How Domain Name Servers Work | HowStuffWorks
If you've ever used the Internet, it's a good bet that you've used the Domain Name System, or DNS, even without realizing it. DNS is a protocol within the set of. In connecting to a host, using the fully qualified domain name shows where the user wants to go. A DNS server can resolve the host name to an IP address. The Domain Name System (DNS) is a hierarchical decentralized naming system for computers, It defines the DNS protocol, a detailed specification of the data structures and . For proper operation of its domain name resolver, a network host is . and IP addresses are not required to match in a one-to-one relationship .
Typically, such caching DNS servers also implement the recursive algorithm necessary to resolve a given name starting with the DNS root through to the authoritative name servers of the queried domain.
With this function implemented in the name server, user applications gain efficiency in design and operation. The combination of DNS caching and recursive functions in a name server is not mandatory; the functions can be implemented independently in servers for special purposes.
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Internet service providers typically provide recursive and caching name servers for their customers. In addition, many home networking routers implement DNS caches and recursors to improve efficiency in the local network. A resolver is responsible for initiating and sequencing the queries that ultimately lead to a full resolution translation of the resource sought, e. DNS resolvers are classified by a variety of query methods, such as recursive, non-recursive, and iterative.
A resolution process may use a combination of these methods. For example, a simple stub resolver running on a home router typically makes a recursive query to the DNS server run by the user's ISP.
A recursive query is one for which the DNS server answers the query completely by querying other name servers as needed.
How Domain Name Servers Work
In typical operation, a client issues a recursive query to a caching recursive DNS server, which subsequently issues non-recursive queries to determine the answer and send a single answer back to the client. The resolver, or another DNS server acting recursively on behalf of the resolver, negotiates use of recursive service using bits in the query headers.
DNS servers are not required to support recursive queries. Each server refers the client to the next server in the chain, until the current server can fully resolve the request. For example, a possible resolution of www. Circular dependencies and glue records[ edit ] Name servers in delegations are identified by name, rather than by IP address. This means that a resolving name server must issue another DNS request to find out the IP address of the server to which it has been referred.
If the name given in the delegation is a subdomain of the domain for which the delegation is being provided, there is a circular dependency.
In this case, the name server providing the delegation must also provide one or more IP addresses for the authoritative name server mentioned in the delegation. This information is called glue. The delegating name server provides this glue in the form of records in the additional section of the DNS response, and provides the delegation in the authority section of the response. A glue record is a combination of the name server and IP address. For example, if the authoritative name server for example.
As ns1 is contained in example. To break the dependency, the name server for the top level domain org includes glue along with the delegation for example. The glue records are address records that provide IP addresses for ns1. The resolver uses one or more of these IP addresses to query one of the domain's authoritative servers, which allows it to complete the DNS query.
Record caching[ edit ] A standard practice in implementing name resolution in applications is to reduce the load on the Domain Name System servers by caching results locally, or in intermediate resolver hosts.
Results obtained from a DNS request are always associated with the time to live TTLan expiration time after which the results must be discarded or refreshed. The period of validity may vary from a few seconds to days or even weeks. As a result of this distributed caching architecture, changes to DNS records do not propagate throughout the network immediately, but require all caches to expire and to be refreshed after the TTL.
Some resolvers may override TTL values, as the protocol supports caching for up to sixty-eight years or no caching at all. Negative cachingi. Multiple domain names may be associated with an IP address.
The DNS stores IP addresses in the form of domain names as specially formatted names in pointer PTR records within the infrastructure top-level domain arpa. For IPv4, the domain is in-addr. For IPv6, the reverse lookup domain is ip6.
What's the difference between a host name, a domain name and a fully qualified domain name (FDQN)?
The IP address is represented as a name in reverse-ordered octet representation for IPv4, and reverse-ordered nibble representation for IPv6. When performing a reverse lookup, the DNS client converts the address into these formats before querying the name for a PTR record following the delegation chain as for any DNS query.
For example, assuming the IPv4 address ARIN's servers delegate Instead DNS resolution takes place transparently in applications such as web browserse-mail clientsand other Internet applications. When an application makes a request that requires a domain name lookup, such programs send a resolution request to the DNS resolver in the local operating system, which in turn handles the communications required.
The DNS resolver will almost invariably have a cache see above containing recent lookups. If the cache can provide the answer to the request, the resolver will return the value in the cache to the program that made the request. If the cache does not contain the answer, the resolver will send the request to one or more designated DNS servers. In the case of most home users, the Internet service provider to which the machine connects will usually supply this DNS server: In any event, the name server thus queried will follow the process outlined aboveuntil it either successfully finds a result or does not.
It then returns its results to the DNS resolver; assuming it has found a result, the resolver duly caches that result for future use, and hands the result back to the software which initiated the request. Broken resolvers[ edit ] Some large ISPs have configured their DNS servers to violate rules, such as by disobeying TTLs, or by indicating that a domain name does not exist just because one of its name servers does not respond. This practice can add extra difficulty when debugging DNS issues, as it obscures the history of such data.
These caches typically use very short caching times — in the order of one minute. Other applications[ edit ] The Domain Name System includes several other functions and features. Hostnames and IP addresses are not required to match in a one-to-one relationship. Multiple hostnames may correspond to a single IP address, which is useful in virtual hostingin which many web sites are served from a single host. Alternatively, a single hostname may resolve to many IP addresses to facilitate fault tolerance and load distribution to multiple server instances across an enterprise or the global Internet.
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DNS serves other purposes in addition to translating names to IP addresses. For instance, mail transfer agents use DNS to find the best mail server to deliver e-mail: An MX record provides a mapping between a domain and a mail exchanger; this can provide an additional layer of fault tolerance and load distribution. A common method is to place the IP address of the subject host into the sub-domain of a higher level domain name, and to resolve that name to a record that indicates a positive or a negative indication.
It points to 5. This hostname is either not configured, or resolves to E-mail servers can query blacklist.
What's the difference between a host name, a domain name and a fully qualified domain name (FDQN)?
Many of such blacklists, either subscription-based or free of cost, are available for use by email administrators and anti-spam software. To provide resilience in the event of computer or network failure, multiple DNS servers are usually provided for coverage of each domain. At the top level of global DNS, thirteen groups of root name servers exist, with additional "copies" of them distributed worldwide via anycast addressing.
Each message consists of a header and four sections: A header field flags controls the content of these four sections. The identification field can be used to match responses with queries. The flag field consists of several sub-fields. These string of numbers, such as Without DNS, your brain would be filled with numbers just trying to remember the IP Addresses for every single website you use!
Local Cache A cache is a block of memory for temporary storage of data that has a high probability of being used again. If you accessed the website recently, it would have the IP address cached in the system.
In that case, the browser can immediately call the IP Address to retrieve the webpage! This setting determines how long the cache may be stored when the website is accessed. We will address how that works later on. It reads the request and locates the appropriate domain server to redirect to. As such, it plays an important role in redirection to the next layer of Domain Servers.
After being passed down by RLDS, this layer works in the same way as the second gatekeeper.
The number of domain names are increasing exponentially. As such, it is redirected to the TLDS to diversify the processing power and memory required. This DNS Server is usually owned by the institute that is responsible for hosting your website.
As such, a request for the record of the domain is sent to this DNS Server.