แสดงบทความที่มีป้ายกำกับ Domain name registrar แสดงบทความทั้งหมด
แสดงบทความที่มีป้ายกำกับ Domain name registrar แสดงบทความทั้งหมด
วันอาทิตย์ที่ 15 มิถุนายน พ.ศ. 2551
Claiming An Infringement On Your Domain Name
Claiming An Infringement On Your Domain NameBy : Richard A. ChapoA domain name is your address on the web, which can make it vitally important. So, what do you do when someone tries to move onto your yard?If you work on the web at all, you know that domain names can be very similar. This raises the question of when one domain crosses the line and infringes on another. The issue is often brought up with larger commercial sites when others register similar domains.A claim for domain name infringement is governed by the Uniform Domain Name Dispute Resolution Policy issued by ICANN. As a domain owner, you might be surprised to learn you have consented to this. It occurred when you purchased your domain, although you probably didn't read the terms and conditions associated with your purchase.To prove your domain is being infringed upon, you have to prove certain things. They are as follows:1. You must prove the domain name in dispute is identical or confusingly similar to a trade or service mark you have registered. In simple terms, this means you must show the other guy is using a domain that most people would associated with your business. If I have a trademark for ZipIt and someone is using a NET suffix of this, it is confusing for the public.2. You must also prove the owner of the other domain has no rights or legitimate interests in the domain.3. Finally, you must prove domain name is registered by the other party and being used in bad faith. Indications of bad faith include the other party tried to sell it to you or one of your competitors for more than they paid for it, the other party has done this repeatedly to other companies, the other party registered the domain in an attempt to intentionally steal your clients.If you decide to pursue an infringement action, it is always handled as a binding arbitration. This essentially means the dispute is heard by a panel of arbitrators and their decision is binding upon you and the other party. The advantage of this is it tends to be less expensive than going to court, but your damages are limited to an act instead of money. Specifically, the panel can only issue instructions for the offending domain to be canceled, modified or left alone. The panel cannot award any monetary damages.At the end of the day, using the infringement arbitration provisions of the Uniform Domain Name Dispute Resolution Policy provided by ICANN is a good choice if you just want to terminate a domain you feel is infringing upon yours. The key to the dispute, however, is the fact you need to have a trademark or service mark first in most cases.About the AuthorProtect yourself with a domain name purchase agreement from SanDiegoBusinessLawFirm.com.
Premium domain names
In the business of marketing domain names, "premium" domain names are often valuable, and have particular characterizations: The names are short and memorable, may contain words that are regularly searched on search engines or keywords that help the name gain a higher ranking on search engines. They may contain generic words, so the word has more than one meaning, and they may contain common typos.
Subdomain
For a protein domain inside of a larger domain, see Protein domains.
In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain.[1] For example, "example.com" is a subdomain of the "com" top-level domain (TLD) while "www.example.com" is a subdomain in the domain "example.com". In fact, the "com" TLD is a subdomain of the root domain, ".". This hierarchical organisation is similar to that in a filesystem; something is a subdomain if it could be equated to a folder, and a record within that subdomain to a file. Note, though, that DNS names are written in descending hierarchy right-to-left, where filesystems are written left-to-right.
Relative to a subdomain, the larger domain that it is a part of is its parent domain, or alternately superdomain (the former term appears to be preferred by the IETF).
Note that "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org comprises a subdomain of the domain wikipedia.org. In theory, this subdivision can go down to 127 levels deep, and each DNS label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice some domain registries have shorter limits than that.
A subdomain is sometimes termed a vanity domain, especially when it is a subdomain of an ISP's own domain aliased to an individual user account. However, the term "vanity domain" has other usages, discussed at that article.
Some websites use a different subdomains to point to different server clusters. For example, www.example.com points to Server Cluster 1 or Datacentre 1, and www2.example.com points to Server Cluster 2 or Datacentre 2, etc.
Subdomains are commonly used by organizations that wish to assign a unique name to a particular department, function, or service related to the organization. For example, a university might assign "cs" to the computer science department, such that a number of hosts could be used inside that subdomain, such as mail.cs.example.edu or www.cs.example.edu.
Depending on application, a record inside a domain, or subdomain might refer to a Host name, or a service provided by a number of machines in a cluster.
In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain.[1] For example, "example.com" is a subdomain of the "com" top-level domain (TLD) while "www.example.com" is a subdomain in the domain "example.com". In fact, the "com" TLD is a subdomain of the root domain, ".". This hierarchical organisation is similar to that in a filesystem; something is a subdomain if it could be equated to a folder, and a record within that subdomain to a file. Note, though, that DNS names are written in descending hierarchy right-to-left, where filesystems are written left-to-right.
Relative to a subdomain, the larger domain that it is a part of is its parent domain, or alternately superdomain (the former term appears to be preferred by the IETF).
Note that "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org comprises a subdomain of the domain wikipedia.org. In theory, this subdivision can go down to 127 levels deep, and each DNS label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice some domain registries have shorter limits than that.
A subdomain is sometimes termed a vanity domain, especially when it is a subdomain of an ISP's own domain aliased to an individual user account. However, the term "vanity domain" has other usages, discussed at that article.
Some websites use a different subdomains to point to different server clusters. For example, www.example.com points to Server Cluster 1 or Datacentre 1, and www2.example.com points to Server Cluster 2 or Datacentre 2, etc.
Subdomains are commonly used by organizations that wish to assign a unique name to a particular department, function, or service related to the organization. For example, a university might assign "cs" to the computer science department, such that a number of hosts could be used inside that subdomain, such as mail.cs.example.edu or www.cs.example.edu.
Depending on application, a record inside a domain, or subdomain might refer to a Host name, or a service provided by a number of machines in a cluster.
Domain Name System
The Domain Name System (DNS) associates various information with domain names; most importantly, it serves as the "phone book" for the Internet by translating human-readable computer hostnames, e.g. www.example.com, into IP addresses, e.g. 208.77.188.166, which networking equipment needs to deliver information. It also stores other information such as the list of mail servers that accept email for a given domain. In providing a worldwide keyword-based redirection service, the Domain Name System is an essential component of contemporary Internet use.
Uses
The most basic task of DNS is to translate hostnames to IP addresses. In very simple terms, it can be compared to a phone book. DNS also has other important uses.
Above all, DNS makes it possible to assign Internet names to organizations (or concerns they represent) independent of the physical routing hierarchy represented by the numerical IP address. Because of this, hyperlinks and Internet contact information can remain the same, whatever the current IP routing arrangements may be, and can take a human-readable form (such as "example.com"), which is easier to remember than the IP address 208.77.188.166. People take advantage of this when they recite meaningful URLs and e-mail addresses without caring how the machine will actually locate them.
The Domain Name System distributes the responsibility for assigning domain names and mapping them to IP networks by allowing an authoritative name server for each domain to keep track of its own changes, avoiding the need for a central register to be continually consulted and updated.
History
The practice of using a name as a more human-legible abstraction of a machine's numerical address on the network predates even TCP/IP, and goes all the way to the ARPAnet era. Back then however, a different system was used, as DNS was invented only in 1983, shortly after TCP/IP was deployed. With the older system, each computer on the network retrieved a file called HOSTS.TXT from a computer at SRI (now SRI International)[1][2]. The HOSTS.TXT file mapped numerical addresses to names. A hosts file still exists on most modern operating systems, either by default or through configuration, and allows users to specify an IP address (eg. 208.77.188.166) to use for a hostname (eg. www.example.net) without checking DNS. Systems based on a hosts file have inherent limitations, because of the obvious requirement that every time a given computer's address changed, every computer that seeks to communicate with it would need an update to its hosts file.
The growth of networking called for a more scalable system, one that recorded a change in a host's address in one place only. Other hosts would learn about the change dynamically through a notification system, thus completing a globally accessible network of all hosts' names and their associated IP Addresses.
At the request of Jon Postel, Paul Mockapetris invented the Domain Name system in 1983 and wrote the first implementation. The original specifications appear in RFC 882 and RFC 883. In November 1987, the publication of RFC 1034 and RFC 1035 updated the DNS specification and made RFC 882 and RFC 883 obsolete. Several more-recent RFCs have proposed various extensions to the core DNS protocols.
In 1984, four Berkeley students — Douglas Terry, Mark Painter, David Riggle and Songnian Zhou — wrote the first UNIX implementation, which was maintained by Ralph Campbell thereafter. In 1985, Kevin Dunlap of DEC significantly re-wrote the DNS implementation and renamed it BIND (Berkeley Internet Name Domain, previously: Berkeley Internet Name Daemon). Mike Karels, Phil Almquist and Paul Vixie have maintained BIND since then. BIND was ported to the Windows NT platform in the early 1990s.
Due to BIND's long history of security issues and exploits, several alternative nameserver and resolver programs have been written and distributed in recent years.
How DNS works in theory
Domain names, arranged in a tree, cut into zones, each served by a nameserver.
The domain name space
The domain name space consists of a tree of domain names. Each node or leaf in the tree has zero or more resource records, which hold information associated with the domain name. The tree sub-divides into zones beginning at the root zone. A DNS zone consists of a collection of connected nodes authoritatively served by an authoritative DNS nameserver. (Note that a single nameserver can host several zones.)
When a system administrator wants to let another administrator control a part of the domain name space within the first administrator’s zone of authority, control can be delegated to the second administrator. This splits off a part of the old zone into a new zone, which comes under the authority of the second administrator's nameservers. The old zone ceases to be authoritative for the new zone.
Parts of a domain name
A domain name usually consists of two or more parts (technically a label), which is conventionally written separated by dots, such as example.com.
The rightmost label conveys the top-level domain (for example, the address www.example.com has the top-level domain com).
Each label to the left specifies a subdivision, or subdomain of the domain above it. Note: “subdomain” expresses relative dependence, not absolute dependence. For example: example.com comprises a subdomain of the com domain, and www.example.com comprises a subdomain of the domain example.com. In theory, this subdivision can go down 127 levels. Each label can contain up to 63 characters. The whole domain name does not exceed a total length of 253 characters. [3] In practice, some domain registries may have shorter limits.
A hostname refers to a domain name that has one or more associated IP addresses; ie: the 'www.example.com' and 'example.com' domains are both hostnames, however, the 'com' domain is not.
DNS servers
Main article: Name server
The Domain Name System consists of a hierarchical set of DNS servers. Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains "beneath" it. The hierarchy of authoritative DNS servers matches the hierarchy of domains. At the top of the hierarchy stand the root nameservers: the servers to query when looking up (resolving) a top-level domain name (TLD).
DNS resolvers
See also: resolv.conf
A resolver looks up the resource record information associated with nodes. A resolver knows how to communicate with name servers by sending DNS queries and heeding DNS responses.
A DNS query may be either a recursive query or a non-recursive query:
A non-recursive query is one where the DNS server may provide a partial answer to the query (or give an error). DNS servers must support non-recursive queries.
A recursive query is one where the DNS server will fully answer the query (or give an error). DNS servers are not required to support recursive queries.
The resolver (or another DNS server acting recursively on behalf of the resolver) negotiates use of recursive service using bits in the query headers.
Resolving usually entails iterating through several name servers to find the needed information. However, some resolvers function simplistically and can communicate only with a single name server. These simple resolvers rely on a recursive query to a recursive name server to perform the work of finding information for them.
Address resolution mechanism
(This description deliberately uses the fictional .example TLD in accordance with the DNS guidelines themselves.)
In theory a full host name may have several name segments, (e.g ahost.ofasubnet.ofabiggernet.inadomain.example). In practice, in the experience of the majority of public users of Internet services, full host names will frequently consist of just three segments (ahost.inadomain.example, and most often www.inadomain.example).
For querying purposes, software interprets the name segment by segment, from right to left, using an iterative search procedure. At each step along the way, the program queries a corresponding DNS server to provide a pointer to the next server which it should consult.
A DNS recursor consults three nameservers to resolve the address www.wikipedia.org.
As originally envisaged, the process was as simple as:
the local system is pre-configured with the known addresses of the root servers in a file of root hints, which need to be updated periodically by the local administrator from a reliable source to be kept up to date with the changes which occur over time.
query one of the root servers to find the server authoritative for the next level down (so in the case of our simple hostname, a root server would be asked for the address of a server with detailed knowledge of the example top level domain).
querying this second server for the address of a DNS server with detailed knowledge of the second-level domain (inadomain.example in our example).
repeating the previous step to progress down the name, until the final step which would, rather than generating the address of the next DNS server, return the final address sought.
The diagram illustrates this process for the real host www.wikipedia.org.
The mechanism in this simple form has a difficulty: it places a huge operating burden on the root servers, with each and every search for an address starting by querying one of them. Being as critical as they are to the overall function of the system such heavy use would create an insurmountable bottleneck for trillions of queries placed every day. The section DNS in practice describes how this is addressed.
Circular dependencies and glue records
Name servers in delegations appear listed 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. Since this can introduce a circular dependency if the nameserver referred to is under the domain that it is authoritative of, it is occasionally necessary for the nameserver providing the delegation to also provide the IP address of the next nameserver. This record is called a glue record.
For example, assume that the sub-domain en.wikipedia.org contains further sub-domains (such as something.en.wikipedia.org) and that the authoritative name server for these lives at ns1.something.en.wikipedia.org. A computer trying to resolve something.en.wikipedia.org will thus first have to resolve ns1.something.en.wikipedia.org. Since ns1 is also under the something.en.wikipedia.org subdomain, resolving ns1.something.en.wikipedia.org requires resolving something.en.wikipedia.org which is exactly the circular dependency mentioned above. The dependency is broken by the glue record in the nameserver of en.wikipedia.org that provides the IP address of ns1.something.en.wikipedia.org directly to the requestor, enabling it to bootstrap the process by figuring out where ns1.something.en.wikipedia.org is located.
In practice
When an application (such as a web browser) tries to find the IP address of a domain name, it doesn't necessarily follow all of the steps outlined in the Theory section above. We will first look at the concept of caching, and then outline the operation of DNS in "the real world."
Caching and time to live
Because of the huge volume of requests generated by a system like DNS, the designers wished to provide a mechanism to reduce the load on individual DNS servers. To this end, the DNS resolution process allows for caching (i.e. the local recording and subsequent consultation of the results of a DNS query) for a given period of time after a successful answer. How long a resolver caches a DNS response (i.e. how long a DNS response remains valid) is determined by a value called the time to live (TTL). The TTL is set by the administrator of the DNS server handing out the response. The period of validity may vary from just seconds to days or even weeks.
Caching time
As a noteworthy consequence of this distributed and caching architecture, changes to DNS do not always take effect immediately and globally. This is best explained with an example: If an administrator has set a TTL of 6 hours for the host www.wikipedia.org, and then changes the IP address to which www.wikipedia.org resolves at 12:01pm, the administrator must consider that a person who cached a response with the old IP address at 12:00noon will not consult the DNS server again until 6:00pm. The period between 12:01pm and 6:00pm in this example is called caching time, which is best defined as a period of time that begins when you make a change to a DNS record and ends after the maximum amount of time specified by the TTL expires. This essentially leads to an important logistical consideration when making changes to DNS: not everyone is necessarily seeing the same thing you're seeing. RFC 1537 helps to convey basic rules for how to set the TTL.
Note that the term "propagation", although very widely used in this context, does not describe the effects of caching well. Specifically, it implies that [1] when you make a DNS change, it somehow spreads to all other DNS servers (instead, other DNS servers check in with yours as needed), and [2] that you do not have control over the amount of time the record is cached (you control the TTL values for all DNS records in your domain, except your NS records and any authoritative DNS servers that use your domain name).
Some resolvers may override TTL values, as the protocol supports caching for up to 68 years or no caching at all. Negative caching (the non-existence of records) is determined by name servers authoritative for a zone which MUST include the Start of Authority (SOA) record when reporting no data of the requested type exists. The MINIMUM field of the SOA record and the TTL of the SOA itself is used to establish the TTL for the negative answer. RFC 2308
Many people incorrectly refer to a mysterious 48 hour or 72 hour propagation time when you make a DNS change. When one changes the NS records for one's domain or the IP addresses for hostnames of authoritative DNS servers using one's domain (if any), there can be a lengthy period of time before all DNS servers use the new information. This is because those records are handled by the zone parent DNS servers (for example, the .com DNS servers if your domain is example.com), which typically cache those records for 48 hours. However, those DNS changes will be immediately available for any DNS servers that do not have them cached. And any DNS changes on your domain other than the NS records and authoritative DNS server names can be nearly instantaneous, if you choose for them to be (by lowering the TTL once or twice ahead of time, and waiting until the old TTL expires before making the change).
In the real world
DNS resolving from program to OS-resolver to ISP-resolver to greater system.
Users generally do not communicate directly with a DNS resolver. Instead DNS-resolution takes place transparently in client-applications such as web-browsers, mail-clients, and other Internet applications. When an application makes a request which requires a DNS lookup, such programs send a resolution request to the local 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: such a user will either have configured that server's address manually or allowed DHCP to set it; however, where systems administrators have configured systems to use their own DNS servers, their DNS resolvers point to separately maintained nameservers of the organization. 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.
Broken resolvers
An additional level of complexity emerges when resolvers violate the rules of the DNS protocol. A number of large ISPs have configured their DNS servers to violate rules (presumably to allow them to run on less-expensive hardware than a fully-compliant resolver), 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.[citation needed]
As a final level of complexity, some applications (such as web-browsers) also have their own DNS cache, in order to reduce the use of the DNS resolver library itself. This practice can add extra difficulty when debugging DNS issues, as it obscures the freshness of data, and/or what data comes from which cache. These caches typically use very short caching times — on the order of one minute. Internet Explorer offers a notable exception: recent versions cache DNS records for half an hour.[4]
Other applications
The system outlined above provides a somewhat simplified scenario. The Domain Name System includes several other functions:
Hostnames and IP addresses do not necessarily match on a one-to-one basis. Many hostnames may correspond to a single IP address: combined with virtual hosting, this allows a single machine to serve many web sites. Alternatively a single hostname may correspond to many IP addresses: this can facilitate fault tolerance and load distribution, and also allows a site to move physical location seamlessly.
There are many uses of DNS besides translating names to IP addresses. For instance, Mail transfer agents use DNS to find out where to deliver e-mail for a particular address. The domain to mail exchanger mapping provided by MX records accommodates another layer of fault tolerance and load distribution on top of the name to IP address mapping.
Sender Policy Framework and DomainKeys instead of creating their own record types were designed to take advantage of another DNS record type, the TXT record.
To provide resilience in the event of computer failure, multiple DNS servers are usually provided for coverage of each domain, and at the top level, thirteen very powerful root servers exist, with additional "copies" of several of them distributed worldwide via Anycast.
Uses
The most basic task of DNS is to translate hostnames to IP addresses. In very simple terms, it can be compared to a phone book. DNS also has other important uses.
Above all, DNS makes it possible to assign Internet names to organizations (or concerns they represent) independent of the physical routing hierarchy represented by the numerical IP address. Because of this, hyperlinks and Internet contact information can remain the same, whatever the current IP routing arrangements may be, and can take a human-readable form (such as "example.com"), which is easier to remember than the IP address 208.77.188.166. People take advantage of this when they recite meaningful URLs and e-mail addresses without caring how the machine will actually locate them.
The Domain Name System distributes the responsibility for assigning domain names and mapping them to IP networks by allowing an authoritative name server for each domain to keep track of its own changes, avoiding the need for a central register to be continually consulted and updated.
History
The practice of using a name as a more human-legible abstraction of a machine's numerical address on the network predates even TCP/IP, and goes all the way to the ARPAnet era. Back then however, a different system was used, as DNS was invented only in 1983, shortly after TCP/IP was deployed. With the older system, each computer on the network retrieved a file called HOSTS.TXT from a computer at SRI (now SRI International)[1][2]. The HOSTS.TXT file mapped numerical addresses to names. A hosts file still exists on most modern operating systems, either by default or through configuration, and allows users to specify an IP address (eg. 208.77.188.166) to use for a hostname (eg. www.example.net) without checking DNS. Systems based on a hosts file have inherent limitations, because of the obvious requirement that every time a given computer's address changed, every computer that seeks to communicate with it would need an update to its hosts file.
The growth of networking called for a more scalable system, one that recorded a change in a host's address in one place only. Other hosts would learn about the change dynamically through a notification system, thus completing a globally accessible network of all hosts' names and their associated IP Addresses.
At the request of Jon Postel, Paul Mockapetris invented the Domain Name system in 1983 and wrote the first implementation. The original specifications appear in RFC 882 and RFC 883. In November 1987, the publication of RFC 1034 and RFC 1035 updated the DNS specification and made RFC 882 and RFC 883 obsolete. Several more-recent RFCs have proposed various extensions to the core DNS protocols.
In 1984, four Berkeley students — Douglas Terry, Mark Painter, David Riggle and Songnian Zhou — wrote the first UNIX implementation, which was maintained by Ralph Campbell thereafter. In 1985, Kevin Dunlap of DEC significantly re-wrote the DNS implementation and renamed it BIND (Berkeley Internet Name Domain, previously: Berkeley Internet Name Daemon). Mike Karels, Phil Almquist and Paul Vixie have maintained BIND since then. BIND was ported to the Windows NT platform in the early 1990s.
Due to BIND's long history of security issues and exploits, several alternative nameserver and resolver programs have been written and distributed in recent years.
How DNS works in theory
Domain names, arranged in a tree, cut into zones, each served by a nameserver.
The domain name space
The domain name space consists of a tree of domain names. Each node or leaf in the tree has zero or more resource records, which hold information associated with the domain name. The tree sub-divides into zones beginning at the root zone. A DNS zone consists of a collection of connected nodes authoritatively served by an authoritative DNS nameserver. (Note that a single nameserver can host several zones.)
When a system administrator wants to let another administrator control a part of the domain name space within the first administrator’s zone of authority, control can be delegated to the second administrator. This splits off a part of the old zone into a new zone, which comes under the authority of the second administrator's nameservers. The old zone ceases to be authoritative for the new zone.
Parts of a domain name
A domain name usually consists of two or more parts (technically a label), which is conventionally written separated by dots, such as example.com.
The rightmost label conveys the top-level domain (for example, the address www.example.com has the top-level domain com).
Each label to the left specifies a subdivision, or subdomain of the domain above it. Note: “subdomain” expresses relative dependence, not absolute dependence. For example: example.com comprises a subdomain of the com domain, and www.example.com comprises a subdomain of the domain example.com. In theory, this subdivision can go down 127 levels. Each label can contain up to 63 characters. The whole domain name does not exceed a total length of 253 characters. [3] In practice, some domain registries may have shorter limits.
A hostname refers to a domain name that has one or more associated IP addresses; ie: the 'www.example.com' and 'example.com' domains are both hostnames, however, the 'com' domain is not.
DNS servers
Main article: Name server
The Domain Name System consists of a hierarchical set of DNS servers. Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains "beneath" it. The hierarchy of authoritative DNS servers matches the hierarchy of domains. At the top of the hierarchy stand the root nameservers: the servers to query when looking up (resolving) a top-level domain name (TLD).
DNS resolvers
See also: resolv.conf
A resolver looks up the resource record information associated with nodes. A resolver knows how to communicate with name servers by sending DNS queries and heeding DNS responses.
A DNS query may be either a recursive query or a non-recursive query:
A non-recursive query is one where the DNS server may provide a partial answer to the query (or give an error). DNS servers must support non-recursive queries.
A recursive query is one where the DNS server will fully answer the query (or give an error). DNS servers are not required to support recursive queries.
The resolver (or another DNS server acting recursively on behalf of the resolver) negotiates use of recursive service using bits in the query headers.
Resolving usually entails iterating through several name servers to find the needed information. However, some resolvers function simplistically and can communicate only with a single name server. These simple resolvers rely on a recursive query to a recursive name server to perform the work of finding information for them.
Address resolution mechanism
(This description deliberately uses the fictional .example TLD in accordance with the DNS guidelines themselves.)
In theory a full host name may have several name segments, (e.g ahost.ofasubnet.ofabiggernet.inadomain.example). In practice, in the experience of the majority of public users of Internet services, full host names will frequently consist of just three segments (ahost.inadomain.example, and most often www.inadomain.example).
For querying purposes, software interprets the name segment by segment, from right to left, using an iterative search procedure. At each step along the way, the program queries a corresponding DNS server to provide a pointer to the next server which it should consult.
A DNS recursor consults three nameservers to resolve the address www.wikipedia.org.
As originally envisaged, the process was as simple as:
the local system is pre-configured with the known addresses of the root servers in a file of root hints, which need to be updated periodically by the local administrator from a reliable source to be kept up to date with the changes which occur over time.
query one of the root servers to find the server authoritative for the next level down (so in the case of our simple hostname, a root server would be asked for the address of a server with detailed knowledge of the example top level domain).
querying this second server for the address of a DNS server with detailed knowledge of the second-level domain (inadomain.example in our example).
repeating the previous step to progress down the name, until the final step which would, rather than generating the address of the next DNS server, return the final address sought.
The diagram illustrates this process for the real host www.wikipedia.org.
The mechanism in this simple form has a difficulty: it places a huge operating burden on the root servers, with each and every search for an address starting by querying one of them. Being as critical as they are to the overall function of the system such heavy use would create an insurmountable bottleneck for trillions of queries placed every day. The section DNS in practice describes how this is addressed.
Circular dependencies and glue records
Name servers in delegations appear listed 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. Since this can introduce a circular dependency if the nameserver referred to is under the domain that it is authoritative of, it is occasionally necessary for the nameserver providing the delegation to also provide the IP address of the next nameserver. This record is called a glue record.
For example, assume that the sub-domain en.wikipedia.org contains further sub-domains (such as something.en.wikipedia.org) and that the authoritative name server for these lives at ns1.something.en.wikipedia.org. A computer trying to resolve something.en.wikipedia.org will thus first have to resolve ns1.something.en.wikipedia.org. Since ns1 is also under the something.en.wikipedia.org subdomain, resolving ns1.something.en.wikipedia.org requires resolving something.en.wikipedia.org which is exactly the circular dependency mentioned above. The dependency is broken by the glue record in the nameserver of en.wikipedia.org that provides the IP address of ns1.something.en.wikipedia.org directly to the requestor, enabling it to bootstrap the process by figuring out where ns1.something.en.wikipedia.org is located.
In practice
When an application (such as a web browser) tries to find the IP address of a domain name, it doesn't necessarily follow all of the steps outlined in the Theory section above. We will first look at the concept of caching, and then outline the operation of DNS in "the real world."
Caching and time to live
Because of the huge volume of requests generated by a system like DNS, the designers wished to provide a mechanism to reduce the load on individual DNS servers. To this end, the DNS resolution process allows for caching (i.e. the local recording and subsequent consultation of the results of a DNS query) for a given period of time after a successful answer. How long a resolver caches a DNS response (i.e. how long a DNS response remains valid) is determined by a value called the time to live (TTL). The TTL is set by the administrator of the DNS server handing out the response. The period of validity may vary from just seconds to days or even weeks.
Caching time
As a noteworthy consequence of this distributed and caching architecture, changes to DNS do not always take effect immediately and globally. This is best explained with an example: If an administrator has set a TTL of 6 hours for the host www.wikipedia.org, and then changes the IP address to which www.wikipedia.org resolves at 12:01pm, the administrator must consider that a person who cached a response with the old IP address at 12:00noon will not consult the DNS server again until 6:00pm. The period between 12:01pm and 6:00pm in this example is called caching time, which is best defined as a period of time that begins when you make a change to a DNS record and ends after the maximum amount of time specified by the TTL expires. This essentially leads to an important logistical consideration when making changes to DNS: not everyone is necessarily seeing the same thing you're seeing. RFC 1537 helps to convey basic rules for how to set the TTL.
Note that the term "propagation", although very widely used in this context, does not describe the effects of caching well. Specifically, it implies that [1] when you make a DNS change, it somehow spreads to all other DNS servers (instead, other DNS servers check in with yours as needed), and [2] that you do not have control over the amount of time the record is cached (you control the TTL values for all DNS records in your domain, except your NS records and any authoritative DNS servers that use your domain name).
Some resolvers may override TTL values, as the protocol supports caching for up to 68 years or no caching at all. Negative caching (the non-existence of records) is determined by name servers authoritative for a zone which MUST include the Start of Authority (SOA) record when reporting no data of the requested type exists. The MINIMUM field of the SOA record and the TTL of the SOA itself is used to establish the TTL for the negative answer. RFC 2308
Many people incorrectly refer to a mysterious 48 hour or 72 hour propagation time when you make a DNS change. When one changes the NS records for one's domain or the IP addresses for hostnames of authoritative DNS servers using one's domain (if any), there can be a lengthy period of time before all DNS servers use the new information. This is because those records are handled by the zone parent DNS servers (for example, the .com DNS servers if your domain is example.com), which typically cache those records for 48 hours. However, those DNS changes will be immediately available for any DNS servers that do not have them cached. And any DNS changes on your domain other than the NS records and authoritative DNS server names can be nearly instantaneous, if you choose for them to be (by lowering the TTL once or twice ahead of time, and waiting until the old TTL expires before making the change).
In the real world
DNS resolving from program to OS-resolver to ISP-resolver to greater system.
Users generally do not communicate directly with a DNS resolver. Instead DNS-resolution takes place transparently in client-applications such as web-browsers, mail-clients, and other Internet applications. When an application makes a request which requires a DNS lookup, such programs send a resolution request to the local 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: such a user will either have configured that server's address manually or allowed DHCP to set it; however, where systems administrators have configured systems to use their own DNS servers, their DNS resolvers point to separately maintained nameservers of the organization. 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.
Broken resolvers
An additional level of complexity emerges when resolvers violate the rules of the DNS protocol. A number of large ISPs have configured their DNS servers to violate rules (presumably to allow them to run on less-expensive hardware than a fully-compliant resolver), 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.[citation needed]
As a final level of complexity, some applications (such as web-browsers) also have their own DNS cache, in order to reduce the use of the DNS resolver library itself. This practice can add extra difficulty when debugging DNS issues, as it obscures the freshness of data, and/or what data comes from which cache. These caches typically use very short caching times — on the order of one minute. Internet Explorer offers a notable exception: recent versions cache DNS records for half an hour.[4]
Other applications
The system outlined above provides a somewhat simplified scenario. The Domain Name System includes several other functions:
Hostnames and IP addresses do not necessarily match on a one-to-one basis. Many hostnames may correspond to a single IP address: combined with virtual hosting, this allows a single machine to serve many web sites. Alternatively a single hostname may correspond to many IP addresses: this can facilitate fault tolerance and load distribution, and also allows a site to move physical location seamlessly.
There are many uses of DNS besides translating names to IP addresses. For instance, Mail transfer agents use DNS to find out where to deliver e-mail for a particular address. The domain to mail exchanger mapping provided by MX records accommodates another layer of fault tolerance and load distribution on top of the name to IP address mapping.
Sender Policy Framework and DomainKeys instead of creating their own record types were designed to take advantage of another DNS record type, the TXT record.
To provide resilience in the event of computer failure, multiple DNS servers are usually provided for coverage of each domain, and at the top level, thirteen very powerful root servers exist, with additional "copies" of several of them distributed worldwide via Anycast.
Domain name registrar
A domain name registrar is a company accredited by the Internet Corporation for Assigned Names and Numbers (ICANN) and/or by a national ccTLD authority to register Internet domain names. These "retail" companies are often distinct from the "wholesale" domain name registry operator.
The Internet Corporation for Assigned Names and Numbers, or ICANN, has authority over generic top-level domains, or gTLDs. Examples of gTLDs include .com, .net, .org and .mobi. ICANN does not have authority over ccTLDs, or Country Code Top-Level Domains, though it is quite common for domain name registrars to offer ccTLD registration services as well. Most registrars provide DNS hosting service, but this is not required, and is often considered a separate service.
Until 1999, there was no Shared Registration System (SRS). Network Solutions (NSI) operated the .com, .net, and .org registries, and was the de jure registrar and domain name registry operator. However, several companies had set up as de facto registrars, including NetNames, who invented the idea of a commercial standalone domain name registration service in 1996. Registrars formed another link in the food chain, introducing the concept of domain name sales, effectively introducing the wholesale model into the industry. NSI followed suit, forcing the issue of separation of Registry and Registrar.
In October 1998, following pressure from the growing domain name registration business and other interested parties, NSI's agreement with the US Department of Commerce was amended, requiring the creation of an SRS that supported multiple registrars. The SRS officially opened on November 30, 1999 under the supervision of ICANN, though there had been several testbed registrars using the system since March 11, 1999. Since then, over 500 registrars have entered the market for domain name registration services.
Designated registrar
An end-user cannot directly register and manage their domain name information with ICANN. A designated registrar must be chosen to have one's domain names registered and managed with the appropriate registry (NIC) on their behalf. Prior to 1999, the only .com registrar was NSI, but the approval of the SRS opened up the opportunity for other companies to be designated as registrars.
Each ICANN-accredited registrar must pay a fixed fee of US$4,000 plus a per-registrar variable fee totaling US$3.8 million divided among all registrars.
Only one designated registrar may modify or delete information about a domain name. The competition that SRS created enables the end user to choose from many registrars offering different services at varying prices. It is not unusual for an end user to wish to switch registrars. Thus, there is the domain name transfer clause.
When a registrar registers a .com domain name for the end-user, it must pay a maximum annual fee of US$6.00 to VeriSign and a US$0.20 administration fee to ICANN. VeriSign is the registry manager for .com gTLD. Low cost bulk registrars like Go Daddy and Tucows must manage their margin after paying these fees and their equipment cost. Therefore, the barrier for entry into the bulk registrar industry is high for new companies without an existing customer base.
An end-user registers either directly with a registrar, or indirectly through one or more layers of resellers. The cost generally ranges from a low of about $10 per year to about $30 per year, as of 2008. The maximum period of registration is generally 10 years ahead. [1] [2] [3] Some registrars are offering longer periods, up to one hundred years, but such services are implemented internally, by promising to renew annually, not in the official registration database. Some packages of services, such as web hosting, include the domain registration in the total package pricing.
Domain name transfers
Domain name transfers is the act of designating a new registrar with the authority to add, modify, and delete information about the domain name. The usual process of a domain name transfer is:
The end user verifies that the whois admin contact info is correct, particularly the email address; obtains the authentication code from the old registrar, and removes any lock that has been placed on the registration.
The end user contacts the new registrar with the wish to transfer the domain name to their service, and supplies the authentication code.
The new registrar will contact the old registrar with this information.
The old registrar will contact the end user to confirm the authenticity of this request. The end user may have to take further action with the old registrar, such as returning to the online management tools, to re-iterate their desire to proceed, in order to expedite the transfer.
The old registrar will release authority to the new registrar.
The new registrar will notify the end user of transfer completion. The new registrar may have automatically copied over the domain server information, and everything may automatically continue to work as before. Otherwise, the domain server information will need to be updated with the new registrar.
After this process, the new registrar becomes one's designated registrar and all correspondence shall be done with them. The process may take about five days. In some cases, the old registrar may intentionally delay the transfer as long as allowable. After transfer, the domain cannot be transferred again for 60 days, except back to the previous registrar.
It is unwise to attempt to transfer a domain immediately before it expires. Because a transfer can in some cases take up to 14 days, the transfer may not complete before the registration expires, resulting in loss of the domain name registration and failure of the transfer. To avoid this, either transfer well before the expiration date, or renew the registration before attempting the transfer.
Transfer scams
With the introduction of SRS, many smaller registrars had to compete with the de facto standard, NSI. Some companies offered value added services or used viral marketing. Some companies decided to trick customers to switch from NSI.
Many of these transfer scams involve a notice sent in the mail, fax, or e-mail. Some scammers may even call by phone (as the contact information is available through WHOIS) to harvest more information. These notices would include information publicly available from the WHOIS database to add to the look of authenticity. The text would include legalese to confuse the end user into thinking that it is an official binding document.
Scam registrars go after domain names that are expiring soon or have recently expired. Expired domain names do not have to go through the authentication process to be transferred, as the previous registrar would have relinquished management rights of the domain name. Domain name expiry dates are readily available via WHOIS.
The Internet Corporation for Assigned Names and Numbers, or ICANN, has authority over generic top-level domains, or gTLDs. Examples of gTLDs include .com, .net, .org and .mobi. ICANN does not have authority over ccTLDs, or Country Code Top-Level Domains, though it is quite common for domain name registrars to offer ccTLD registration services as well. Most registrars provide DNS hosting service, but this is not required, and is often considered a separate service.
Until 1999, there was no Shared Registration System (SRS). Network Solutions (NSI) operated the .com, .net, and .org registries, and was the de jure registrar and domain name registry operator. However, several companies had set up as de facto registrars, including NetNames, who invented the idea of a commercial standalone domain name registration service in 1996. Registrars formed another link in the food chain, introducing the concept of domain name sales, effectively introducing the wholesale model into the industry. NSI followed suit, forcing the issue of separation of Registry and Registrar.
In October 1998, following pressure from the growing domain name registration business and other interested parties, NSI's agreement with the US Department of Commerce was amended, requiring the creation of an SRS that supported multiple registrars. The SRS officially opened on November 30, 1999 under the supervision of ICANN, though there had been several testbed registrars using the system since March 11, 1999. Since then, over 500 registrars have entered the market for domain name registration services.
Designated registrar
An end-user cannot directly register and manage their domain name information with ICANN. A designated registrar must be chosen to have one's domain names registered and managed with the appropriate registry (NIC) on their behalf. Prior to 1999, the only .com registrar was NSI, but the approval of the SRS opened up the opportunity for other companies to be designated as registrars.
Each ICANN-accredited registrar must pay a fixed fee of US$4,000 plus a per-registrar variable fee totaling US$3.8 million divided among all registrars.
Only one designated registrar may modify or delete information about a domain name. The competition that SRS created enables the end user to choose from many registrars offering different services at varying prices. It is not unusual for an end user to wish to switch registrars. Thus, there is the domain name transfer clause.
When a registrar registers a .com domain name for the end-user, it must pay a maximum annual fee of US$6.00 to VeriSign and a US$0.20 administration fee to ICANN. VeriSign is the registry manager for .com gTLD. Low cost bulk registrars like Go Daddy and Tucows must manage their margin after paying these fees and their equipment cost. Therefore, the barrier for entry into the bulk registrar industry is high for new companies without an existing customer base.
An end-user registers either directly with a registrar, or indirectly through one or more layers of resellers. The cost generally ranges from a low of about $10 per year to about $30 per year, as of 2008. The maximum period of registration is generally 10 years ahead. [1] [2] [3] Some registrars are offering longer periods, up to one hundred years, but such services are implemented internally, by promising to renew annually, not in the official registration database. Some packages of services, such as web hosting, include the domain registration in the total package pricing.
Domain name transfers
Domain name transfers is the act of designating a new registrar with the authority to add, modify, and delete information about the domain name. The usual process of a domain name transfer is:
The end user verifies that the whois admin contact info is correct, particularly the email address; obtains the authentication code from the old registrar, and removes any lock that has been placed on the registration.
The end user contacts the new registrar with the wish to transfer the domain name to their service, and supplies the authentication code.
The new registrar will contact the old registrar with this information.
The old registrar will contact the end user to confirm the authenticity of this request. The end user may have to take further action with the old registrar, such as returning to the online management tools, to re-iterate their desire to proceed, in order to expedite the transfer.
The old registrar will release authority to the new registrar.
The new registrar will notify the end user of transfer completion. The new registrar may have automatically copied over the domain server information, and everything may automatically continue to work as before. Otherwise, the domain server information will need to be updated with the new registrar.
After this process, the new registrar becomes one's designated registrar and all correspondence shall be done with them. The process may take about five days. In some cases, the old registrar may intentionally delay the transfer as long as allowable. After transfer, the domain cannot be transferred again for 60 days, except back to the previous registrar.
It is unwise to attempt to transfer a domain immediately before it expires. Because a transfer can in some cases take up to 14 days, the transfer may not complete before the registration expires, resulting in loss of the domain name registration and failure of the transfer. To avoid this, either transfer well before the expiration date, or renew the registration before attempting the transfer.
Transfer scams
With the introduction of SRS, many smaller registrars had to compete with the de facto standard, NSI. Some companies offered value added services or used viral marketing. Some companies decided to trick customers to switch from NSI.
Many of these transfer scams involve a notice sent in the mail, fax, or e-mail. Some scammers may even call by phone (as the contact information is available through WHOIS) to harvest more information. These notices would include information publicly available from the WHOIS database to add to the look of authenticity. The text would include legalese to confuse the end user into thinking that it is an official binding document.
Scam registrars go after domain names that are expiring soon or have recently expired. Expired domain names do not have to go through the authentication process to be transferred, as the previous registrar would have relinquished management rights of the domain name. Domain name expiry dates are readily available via WHOIS.
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