DNS Records (A, AAAA, CNAME, MX, NS, SRV, TXT)
Posted by Sean Syed on 12 February 2013 06:54 PM
The most common types of DNS records are:
- SOA record. Defined by provider and read-only. The SOA (start of authority) record is the most crucial record in a DNS entry. It conveys more information than all the other records combined. This record is called the start of authority because it denotes the DNS entry as the official source of information for its domain.
- Address records (A) that map the name of a machine to its numeric IP address. In clearer terms, this record states the hostname and IP address of a certain machine. To "resolve" a hostname means to find its matching IP address.
- IPv6 address records (AAAA) that map a hostname to its 128 bit IP address (IPv6 address). This record states that all requests for a hostname will be sent to a server with this IP address.
Note: IPv4 and IPv6 resource records can be freely mixed in the zone file if host is running dual (IPv4 and IPv6) addressing.
- CNAME records allow a machine to be known by more than one hostname. There must always be an A record for the machine before aliases can be added. The host name of a machine that is stated in an A record is called the canonical, or official name of the machine. Other records should point to the canonical name
- MX (Mail eXchanger) resource record identifies the mail server that is responsible for handling e-mails for a given domain name. When more than one MX record is entered for any single domain name that is using more than one mail server, the MX record can be prioritized with a preference number that indicates the order in which the mail servers should be used. This enables the use of primary and backup mail servers, which makes mail delivery more reliable. Please, keep in mind the following rules about MX records:
- MX record must point to hosts defined by A records, not CNAMEs.
- MX record cannot point to IP addresses.
- If you create a single MX record, the priority doesn't matter If you create multiple MX records, your primary mail server must have the lowest preference.
- Name Server records (NS) state the authoritative name servers for the given domain. There must be at least two NS records in every DNS zone.
- SRV-records are used to specify the location of a service. They are recently were used in connection with different directory servers such as LDAP (Lightweight Directory Access Protocol). In addition, SRV records can be used for advanced load balancing and to specify specific ports for services - for example that a web-server is running on port 8080 instead of the usual port 80 (theoretical example - this is not yet supported by any major browsers). The "service location" is specified through a target, priority, weight, and port:
- Target is the domain name of the server (referencing an A-record).
- Priority is a preference number used when more servers are providing the same service (lower numbers are tried first).
- Weight is used for advanced load balancing.
- Port is the TCP/UDP port number on the server that provides this service.
- TXT records - strictly informational, not functional. Used to provide up to 255 characters of free form text (quoted string). Provides the ability to associate some text with a host or other name.
The form for resource records input is organized as follows:
- Enter a record subject into the Name field.
- Enter a record time-to live in seconds into the TTL field.
- Select a record type from the Type drop-down menu.
- Enter priority (for MX or SRV record only) into the Priority field.
- Enter the rest of a record in one string, in the same way as a record is entered in zone file into the Value field.