What is DNS Server?

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How Domain Name Servers Work
How Domain Name Servers Work. For example, when you enter "https: If so, click the Continue button. Your computer stores the record in its cache, reads the IP address from the record, then passes this information to your browser. With this function implemented in the name server, user applications gain efficiency in design and operation. To request a hostname and an address and add a computer to the master file, users contacted the SRI's Network Information Center NIC , directed by Elizabeth Feinler , by telephone during business hours. Retrieved 28 July

Your Answer

What is DNS?

Click the radio button Use the following DNS server addresses and type Click on View network status and tasks. Click on View status. Vista may ask for your permission to make changes. If so, click the Continue button. Click the radio button "Use the following DNS server addresses: Click the OK button, then the Close button, and the Close button again.

Close the Network and Sharing Center window. Generic Router Open the preferences for your router. Find the DNS server settings. Scan for the letters DNS next to a field which allows two or three sets of numbers, each broken into four groups of one to three numbers. Put in the OpenDNS server addresses, Linksys Router Visit the router's IP address in a new browser window.

Enter the Network password. The "Enter Network Password" window will appear. Skip user name and type the router's password admin is the default password, if you haven't changed it and click the OK button. Type in OpenDNS addresses, Repeat the procedure for additional network connections you want to change.

If the lock icon in the lower left-hand corner of the window is locked, click the icon to make changes, and when prompted to authenticate, enter your password. To change the settings for a wireless connection, select Airport , and click Advanced. Select the DNS tab. To change the settings for an Ethernet connection, select the Wired tab, then select your network interface in the list.

It is usually called eth0. To change the settings for a wireless connection, select the Wireless tab, then select the appropriate wireless network. If the method is set to something else, do not change it. Click Apply to save the change.

If you are prompted for a password or confirmation, type the password or provide confirmation. Replace the nameserver lines with, or add, the following lines: Restart any Internet clients you are using. Replace that line with, or add, the following line: Routers Every router uses a different user interface for configuring DNS server settings; we provide only a generic procedure below. To change your settings on a router: In your browser, enter the IP address to access the router's administration console.

When prompted, enter the password to access network settings. Find the screen in which DNS server settings are specified. If there are IP addresses specified in the fields for the primary and seconday DNS servers, write them down for future reference. Replace those addresses with the Google IP addresses: To change your settings on a mobile device: A resolution process may use a combination of these methods.

In a non-recursive query , a DNS resolver queries a DNS server that provides a record either for which the server is authoritative, or it provides a partial result without querying other servers. 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.

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. 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.

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 TTL , an 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 caching , i. 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.

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 Users generally do not communicate directly with a DNS resolver. Instead DNS resolution takes place transparently in applications such as web browsers , e-mail clients , and 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 above , until 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. 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.

Some applications, such as web browsers, maintain an internal DNS cache to avoid repeated lookups via the network. 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.

Internet Explorer represents a notable exception: Google Chrome triggers a specific error message for DNS issues. 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 hosting , in 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. 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. E-mail servers can query blacklist. 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 header section contains the following fields: The identification field can be used to match responses with queries. The flag field consists of several sub-fields.

The first is a single bit which indicates if the message is a query 0 or a reply 1. The second sub-field consists of four bits indicating the type of query, or the type of query this message is a response to. A single-bit sub-field indicates if the DNS server is authoritative for the queried hostname. Another single-bit sub-field indicates if the client wants to send a recursive query "RD". Another sub-field indicates if the message was truncated for some reason "TC" , and a four-bit sub-field is used for error codes.

The domain name is broken into discrete labels which are concatenated; each label is prefixed by the length of that label. The answer section has the resource records of the queried name. A domain name may occur in multiple records if it has multiple IP addresses associated. TCP is also used for tasks such as zone transfers.

Some resolver implementations use TCP for all queries.

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