CDN services continues to grow, and today the majority of web traffic is served through CDNs, including traffic from major sites like Facebook, Netflix, and Amazon.

1. How do CDNs work?

A properly configured CDN may also help protect websites against some common malicious attacks, such as Distributed Denial of Service (DDOS) attacks.

Is a CDN the same as a web host?

While a CDN does not host content and can’t replace the need for proper web hosting, it does help cache content at the network edge, which improves website performance. Many websites struggle to have their performance needs met by traditional hosting services, which is why they opt for CDNs.

By utilizing caching to reduce hosting bandwidth, helping to prevent interruptions in service, and improving security, CDNs are a popular choice to relieve some of the major pain points that come with traditional web hosting.

What are the benefits of using a CDN?

Although the benefits of using a CDN vary depending on the size and needs of an Internet property, the primary benefits for most users can be broken down into 4 different components:

  1. Improving website load times – By distributing content closer to website visitors by using a nearby CDN server (among other optimizations), visitors experience faster page loading times. As visitors are more inclined to click away from a slow-loading site, a CDN can reduce bounce rates and increase the amount of time that people spend on the site. In other words, a faster a website means more visitors will stay and stick around longer.
  2. Reducing bandwidth costs – Bandwidth consumption costs for website hosting is a primary expense for websites. Through caching and other optimizations, CDNs are able to reduce the amount of data an origin server must provide, thus reducing hosting costs for website owners.
  3. Increasing content availability and redundancy – Large amounts of traffic or hardware failures can interrupt normal website function. Thanks to their distributed nature, a CDN can handle more traffic and withstand hardware failure better than many origin servers.
  4. Improving website security – A CDN may improve security by providing DDoS mitigation, improvements to security certificates, and other optimizations.

How does a CDN work?

At its core, a CDN is a network of servers linked together with the goal of delivering content as quickly, cheaply, reliably, and securely as possible. In order to improve speed and connectivity, a CDN will place servers at the exchange points between different networks.

These Internet exchange points (IXPs) are the primary locations where different Internet providers connect in order to provide each other access to traffic originating on their different networks. By having a connection to these high speed and highly interconnected locations, a CDN provider is able to reduce costs and transit times in high speed data delivery.

Beyond placement of servers in IXPs, a CDN makes a number of optimizations on standard client/server data transfers. CDNs place Data Centers at strategic locations across the globe, enhance security, and are designed to survive various types of failures and Internet congestion.

Latency – How does a CDN improve website load times?

When it comes to websites loading content, users drop off quickly as a site slows down. CDN services can help to reduce load times in the following ways:

  • The globally distributed nature of a CDN means reduce distance between users and website resources. Instead of having to connect to wherever a website’s origin server may live, a CDN lets users connect to a geographically closer data center. Less travel time means faster service.
  • Hardware and software optimizations such as efficient load balancing and solid-state hard drives can help data reach the user faster.
  • CDNs can reduce the amount of data that’s transferred by reducing file sizes using tactics such as minification and file compression. Smaller file sizes mean quicker load times.
  • CDNs can also speed up sites which use TLS/SSL certificates by optimizing connection reuse and enabling TLS false start.

Reliability and Redundancy – How does a CDN keep a website always online?

Uptime is a critical component for anyone with an Internet property. Hardware failures and spikes in traffic, as a result of either malicious attacks or just a boost in popularity, have the potential to bring down a web server and prevent users from accessing a site or service. A well-rounded CDN has several features that will minimize downtime:

  • Load balancing distributes network traffic evenly across several servers, making it easier to scale rapid boosts in traffic.
  • Intelligent failover provides uninterrupted service even if one or more of the CDN servers go offline due to hardware malfunction; the failover can redistribute the traffic to the other operational servers.
  • In the event that an entire data center is having technical issues, Anycast routing transfers the traffic to another available data center, ensuring that no users lose access to the website.

Data Security – How does a CDN protect data?

Information security is an integral part of a CDN. a CDN can keep a site secured with fresh TLS/SSL certificates which will ensure a high standard of authentication, encryption, and integrity. Investigate the security concerns surrounding CDNs, and explore what can be done to securely deliver content.

Bandwidth Expense – How does a CDN reduce bandwidth costs?

Every time an origin server responds to a request, bandwidth is consumed. See how a CDN, like the Cloudflare CDN, cuts down on origin requests and reduces bandwidth costs.

2. CDN reliability and redundancy

A CDN is designed to circumvent network congestion and be resilient against service interruption. Lean more about CDN reliability.

CDN Benefits – Reliability & Redundancy

One of the important characteristics about a CDN is its ability to keep website content online in the face of the common network problems including hardware failures and network congestion. By load balancing Internet traffic, using intelligent failover, and by maintaining servers across many data centers, a CDN is designed to circumvent network congestion and be resilient against service interruption.

What is load balancing? How does a CDN load balance traffic?

The purpose of a load balancer is to distribute network traffic equally across a number of servers. Load balancing can be either hardware or software based. A CDN uses load balancing in a data center to distribute incoming requests across the available server pool to ensure that spikes in traffic are handled in the most efficient manner possible. By efficiently using available resources, load balancing is able to increase processing speeds and effectively utilize server capacity. Properly load balancing incoming traffic is a key component in mitigating spikes in traffic that occur during atypical Internet activity such as when a website is experiencing an unusually high number of visitors or during a distributed denial-of-service attack.

A CDN also uses load balancing to make changes quickly and efficiently when the availability of server resources fluctuates up or down. In the event that a server fails and failover occurs, a load balancer will redirect the traffic allocated for the failed server and distribute it proportionally across the remaining servers. This provides resiliency and reliability by increasing likelihood that hardware failures will not disrupt the flow of traffic. When a new server comes online in the data center, a load balancer will proportionately remove load from other servers and increase the utilization of the new hardware. Software-based load balancing services allow a CDN to scale load balancing capacity quickly without the bottlenecks present when using physical load-balancing hardware.

What is failover? How does a CDN failover between servers?

In computer systems that require a high degree of reliability and near continuous availability, failover is used to prevent traffic from being lost when a server is unavailable. When a server goes down, the traffic needs to be rerouted to a server that is still functional. By automatically offloading tasks to a standby system or another machine with available capacity, intelligent failover can prevent disruption of service to users.

How does a CDN reliably serve content across the Internet?

A CDN is like a GPS system coupled with express toll roads; a CDN will be able to find the optimal path to reach a distant location and will be able to use its own network to find the optimal route to get there as quickly as possible.

When a user loads content from an Internet property such a webpage or web application, a series of connections are made in order to reach the location at which the content is served. Network traffic can be thought of metaphorically as a road and highway system; smaller surface streets move local traffic around the same area and interstate highways transfer traffic into different states. When something goes wrong, like a tanker truck blocks all lanes of an interstate highway on the primary path into different state, traffic must find another path around. Like a highway crossing different states, traffic often has to move across different networks to reach its final destination. If a blockage exists in a particular network, the traffic must be redirected down a different pathway. This process can be time-consuming and inefficient.

Let’s say a user in San Francisco is loading a website in Los Angeles. The connection makes many steps, but in this example, one of the most important steps is where the network signal passes through a telecommunications provider based in in San Jose on its way towards the final destination. When a network engineer accidentally pours coffee on routing equipment in San Jose, the provider goes offline, breaking the connection (stranger things have happened). When this occurs the user is no longer able to load their Internet content unless the networked traffic is rerouted to accommodate for the new network landscape. The user’s request now needs to go through a different telecom provider if it ever wants to arrive in Los Angeles.

Now that the traffic is no longer able to pass through the intended network, it must instead step into an entirely different network maintained by a different organization. This process of renegotiation and switching networks may occur multiple times in a network request and instances like this can add latency and may push the traffic onto a congested pathway, resulting in a delay. A CDN of sufficient size will typically control its own network connections by placing servers in Internet exchange points (IXPs) and other strategic locations. These optimized network schemas allow CDN providers to optimize the route and reduce latency.

How can a CDN use an Anycast network to increase reliability?

Some CDNs will use an Anycast routing method to transfer Internet traffic to specific available data centers. This occurs in order to ensure improved response time and to prevent any one data center from becoming overwhelmed with traffic in the event of extraordinary demand such as during a DDoS attack.

With Anycast, multiple machines can share the same IP address. When a request is sent to an Anycast IP address, routers will direct it to the machine on the network that is closest. In the event that an entire data center fails or is otherwise incapacitated with heavy traffic, an Anycast network can respond to the outage somewhat similarly to how a load balancer transfers traffic across multiple servers inside a data center; the data is routed away from the failing location and instead is routed towards another data center that is still online and functional.

DDoS attacks are currently one of the most substantial threats to the reliability of Internet properties. CDNs that use Anycast have additional flexibility in mitigating DDoS attacks. In most modern DDoS attacks many compromise computers or “bots” are used to form what is known as a botnet. These compromised machines can generate so much Internet traffic that they can overwhelm a typical Unicast-connected machine. With an Anycast network, a portion of the botnet attack traffic can be distributed across multiple data centers, reducing the impact of the attack. Learn about the Cloudflare CDN with Anycast routing.

3. What is Anycast?

Anycast is a network addressing and routing method in which incoming requests can be routed to a variety of different locations or “nodes.” In the context of a CDN, Anycast typically routes incoming traffic to the nearest data center with the capacity to process the request efficiently. Selective routing allows an Anycast network to be resilient in the face of high traffic volume, network congestion, and DDoS attacks.

How does Anycast Work?

Anycast network routing is able to route incoming connection requests across multiple data centers. When requests come into a single IP address associated with the Anycast network, the network distributes the data based on some prioritization methodology. The selection process behind choosing a particular data center will typically be optimized to reduce latency by selecting the data center with the shortest distance from the requester. Anycast is characterized by a 1-to-1 of many association, and is one of the 5 main network protocol methods used in the Internet protocol.

Why Use an Anycast Network?

If many requests are made simultaneously to the same origin server, the server may become overwhelmed with traffic and be unable to respond efficiently to additional incoming requests. With an Anycast network, instead of one origin server taking the brunt of the traffic, the load can also be spread across other available data centers, each of which will have servers capable of processing and responding to the incoming request. This routing method can prevent an origin server from extending capacity and avoids service interruptions to clients requesting content from the origin server.

What is the Difference between Anycast and Unicast?

Most of the Internet works via a routing scheme called Unicast. Under Unicast, every node on the network gets a unique IP address. Home and office networks use Unicast; when a computer is connected to a wireless network and gets a message saying the IP address is already in use, an IP address conflict has occurred because another computer on the same Unicast network is already using the same IP. In most cases, that isn’t allowed.

When a CDN is using a unicast address, traffic is routed directly to the specific node. This creates a vulnerability when the network experiences extraordinary traffic such as during a DDoS attack. Because the traffic is routed directly to a particular data center, the location or its surrounding infrastructure may become overwhelmed with traffic, potentially resulting in denial-of-service to legitimate requests.

Using Anycast means the network can be extremely resilient. Because traffic will find the best path, an entire data center can be taken offline and traffic will automatically flow to a proximal data center.

How does an Anycast network mitigate a DDoS attack?

After other DDoS mitigation tools filter out some of the attack traffic, Anycast distributes the remaining attack traffic across multiple data centers, preventing any one location from becoming overwhelmed with requests. If the capacity of the Anycast network is greater than the attack traffic, the attack is effectively mitigated. In most DDoS attacks, many compromised “zombie” or “bot” computers are used to form what is known as a botnet. These machines can be scattered around the web and generate so much traffic that they can overwhelm a typical Unicast-connected machine.

A properly Anycasted CDN increases the surface area of the receiving network so that the unfiltered denial-of-service traffic from a distributed botnet will be absorbed by each of the CDN’s data centers. As a result, as a network continues to grow in size and capacity it becomes harder and harder to launch an effective DDoS against anyone using the CDN.

It is not easy to setup a true Anycasted network. Proper implementation requires that a CDN provider maintains their own network hardware, builds direct relationships with their upstream carriers, and tunes their networking routes to ensure traffic doesn’t “flap” between multiple locations. This Cloudflare blog post explains how Cloudflare uses Anycast to load balance without load balancers.

4. What is a CDN Data Center?

A data center is a facility housing many networked computers that work together to process, store, and share data. Most major tech companies rely heavily upon data centers as a central component in delivering online services.

What is the difference between a data center and a point-of-presence (PoP)?

The terms data center and point-of-presence (PoP) are sometimes used interchangeably, though distinctions can be made between them. Speaking generally, a PoP may refer to a company having a single server presence in a location while a data center may refer to a location that houses multiple servers. Instead of referring to multiple PoPs in one location, Cloudflare uses the term data center to indicate a location in which many of our servers are maintained.

The concept of a point-of-presence rose to prominence during the court ordered breakup of the Bell telephone system. In the court decision, a point-of-presence referred to a location where long-distance carriers terminate services and shift connections onto a local network. Similarly, on the modern Internet a PoP typically refers to where CDNs have a physical presence in a location, often in the junctures between networks known as Internet exchange points (IxP).

A data center refers to a physical location in which computers are networked together in order to improve usability and reduce costs related to storage, bandwidth, and other networking components. Data centers such as IxP co-location facilities allow different Internet service providers, CDN’s, and other infrastructure companies to connect with each other to share transit.

What are the common concerns in the design of a data center?

Many components and factors are taken into consideration when creating a modern data center. With proper planning, maintenance, and security, a data center is at lower risk of both downtime and data breaches.

Data center considerations include:

  • Redundancy/backup – the level of redundancy varies widely based on the quality of a data center; in high tier data centers, multiple redundancies in power and backup servers are built into the infrastructure.
  • Efficiency – the amount of electricity used at a large data center rivals that of a small town. Whenever possible, data centers attempt to cut down on costs by optimizing cooling processes and using energy-efficient hardware.
  • Security – proper physical security, both in terms of electronic surveillance, access controls, and on-site security guards reduce the risk associated with bad actors attempting to gain site access.
  • Environmental controls/factors – maintaining the right environmental conditions is necessary for the proper functioning of electronic hardware. Keeping both temperature and humidity within acceptable parameters requires the proper balance of air conditioning, humidity control, and airflow regulation. In areas that are vulnerable to earthquakes, properly secured servers are also a necessary concern.
  • Maintenance and monitoring – on-site or on-call network engineers are needed in order stay on top of server crashes and other hardware failures. Proper response helps to ensure server uptime and eliminate reductions in quality of service.
  • Bandwidth – a data center is incomplete without the bandwidth necessary to handle all the requisite network traffic. Bandwidth considerations are a central component in data center infrastructure, with external network connections and internal data center topology both designed around sufficient network capacity.

5. What is an Origin Server?

The purpose of an origin server is to process and respond to incoming internet requests from internet clients. The concept of an origin server is typically used in conjunction with the concept of an edge server or caching server. At its core, an origin server is a computer running one or more programs that are designed to listen for and process incoming internet requests. An origin server can take on all the responsibility of serving up the content for an internet property such as a website, provided that the traffic does not extend beyond what the server is capable of processing and latency is not a primary concern.

The physical distance between an origin server and a client making a request adds latency to the connection, increasing the time it takes for an internet resource such as a webpage to be loaded. The additional round-trip time (RTT) between client and origin server required for a secure internet connection using SSL/TLS also add additional latency to the request, directly impacting the experience of the client requesting data from the origin. By using a Content Distribution Network (CDN) round-trip time is able to be reduced, and the amount of requests to an origin server are also able to be reduced.

What is the difference between an Origin Server and a CDN Edge server?

Put simply, CDN edge servers are computers placed in important junctures between major internet providers in locations across the globe in order to deliver content as quickly as possible. An edge server lives inside a CDN on the “edge” of a network and is specifically designed to quickly process requests. By placing edge servers strategically inside of the Internet Exchange Points (IxPs) that exist between networks, a CDN is able to reduce the amount of time it takes to get to a particular location on the Internet.

These edge servers cache content in order to take the load off of one or more origin servers. By moving static assets like images, HTML and JavaScript files (and potentially other content) as close as possible to the requesting client machine, an edge server cache is able to reduce the amount of time it takes for a web resource to load. Origin servers still have an important function to play when using a CDN, as important server-side code such as the database of hashed client credentials used for authentication is typically maintained inside an origin server.

Here’s a simple example of how an edge server and an origin server work together to serve up a login page and allow a user to login to a service. A very simple login page requires the following static assets to be downloaded for the webpage to render properly:

  1. A HTML file for the webpage
  2. A CSS file for the webpage styling
  3. Several image files
  4. Several JavaScript libraries

These files are all static files; they are not dynamically generated and are the same for all visitors to the website. As a result, these files can be both cached and served to the client from the edge server. All of these files can be loaded closer to the client machine and without any bandwidth consumption by the origin.

Next, when the user enters their login and password and presses “login,” the request for dynamic content travels back to the edge server who then proxies the request back to the origin server. The origin then verifies the user’s identity in the associated database table before sending back the specific account information.

This interplay between edge servers handling static content and origin servers serving up dynamic content is a typical separation of concerns when using a CDN. The capability of some CDNs can also extend beyond this simplistic model.

Can an origin server still be attacked while using a CDN?

The short answer is yes. A CDN does not render an origin server invincible, but when used properly it can render an origin server invisible, acting as a shield for incoming requests. Hiding the real IP address of an origin server is an important part of setting up a CDN. As such, a CDN provider should recommend that the IP address of the origin server be changed when implementing a CDN strategy in order to prevent DDoS attacks from going around the shield and hitting the origin directly. Cloudflare’s CDN includes comprehensive DDoS protection.

6. What is a CDN edge server?

CDN edge server is a computer that exists at the logical extreme or “edge” of a network. An edge server often serves as the connection between separate networks. A primary purpose of a CDN edge server is to store content as close as possible to a requesting client machine, thereby reducing latency and improving page load times.

An edge server is a type of edge device that provides an entry point into a network. Other edges devices include routers and routing switches. Edge devices are often placed inside Internet exchange points (IxPs) to allow different networks to connect and share transit.

How does an edge server work?

In any particular network layout, a number of different devices will connect to each other using one or more predefined network pattern. If a network wants to connect to another network or the larger Internet, it must have some form of bridge in order for traffic to flow from one location to another. Hardware devices that creates this bridge on the edge of a network are called edge devices.

Networks connect across the edge

In a typical home or office network with many devices connected, devices such as mobile phones or computers connect and disconnect to the network through a hub-and-spoke network model. All of the devices exist within the same local area network (LAN), and each device connects to a central router, through which they are able to connect with each other.

In order to connect a second network to the first network, at some point the connection must be made between the networks. The device through which the networks are able to connect with each other is, by definition, an edge device.

Now, if a computer inside Network A needs to connect to a computer inside Network B, the connection must pass from network A, across the network edge, and into the second network. This same paradigm also works in more complex contexts, such when a connection is made across the Internet. The ability for networks to share transit is bottlenecked by the availability of edge devices between them.

When a connection must traverse the Internet, even more intermediary steps must be taken between network A and network B. For the sake of simplicity, let’s imagine that each network is a circle, and the place in which the circles touch is the edge of the network. In order for connection to move across the Internet, it will typically touch many networks and move across many network edge nodes. Generally speaking, the farther the connection must travel, the greater the number of networks that must be traversed. A connection may traverse different Internet service providers and Internet backbone infrastructure hardware before reaching its target.

A CDN provider will place servers in many locations, but some of the most important are the connection points at the edge between different networks. These edge servers will connect with multiple different networks and allow for traffic to pass quickly and efficiently between networks. Without a CDN, transit may take a slower and/or more convoluted route between source and destination. In worst case scenarios, traffic will “trombone” large distances; when connecting to another device across the street, a connection may move across the country and back again. By placing edge servers in key locations, a CDN is able to quickly deliver content to users inside different networks. To learn more about the improvements of using CDN, explore how CDN performance works.

What is the difference between an edge server and an origin server?

An origin server is the web server that receives all Internet traffic when a web property is not using a CDN. Using an origin server without a CDN means that each Internet request must return to the physical location of that origin server, regardless of where in the world it resides. This creates an increase in load times which increases the further the server is from the requesting client machine.

CDN edge servers store (cache) content in strategic locations in order to take the load off of one or more origin servers. By moving static assets like images, HTML and JavaScript files (and potentially other content) as close as possible to the requesting client machine, an edge server cache is able to reduce the amount of time it takes for a web resource to load. Origin servers still have an important function to play when using a CDN, as important server-side code such as a database of hashed client credentials used for authentication, typically is maintained at the origin. Learn about the Cloudflare CDN with edge servers all over the globe.

7. What is a DDoS attack?

A distributed denial-of-service (DDoS) attack is a malicious attempt to disrupt the normal traffic of a targeted server, service or network by overwhelming the target or its surrounding infrastructure with a flood of Internet traffic.

DDoS attacks achieve effectiveness by utilizing multiple compromised computer systems as sources of attack traffic. Exploited machines can include computers and other networked resources such as IoT devices.

From a high level, a DDoS attack is like an unexpected traffic jam clogging up the highway, preventing regular traffic from arriving at its destination.

How does a DDoS attack work?

DDoS attacks are carried out with networks of Internet-connected machines.

These networks consist of computers and other devices (such as IoT devices)which have been infected with malware, allowing them to be controlled remotely by an attacker. These individual devices are referred to as bots (or zombies), and a group of bots is called a botnet.

Once a botnet has been established, the attacker is able to direct an attack by sending remote instructions to each bot.

When a victim’s server or network is targeted by the botnet, each bot sends requests to the target’s IP address, potentially causing the server or network to become overwhelmed, resulting in a denial-of-service to normal traffic.

Because each bot is a legitimate Internet device, separating the attack traffic from normal traffic can be difficult.

How to identify a DDoS attack

The most obvious symptom of a DDoS attack is a site or service suddenly becoming slow or unavailable. But since a number of causes — such a legitimate spike in traffic — can create similar performance issues, further investigation is usually required. Traffic analytics tools can help you spot some of these telltale signs of a DDoS attack:

  • Suspicious amounts of traffic originating from a single IP address or IP range
  • A flood of traffic from users who share a single behavioral profile, such as device type, geolocation, or web browser version
  • An unexplained surge in requests to a single page or endpoint
  • Odd traffic patterns such as spikes at odd hours of the day or patterns that appear to be unnatural (e.g. a spike every 10 minutes)

There are other, more specific signs of DDoS attack that can vary depending on the type of attack.

What are some common types of DDoS attacks?

Different types of DDoS attacks target varying components of a network connection. In order to understand how different DDoS attacks work, it is necessary to know how a network connection is made.

A network connection on the Internet is composed of many different components or “layers”. Like building a house from the ground up, each layer in the model has a different purpose.

The OSI model, shown below, is a conceptual framework used to describe network connectivity in 7 distinct layers.

While nearly all DDoS attacks involve overwhelming a target device or network with traffic, attacks can be divided into three categories. An attacker may use one or more different attack vectors, or cycle attack vectors in response to counter measures taken by the target.

Application layer attacks

The goal of the attack:

Sometimes referred to as a layer 7 DDoS attack (in reference to the 7th layer of the OSI model), the goal of these attacks is to exhaust the target’s resources to create a denial-of-service.

The attacks target the layer where web pages are generated on the server and delivered in response to HTTP requests. A single HTTP request is computationally cheap to execute on the client side, but it can be expensive for the target server to respond to, as the server often loads multiple files and runs database queries in order to create a web page.

Layer 7 attacks are difficult to defend against, since it can be hard to differentiate malicious traffic from legitimate traffic.

Application layer attack example:

HTTP flood

This attack is similar to pressing refresh in a web browser over and over on many different computers at once – large numbers of HTTP requests flood the server, resulting in denial-of-service.

This type of attack ranges from simple to complex.

Simpler implementations may access one URL with the same range of attacking IP addresses, referrers and user agents. Complex versions may use a large number of attacking IP addresses, and target random urls using random referrers and user agents.

Protocol attacks

The goal of the attack:

Protocol attacks, also known as a state-exhaustion attacks, cause a service disruption by over-consuming server resources and/or the resources of network equipment like firewalls and load balancers.

Protocol attacks utilize weaknesses in layer 3 and layer 4 of the protocol stack to render the target inaccessible.

Protocol attack example:

SYN flood

A SYN Flood is analogous to a worker in a supply room receiving requests from the front of the store.

The worker receives a request, goes and gets the package, and waits for confirmation before bringing the package out front. The worker then gets many more package requests without confirmation until they can’t carry any more packages, become overwhelmed, and requests start going unanswered.

This attack exploits the TCP handshake — the sequence of communications by which two computers initiate a network connection — by sending a target a large number of TCP “Initial Connection Request” SYN packets with spoofed source IP addresses.

The target machine responds to each connection request and then waits for the final step in the handshake, which never occurs, exhausting the target’s resources in the process.

Volumetric attacks

The goal of the attack:

This category of attacks attempts to create congestion by consuming all available bandwidth between the target and the larger Internet. Large amounts of data are sent to a target by using a form of amplification or another means of creating massive traffic, such as requests from a botnet.

Amplification example:

DNS Amplification

A DNS amplification is like if someone were to call a restaurant and say “I’ll have one of everything, please call me back and repeat my whole order,” where the callback number actually belongs to the victim. With very little effort, a long response is generated and sent to the victim.

By making a request to an open DNS server with a spoofed IP address (the IP address of the victim), the target IP address then receives a response from the server.

What is the process for mitigating a DDoS attack?

The key concern in mitigating a DDoS attack is differentiating between attack traffic and normal traffic.

For example, if a product release has a company’s website swamped with eager customers, cutting off all traffic is a mistake. If that company suddenly has a surge in traffic from known attackers, efforts to alleviate an attack are probably necessary.

The difficulty lies in telling the real customers apart from the attack traffic.

In the modern Internet, DDoS traffic comes in many forms. The traffic can vary in design from un-spoofed single source attacks to complex and adaptive multi-vector attacks.

A multi-vector DDoS attack uses multiple attack pathways in order to overwhelm a target in different ways, potentially distracting mitigation efforts on any one trajectory.

An attack that targets multiple layers of the protocol stack at the same time, such as a DNS amplification (targeting layers 3/4) coupled with an HTTP flood (targeting layer 7) is an example of multi-vector DDoS.

Mitigating a multi-vector DDoS attack requires a variety of strategies in order to counter different trajectories.

Generally speaking, the more complex the attack, the more likely it is that the attack traffic will be difficult to separate from normal traffic – the goal of the attacker is to blend in as much as possible, making mitigation efforts as inefficient as possible.

Mitigation attempts that involve dropping or limiting traffic indiscriminately may throw good traffic out with the bad, and the attack may also modify and adapt to circumvent countermeasures. In order to overcome a complex attempt at disruption, a layered solution will give the greatest benefit.

Blackhole routing

One solution available to virtually all network admins is to create a blackhole route and funnel traffic into that route. In its simplest form, when blackhole filtering is implemented without specific restriction criteria, both legitimate and malicious network traffic is routed to a null route, or blackhole, and dropped from the network.

If an Internet property is experiencing a DDoS attack, the property’s Internet service provider (ISP) may send all the site’s traffic into a blackhole as a defense. This is not an ideal solution, as it effectively gives the attacker their desired goal: it makes the network inaccessible.

Rate limiting

Limiting the number of requests a server will accept over a certain time window is also a way of mitigating denial-of-service attacks.

While rate limiting is useful in slowing web scrapers from stealing content and for mitigating brute force login attempts, it alone will likely be insufficient to handle a complex DDoS attack effectively.

Nevertheless, rate limiting is a useful component in an effective DDoS mitigation strategy.

Web application firewall

A Web Application Firewall (WAF) is a tool that can assist in mitigating a layer 7 DDoS attack. By putting a WAF between the Internet and an origin server, the WAF may act as a reverse proxy, protecting the targeted server from certain types of malicious traffic.

By filtering requests based on a series of rules used to identify DDoS tools, layer 7 attacks can be impeded. One key value of an effective WAF is the ability to quickly implement custom rules in response to an attack.

Anycast network diffusion

This mitigation approach uses an Anycast network to scatter the attack traffic across a network of distributed servers to the point where the traffic is absorbed by the network.

Like channeling a rushing river down separate smaller channels, this approach spreads the impact of the distributed attack traffic to the point where it becomes manageable, diffusing any disruptive capability.

The reliability of an Anycast network to mitigate a DDoS attack is dependent on the size of the attack and the size and efficiency of the network. An important part of the DDoS mitigation implemented by Cloudflare is the use of an Anycast distributed network.

Cloudflare has a 142 Tbps network, which is an order of magnitude greater than the largest DDoS attack recorded.

If you are currently under attack, there are steps you can take to get out from under the pressure. If you are on Cloudflare already, you can follow these steps to mitigate your attack.

The DDoS protection that we implement at Cloudflare is multifaceted in order to mitigate the many possible attack vectors. Learn more about Cloudflare’s DDoS protection and how it works.

8. What is caching?

Caching is the process of storing copies of files in a cache, or temporary storage location, so that they can be accessed more quickly. Technically, a cache is any temporary storage location for copies of files or data, but the term is often used in reference to Internet technologies. Web browsers cache HTML files, JavaScript, and images in order to load websites more quickly, while DNS servers cache DNS records for faster lookups and CDN servers cache content to reduce latency.

To understand how caches work, consider real-world caches of food and other supplies. When explorer Roald Amundsen made his return journey from his trip to the South Pole in 1912, he and his men subsisted on the caches of food they had stored along the way. This was much more efficient than waiting for supplies to be delivered from their base camp as they traveled. Caches on the Internet serve a similar purpose; they temporarily store the ‘supplies’, or content, needed for users to make their journey across the web.

What does a browser cache do?

Every time a user loads a webpage, their browser has to download quite a lot of data in order to display that webpage. To shorten page load times, browsers cache most of the content that appears on the webpage, saving a copy of the webpage’s content on the device’s hard drive. This way, the next time the user loads the page, most of the content is already stored locally and the page will load much more quickly.

Browsers store these files until their time to live (TTL) expires or until the hard drive cache is full. (TTL is an indication of how long content should be cached.) Users can also clear their browser cache if desired.

What does clearing a browser cache accomplish?

Once a browser cache is cleared, every webpage that loads will load as if it is the first time the user has visited the page. If something loaded incorrectly the first time and was cached, clearing the cache can allow it to load correctly. However, clearing one’s browser cache can also temporarily slow page load times.

What is CDN caching?

A CDN, or content delivery network, caches content (such as images, videos, or webpages) in proxy servers that are located closer to end users than origin servers. (A proxy server is a server that receives requests from clients and passes them along to other servers.) Because the servers are closer to the user making the request, a CDN is able to deliver content more quickly.

Think of a CDN as being like a chain of grocery stores: Instead of going all the way to the farms where food is grown, which could be hundreds of miles away, shoppers go to their local grocery store, which still requires some travel but is much closer. Because grocery stores stock food from faraway farms, grocery shopping takes minutes instead of days. Similarly, CDN caches ‘stock’ the content that appears on the Internet so that webpages load much more quickly.

When a user requests content from a website using a CDN, the CDN fetches that content from an origin server, and then saves a copy of the content for future requests. Cached content remains in the CDN cache as long as users continue to request it.

What is a CDN cache hit? What is a cache miss?

cache hit is when a client device makes a request to the cache for content, and the cache has that content saved. A cache miss occurs when the cache does not have the requested content.

A cache hit means that the content will be able to load much more quickly, since the CDN can immediately deliver it to the end user. In the case of a cache miss, a CDN server will pass the request along to the origin server, then cache the content once the origin server responds, so that subsequent requests will result in a cache hit.

Where are CDN caching servers located?

CDN caching servers are located in data centers all over the globe. Cloudflare has CDN servers in 270 cities spread out throughout the world in order to be as close to end users accessing the content as possible. A location where CDN servers are present is also called a data center.

How long does cached data remain in a CDN server?

When websites respond to CDN servers with the requested content, they attach the content’s TTL as well, letting the servers know how long to store it. The TTL is stored in a part of the response called the HTTP header, and it specifies for how many seconds, minutes, or hours content will be cached. When the TTL expires, the cache removes the content. Some CDNs will also purge files from the cache early if the content is not requested for a while, or if a CDN customer manually purges certain content.

How do other kinds of caching work?

DNS caching takes place on DNS servers. The servers store recent DNS lookups in their cache so that they do not have to query nameservers and can instantly reply with the IP address of a domain.

Search engines may cache webpages that frequently appear in search results in order to answer user queries even if the website they are attempting to access is temporarily down or unable to respond.

How does Cloudflare use caching?

Cloudflare offers a CDN with 270 PoPs distributed internationally. Cloudflare offers free CDN caching services, while paid CDN customers are able to customize how their content is cached. The network is Anycast, meaning the same content can be delivered from any of these data centers. A user in London and a user in Sydney can both view the same content loaded from CDN servers only a few miles away.

9. What is a reverse proxy?

A reverse proxy is a server that sits in front of web servers and forwards client (e.g. web browser) requests to those web servers. Reverse proxies are typically implemented to help increase securityperformance, and reliability. In order to better understand how a reverse proxy works and the benefits it can provide, let’s first define what a proxy server is.

What’s a proxy server?

A forward proxy, often called a proxy, proxy server, or web proxy, is a server that sits in front of a group of client machines. When those computers make requests to sites and services on the Internet, the proxy server intercepts those requests and then communicates with web servers on behalf of those clients, like a middleman.

For example, let’s name 3 computers involved in a typical forward proxy communication:

  • A: This is a user’s home computer
  • B: This is a forward proxy server
  • C: This is a website’s origin server (where the website data is stored)

In a standard Internet communication, computer A would reach out directly to computer C, with the client sending requests to the origin server and the origin server responding to the client. When a forward proxy is in place, A will instead send requests to B, which will then forward the request to C. C will then send a response to B, which will forward the response back to A.

Why would anyone add this extra middleman to their Internet activity? There are a few reasons one might want to use a forward proxy:

  • To avoid state or institutional browsing restrictions – Some governments, schools, and other organizations use firewalls to give their users access to a limited version of the Internet. A forward proxy can be used to get around these restrictions, as they let the user connect to the proxy rather than directly to the sites they are visiting.
  • To block access to certain content – Conversely, proxies can also be set up to block a group of users from accessing certain sites. For example, a school network might be configured to connect to the web through a proxy which enables content filtering rules, refusing to forward responses from Facebook and other social media sites.
  • To protect their identity online – In some cases, regular Internet users simply desire increased anonymity online, but in other cases, Internet users live in places where the government can impose serious consequences to political dissidents. Criticizing the government in a web forum or on social media can lead to fines or imprisonment for these users. If one of these dissidents uses a forward proxy to connect to a website where they post politically sensitive comments, the IP address used to post the comments will be harder to trace back to the dissident. Only the IP address of the proxy server will be visible.

How is a reverse proxy different?

A reverse proxy is a server that sits in front of one or more web servers, intercepting requests from clients. This is different from a forward proxy, where the proxy sits in front of the clients. With a reverse proxy, when clients send requests to the origin server of a website, those requests are intercepted at the network edge by the reverse proxy server. The reverse proxy server will then send requests to and receive responses from the origin server.

The difference between a forward and reverse proxy is subtle but important. A simplified way to sum it up would be to say that a forward proxy sits in front of a client and ensures that no origin server ever communicates directly with that specific client. On the other hand, a reverse proxy sits in front of an origin server and ensures that no client ever communicates directly with that origin server.

Once again, let’s illustrate by naming the computers involved:

  • D: Any number of users’ home computers
  • E: This is a reverse proxy server
  • F: One or more origin servers

Typically all requests from D would go directly to F, and F would send responses directly to D. With a reverse proxy, all requests from D will go directly to E, and E will send its requests to and receive responses from F. E will then pass along the appropriate responses to D.

Below we outline some of the benefits of a reverse proxy:

  • Load balancing – A popular website that gets millions of users every day may not be able to handle all of its incoming site traffic with a single origin server. Instead, the site can be distributed among a pool of different servers, all handling requests for the same site. In this case, a reverse proxy can provide a load balancing solution which will distribute the incoming traffic evenly among the different servers to prevent any single server from becoming overloaded. In the event that a server fails completely, other servers can step up to handle the traffic.
  • Protection from attacks – With a reverse proxy in place, a web site or service never needs to reveal the IP address of their origin server(s). This makes it much harder for attackers to leverage a targeted attack against them, such as a DDoS attack. Instead the attackers will only be able to target the reverse proxy, such as Cloudflare’s CDN, which will have tighter security and more resources to fend off a cyber attack.
  • Global Server Load Balancing (GSLB) – In this form of load balancing, a website can be distributed on several servers around the globe and the reverse proxy will send clients to the server that’s geographically closest to them. This decreases the distances that requests and responses need to travel, minimizing load times.
  • Caching – A reverse proxy can also cache content, resulting in faster performance. For example, if a user in Paris visits a reverse-proxied website with web servers in Los Angeles, the user might actually connect to a local reverse proxy server in Paris, which will then have to communicate with an origin server in L.A. The proxy server can then cache (or temporarily save) the response data. Subsequent Parisian users who browse the site will then get the locally cached version from the Parisian reverse proxy server, resulting in much faster performance.
  • SSL encryption – Encrypting and decrypting SSL (or TLS) communications for each client can be computationally expensive for an origin server. A reverse proxy can be configured to decrypt all incoming requests and encrypt all outgoing responses, freeing up valuable resources on the origin server.

How to implement a reverse proxy

Some companies build their own reverse proxies, but this requires intensive software and hardware engineering resources, as well as a significant investment in physical hardware. One of the easiest and most cost-effective ways to reap all the benefits of a reverse proxy is by signing up for a CDN service. For example, the Cloudflare CDN provides all the performance and security features listed above, as well as many others.









What Is A CDN & Where Does It Shine?



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