The practice of using a name as a simpler, more memorable abstraction of a host's numerical address on a network dates back to the ARPANET era. Before the DNS was invented in 1982, each computer on the network retrieved a file called HOSTS.TXT from a computer at SRI (now SRI International). The HOSTS.TXT file mapped names to numerical addresses. A hosts file still exists on most modern operating systems by default and generally contains a mapping of "localhost" to the IP address 127.0.0.1. Many operating systems use name resolution logic that allows the administrator to configure selection priorities for available name resolution methods.History
Domain name space
Domain name syntax
- The right-most label conveys the top-level domain; for example, the domain name www.example.com belongs to the top-level domain com.
- The hierarchy of domains descends from right to left; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example: the label example specifies a subdomain of the comdomain, and www is a sub domain of example.com. This tree of subdivisions may have up to 127 levels.
- Each label may contain up to 63 characters. The full domain name may not exceed a total length of 255 octets. In the internal binary representation of the DNS the maximum length requires 255 octets of storage. In practice, some domain registries may have shorter limits.
- DNS names may technically consist of any character representable in an octet. However, the allowed formulation of domain names in the DNS root zone, and most other sub domains, uses a preferred format and character set. The characters allowed in a label are a subset of the ASCII character set, and includes the characters a through z, A through Z, digits 0 through 9, and the hyphen. This rule is known as the LDH rule(letters, digits, hyphen). Domain names are interpreted in case-independent manner. Labels may not start or end with a hyphen. There is an additional rule that essentially requires that top-level domain names not be all-numeric.
- A hostname is a domain name that has at least one IP address associated. For example, the domain names www.example.com and example.com are also hostnames, whereas the com domain is not.
Internationalized domain names
Authoritative name server
Address resolution mechanism
- A network host is configured with an initial cache (so called hints) of the known addresses of the root nameservers. Such a hint file is updated periodically by an administrator from a reliable source.
- A query to one of the root servers to find the server authoritative for the top-level domain.
- A query to the obtained TLD server for the address of a DNS server authoritative for the second-level domain.
- Repetition of the previous step to process each domain name label in sequence, until the final step which returns the IP address of the host sought.
Recursive and caching name server
- A non-recursive query is one in which the DNS server provides a record for a domain for which it is authoritative itself, or it provides a partial result without querying other servers.
- A recursive query is one for which the DNS server will fully answer the query (or give an error) by querying other name servers as needed. DNS servers are not required to support recursive queries.
Circular dependencies and glue records
- Hostnames and IP addresses do not necessarily match on a one-to-one basis. Multiple 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 locations 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.
- E-mail Blacklists: The DNS is used for efficient storage and distribution of IP addresses of blacklisted e-mail hosts. The usual method is putting the IP address of the subject host into the sub-domain of a higher level domain name, and resolve that name to different records to indicate a positive or a negative. Here is a hypothetical example blacklist:
- 184.108.40.206 is blacklisted => Creates 220.127.116.11.blacklist.example and resolves to 127.0.0.1
- 18.104.22.168 is not => 22.214.171.124.blacklist.example is not found, or default to 127.0.0.2
- E-mail servers can then query blacklist.example through the DNS mechanism to find out if a specific host connecting to them is in the blacklist. Today many of such blacklists, either free or subscription-based, are available mainly for use by email administrators and anti-spam software.
- 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.
- Dynamic DNS (sometimes called DDNS) allows clients to update their DNS entry as their IP address changes, as it does, for example, when moving between ISPs or mobile hot spots.
DNS resource records
|NAME||Name of the node to which this record pertains||(variable)|
|TYPE||Type of RR in numeric form (e.g. 15 for MX RRs)||2|
|TTL||Count of seconds that the RR stays valid (The maximum is 231-1, which is about 68 years.)||4|
|RDLENGTH||Length of RDATA field||2|
|RDATA||Additional RR-specific data||(variable)|