Caching time Domain

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 12:00pm 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)

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.

Parts of a domain name

Parts of a domain name

A domain name usually consists of two or more parts (technically labels), separated by dots. For example 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 to 127 levels deep. Each label can contain up to 63 characters. The whole domain name does not exceed a total length of 255 characters. 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

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

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 only communicate 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.

As originally envisaged, the process was as simple as:

1. 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.
2. 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).
3. querying this second server for the address of a DNS server with detailed knowledge of the second-level domain (inadomain.example in our example).
4. 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.

Server hosting

The Domain Name System (DNS) associates various sorts of information with so-called 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 the IP addresses, e.g. 208.77.188.166, that networking equipment needs to deliver information. It also stores other information such as the list of mail exchange 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.

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), independently 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 server for each domain to keep track of its own changes, avoiding the need for a central registrar to be continually consulted and updated.

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). 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. As of 2006, the hosts file serves primarily for troubleshooting DNS errors or for mapping local addresses to more organic names. 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[2] 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.

The domain name space

The domain name space consists of a tree of domain names. Each node or leaf in the tree has one or more resource records, which hold information associated with the domain name. The tree sub-divides into zones. A 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 his zone of authority, he can delegate control to the other administrator. This splits a part of the old zone off into a new zone, which comes under the authority of the second administrator's nameservers. The old zone ceases to be authoritative for what goes under the authority of the new zone.