What is DNS

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Short Description of DNS
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    Classification: //Dell SecureWorks/Confidential - Limited External Distribution: What is DNS? Invented in 1982, the Domain Name System is a replacement for a centralized 'hostname to numerical address database' (in reality it was a simple text file) that was used in the days of ARPANET, the precursor to what is known today as the Internet. The purpose of the srcinal database was to provide an easy way to identify a network connected computer, instead of the unfriendly numerical addresses. It is much easier to remember 'google.com' instead of '74.125.237.130'.  As the number of interconnected machines grew, maintaining this information in a central database, as well as ensuring that all clients on the network had an identical copy of this database at all times, became increasingly difficult. The modern DNS addressed this issue by replacing the previous central database with a distributed network of systems. By design, the DNS is more than a simple name-to-IP address mapping database. It allows for many additional properties to be assigned to a domain name, including associated email addresses, anti-spam information, and much more. Domain Name Hierarchy Domain names are the most common method used for accessing websites or any other host on the Internet. Each domain name is made up of a number of elements (called 'labels') separated by a dot. For example: www.google.com The domain name system works in a hierarchical model, with the right-most elements classed as the 'Top Level Domain' or TLD, followed by the second element, which is classed as the 'Second Level Domain.' This structure continues from right to left with each element being classed as a subdomain of the element to its right. In addition to the structure above, any element may be classed as a hostname should it be associated with one or more IP addresses, and provided it meets the following basic rules as stated in the DoD Host Table Specification as well as RFC1123:    A hostname must be a text string consisting of only the letters A through Z (upper or lower case), digits 0 through 9, the minus sign (-), and the period (.)    Classification: //Dell SecureWorks/Confidential - Limited External Distribution:    A hostname cannot contain any spaces    The first character must be an alphabetic character or a digit    The last character cannot be a minus sign or a period    The recommended length for a hostname is up to 24 characters HOW DNS WORKS DNS Request Process When a user attempts to load a website by entering the site's URL into their browser (or any URI with a hostname), a series of requests take place in the background. The illustration below outlines the structure and sequence of these events, and demonstrates how entering a domain name into a browser eventually results in the browser knowing exactly what IP address to connect to at the network layer. 1.   The user queries their Internet Service Provider's (ISP) DNS resolver asking for the IP address for 'www.google.com'. 2.   The DNS resolver asks the root nameserver where it can find details for 'www.google.com', unless it already has the information cached. 3.   If it is asked, the root nameserver responds that this information is handled by the .com nameserver. 4.   The DNS resolver asks the .com nameserver where it can find details for 'www.google.com', unless it already has the information cached. 5.   If it is asked, the .com nameserver responds that this information can be found at the nameservers of google.com. 6.   The DNS resolver asks the google.com nameservers where it can find details for 'www.google.com', unless it already has the information cached. 7.   If it is asked, the google.com nameservers have this information and respond with a DNS record containing the IP address for 'www.google.com'.    Classification: //Dell SecureWorks/Confidential - Limited External Distribution: 8.   The ISP's DNS resolver then sends this information back to the user. The user then knows what IP address to connect to in order to access 'www.google.com'. Once the user knows which IP address to connect to in order to retrieve the content of 'www.google.com' the DNS job is complete. DNS Root Servers As demonstrated in the previous diagram, DNS infrastructure is designed in a distributed manner, with different servers responsible for different sections of the DNS namespace. At the top of the hierarchical structure are the DNS 'root' servers. These servers are responsible for the initial delegation of requests received from DNS resolvers to the correct top level domain nameservers. At the time of writing of this guide, there are 13 root servers defined in the DNS root zone. The hostnames of these servers are in the following format: <LETTER>.ROOT-SERVERS.NET   The letter values are A-M and each of these root server hostnames is managed by a different organization (with the exception of A and J, which are both currently managed by Verisign). The following table represents the current details of the DNS root servers: Letter IPv4 address IPv6 address Operator   A 198.41.0.4 2001:503:ba3e::2:30 Verisign   B 192.228.79.201 N/A USC-ISI   C 192.33.4.12 N/A Cogent Communications D 199.7.91.13 2001:500:2d::d University of Maryland   E 192.203.230.10 N/A NASA   F 192.5.5.241 2001:500:2f::f Internet Systems Consortium   G 192.112.36.4 N/A Defense Information Systems Agency   H 128.63.2.53 2001:500:1::803f:235 U.S. Army Research Lab   I 192.36.148.17 2001:7fe::53 Netnod   J 192.58.128.30 2001:503:c27::2:30 Verisign   K 193.0.14.129 2001:7fd::1 RIPE NCC   L 199.7.83.42 2001:500:3::42 ICANN   M 202.12.27.33 2001:dc3::35 WIDE Project    Classification: //Dell SecureWorks/Confidential - Limited External Distribution: The reason the number 13 was chosen as the number of root server hostnames is primarily the desire for the details of all root servers to fit in one DNS packet. DNS messages were srcinally limited to 512 bytes of data, and each IPv4 address is exactly 32 bytes long, so the number 13 was chosen. This requires 416 bytes of space, leaving 96 bytes for future use or other supporting information. While there are only 13 root server names , in reality there are many more DNS root servers than this. A majority of the IP addresses assigned to the root server hostnames (as seen in the above table) are actually 'anycast' addresses. Anycast is a technique that uses the same address at multiple (physical) servers at the same time. For example, the root server 'L.ROOT-SERVERS.NET' which is managed by ICANN is actually a cluster of over 130 physical servers distributed around the globe. This type of redundancy serves two main purposes: 1.   To ensure speedy responses to DNS queries (by providing network-topologically short distances to the root servers); and 2.   To minimize the likelihood of an outage of the entire DNS system. Management of the DNS root zone itself is actually controlled by the United States Government's Department of Commerce. The zone is managed by the Internet Assigned Numbers Authority (IANA); the IANA contract is currently held by the Internet Corporation for Assigned Names and Numbers (ICANN). Any changes proposed for the DNS root zone must be approved by the US Department of Commerce before they can be implemented. CONFIGURING DNS Nameservers In the example given above, we can see that nameservers play a key role in the DNS request process. When you register a new domain name, the nameservers for that domain are one of the first items you may configure. This is normally done through the same company with which you registered your domain name. Nameserver configuration page at Godaddy.com  Configuring the nameservers provides the parent domains (e.g. .com) with an address at which the rest of the records for that domain name can be found. As was seen in
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