The Internet Flashcards
Domain Name System (DNS)
the phonebook of the Internet. Humans access information online through domain names, like nytimes.com or espn.com. Web browsers interact through Internet Protocol (IP) addresses. DNS translates domain names to IP addresses so browsers can load Internet resources.
Each device connected to the Internet has a unique IP address which other machines use to find the device. DNS servers eliminate the need for humans to memorize IP addresses such as 192.168.1.1 (in IPv4), or more complex newer alphanumeric IP addresses such as 2400:cb00:2048:1::c629:d7a2 (in IPv6).
How Does DNS Work?
The process of DNS resolution involves converting a hostname (such as www.example.com) into a computer-friendly IP address (such as 192.168.1.1). An IP address is given to each device on the Internet, and that address is necessary to find the appropriate Internet device - like a street address is used to find a particular home. When a user wants to load a webpage, a translation must occur between what a user types into their web browser (example.com) and the machine-friendly address necessary to locate the example.com webpage.
DNS server to load webpage:
DNS Recursor
The recursor can be thought of as a librarian who is asked to go find a particular book somewhere in a library. The DNS recursor is a server designed to receive queries from client machines through applications such as web browsers. Typically the recursor is then responsible for making additional requests in order to satisfy the client’s DNS query.
DNS server to load webpage:
Root nameserver
The root server is the first step in translating (resolving) human readable host names into IP addresses. It can be thought of like an index in a library that points to different racks of books - typically it serves as a reference to other more specific locations.
DNS server to load webpage:
TLD nameserver
The top level domain server (TLD) can be thought of as a specific rack of books in a library. This nameserver is the next step in the search for a specific IP address, and it hosts the last portion of a hostname (In example.com, the TLD server is “com”).
DNS server to load webpage:
Authoritative nameserver
This final nameserver can be thought of as a dictionary on a rack of books, in which a specific name can be translated into its definition. The authoritative nameserver is the last stop in the nameserver query. If the authoritative name server has access to the requested record, it will return the IP address for the requested hostname back to the DNS Recursor (the librarian) that made the initial request.
What’s the difference between an authoritative DNS server and a recursive DNS resolver?
Both concepts refer to servers (groups of servers) that are integral to the DNS infrastructure, but each performs a different role and lives in different locations inside the pipeline of a DNS query. One way to think about the difference is the recursive resolver is at the beginning of the DNS query and the authoritative nameserver is at the end.
Recursive DNS resolver
The recursive resolver is the computer that responds to a recursive request from a client and takes the time to track down the DNS record. It does this by making a series of requests until it reaches the authoritative DNS nameserver for the requested record (or times out or returns an error if no record is found). Luckily, recursive DNS resolvers do not always need to make multiple requests in order to track down the records needed to respond to a client; caching is a data persistence process that helps short-circuit the necessary requests by serving the requested resource record earlier in the DNS lookup.
Authoritative DNS Server
Put simply, an authoritative DNS server is a server that actually holds, and is responsible for, DNS resource records. This is the server at the bottom of the DNS lookup chain that will respond with the queried resource record, ultimately allowing the web browser making the request to reach the IP address needed to access a website or other web resources. An authoritative nameserver can satisfy queries from its own data without needing to query another source, as it is the final source of truth for certain DNS records.
in instances where the query is for a subdomain such as foo.example.com or blog.cloudflare.com, an additional nameserver will be added to the sequence after the authoritative nameserver, which is responsible for storing the subdomain’s CNAME record.
The 8 steps in a DNS lookup
A user types ‘example.com’ into a web browser and the query travels into the Internet and is received by a DNS recursive resolver.
The resolver then queries a DNS root nameserver (.).
The root server then responds to the resolver with the address of a Top Level Domain (TLD) DNS server (such as .com or .net), which stores the information for its domains. When searching for example.com, our request is pointed toward the .com TLD.
The resolver then makes a request to the .com TLD.
The TLD server then responds with the IP address of the domain’s nameserver, example.com.
Lastly, the recursive resolver sends a query to the domain’s nameserver.
The IP address for example.com is then returned to the resolver from the nameserver.
The DNS resolver then responds to the web browser with the IP address of the domain requested initially.
Once the 8 steps of the DNS lookup have returned the IP address for example.com, the browser is able to make the request for the web page:
The browser makes a HTTP request to the IP address.
The server at that IP returns the webpage to be rendered in the browser (step 10).
What is a DNS resolver?
The DNS resolver is the first stop in the DNS lookup, and it is responsible for dealing with the client that made the initial request. The resolver starts the sequence of queries that ultimately leads to a URL being translated into the necessary IP address.
Note: A typical uncached DNS lookup will involve both recursive and iterative queries.
It’s important to differentiate between a recursive DNS query and a recursive DNS resolver. The query refers to the request made to a DNS resolver requiring the resolution of the query. A DNS recursive resolver is the computer that accepts a recursive query and processes the response by making the necessary requests.
DNS query:
Recursive query
In a recursive query, a DNS client requires that a DNS server (typically a DNS recursive resolver) will respond to the client with either the requested resource record or an error message if the resolver can’t find the record.
DNS query:
Iterative query
in this situation the DNS client will allow a DNS server to return the best answer it can. If the queried DNS server does not have a match for the query name, it will return a referral to a DNS server authoritative for a lower level of the domain namespace. The DNS client will then make a query to the referral address. This process continues with additional DNS servers down the query chain until either an error or timeout occurs.
DNS query:
Non-recursive query
typically this will occur when a DNS resolver client queries a DNS server for a record that it has access to either because it’s authoritative for the record or the record exists inside of its cache. Typically, a DNS server will cache DNS records to prevent additional bandwidth consumption and load on upstream servers.
What is DNS caching? Where does DNS caching occur?
The purpose of caching is to temporarily stored data in a location that results in improvements in performance and reliability for data requests. DNS caching involves storing data closer to the requesting client so that the DNS query can be resolved earlier and additional queries further down the DNS lookup chain can be avoided, thereby improving load times and reducing bandwidth/CPU consumption. DNS data can be cached in a variety of locations, each of which will store DNS records for a set amount of time determined by a time-to-live (TTL).
Browser DNS caching
Modern web browsers are designed by default to cache DNS records for a set amount of time. The purpose here is obvious; the closer the DNS caching occurs to the web browser, the fewer processing steps must be taken in order to check the cache and make the correct requests to an IP address. When a request is made for a DNS record, the browser cache is the first location checked for the requested record.
In Chrome, you can see the status of your DNS cache by going to chrome://net-internals/#dns.
Operating system (OS) level DNS caching
The operating system level DNS resolver is the second and last local stop before a DNS query leaves your machine. The process inside your operating system that is designed to handle this query is commonly called a “stub resolver” or DNS client. When a stub resolver gets a request from an application, it first checks its own cache to see if it has the record. If it does not, it then sends a DNS query (with a recursive flag set), outside the local network to a DNS recursive resolver inside the Internet service provider (ISP).
When the recursive resolver inside the ISP receives a DNS query, like all previous steps, it will also check to see if the requested host-to-IP-address translation is already stored inside its local persistence layer.
The recursive resolver also has additional functionality depending on the types of records it has in its cache:
If the resolver does not have the A records, but does have the NS records for the authoritative nameservers, it will query those name servers directly, bypassing several steps in the DNS query. This shortcut prevents lookups from the root and .com nameservers (in our search for example.com) and helps the resolution of the DNS query occur more quickly.
If the resolver does not have the NS records, it will send a query to the TLD servers (.com in our case), skipping the root server.
In the unlikely event that the resolver does not have records pointing to the TLD servers, it will then query the root servers. This event typically occurs after a DNS cache has been purged.
What is a domain name?
a string of text that maps to an alphanumeric IP address, used to access a website from client software. In plain English, a domain name is the text that a user types into a browser window to reach a particular website. For instance, the domain name for Google is ‘google.com’.
The actual address of a website is a complex numerical IP address (e.g. 192.0.2.2), but thanks to DNS, users are able to enter human-friendly domain names and be routed to the websites they are looking for. This process is known as a DNS lookup.
Who managed domain names?
Domain names are all managed by domain registries, which delegate the reservation of domain names to registrars. Anyone who wants to create a website can register a domain name with a registrar, and there are currently over 300 million registered domain names.
What is the difference between a domain name and a URL?
A uniform resource locator (URL), sometimes called a web address, contains the domain name of a site as well as other information, including the protocol and the path. For example, in the URL ‘https://cloudflare.com/learning/’, ‘cloudflare.com’ is the domain name, while ‘https’ is the protocol and ‘/learning/’ is the path to a specific page on the website.
What are the parts of a domain name?
Domain names are typically broken up into two or three parts, each separated by a dot. When read right-to-left, the identifiers in domain names go from most general to most specific. The section to the right of the last dot in a domain name is the top-level domain (TLD). These include the ‘generic’ TLDs such as ‘.com’, ‘.net’, and ‘.org’, as well as country-specific TLDs like ‘.uk’ and ‘.jp’.
To the left of the TLD is the second-level domain (2LD) and if there is anything to the left of the 2LD, it is called the third-level domain (3LD). Let’s look at a couple of examples:
For Google’s US domain name, ‘google.com’:
’.com’ is the TLD (most general)
’google’ is the 2LD (most specific)
But for Google UK’s domain name, ‘google.co.uk’:
’.com’ is the TLD (most general)
’.co’* is the 2LD
’google’ is the 3LD (most specific)
*In this case, the 2LD indicates the type of organization that registered the domain (.co in the UK is for sites registered by companies).
How to keep a domain name secure?
Once a domain name has been registered with a registrar, that registrar is in charge of notifying the registrant when their domain is about to expire and giving them the chance to renew, ensuring they do not lose their domain name. In some cases, registrars will prey on their users’ expired domain names by buying those domains the second they expire and then selling them back to the original registrant at an exorbitant price. It is important to choose an honest and trustworthy registrar to avoid these kinds of predatory practices.
web server
A computer that hosts a website on the Internet.
web page
simple document displayable by a browser. Such documents are written in the HTML language (which we look into in more detail in other articles). A web page can embed a variety of different types of resources such as:
style information — controlling a page’s look-and-feel
scripts — which add interactivity to the page
media — images, sounds, and videos.
The term web server can refer to hardware or software, or both of them working together.
On the hardware side, a web server is a computer that stores web server software and a website’s component files (for example, HTML documents, images, CSS stylesheets, and JavaScript files). A web server connects to the Internet and supports physical data interchange with other devices connected to the web.
On the software side, a web server includes several parts that control how web users access hosted files. At a minimum, this is an HTTP server. An HTTP server is software that understands URLs (web addresses) and HTTP (the protocol your browser uses to view webpages). An HTTP server can be accessed through the domain names of the websites it stores, and it delivers the content of these hosted websites to the end user’s device.
At the most basic level, whenever a browser needs a file that is hosted on a web server, the browser requests the file via HTTP. When the request reaches the correct (hardware) web server, the (software) HTTP server accepts the request, finds the requested document, and sends it back to the browser, also through HTTP. (If the server doesn’t find the requested document, it returns a 404 response instead.)
To publish a website, you need either a static or a dynamic web server.
static web server
or stack, consists of a computer (hardware) with an HTTP server (software). We call it “static” because the server sends its hosted files as-is to your browser.
dynamic web server
consists of a static web server plus extra software, most commonly an application server and a database. We call it “dynamic” because the application server updates the hosted files before sending content to your browser via the HTTP server.
