What is DNS: Difference between revisions
(Created page with 'DNS (Domain Names Services) is the system that maps a host name (such as a web server) to an IP address. An IP address is a unique address on a network. In the case of a publicl...') |
No edit summary |
||
| (2 intermediate revisions by one other user not shown) | |||
| Line 3: | Line 3: | ||
An IP address is a unique address on a network. In the case of a publicly accessible server, the IP address is referred to as a ''public'' IP address and must be issued by an ISP. There are also a number of ranges that are reserved for private networks. |
An IP address is a unique address on a network. In the case of a publicly accessible server, the IP address is referred to as a ''public'' IP address and must be issued by an ISP. There are also a number of ranges that are reserved for private networks. |
||
Every server must have at least one IP address, but a server can be known by multiple names. These names can be mapped to an alias, known as a '''CNAME''', or to a specific IP address, called an '''A''' record. Good practice usually means that only the primary server name is mapped to an IP address, and all other names use that primary server name as an alias. This means you only have to change it's IP address in one place. |
Every server must have at least one IP address, but a server can be known by multiple names (also known as host names). These names can be mapped to an alias, known as a '''CNAME''', or to a specific IP address, called an '''A''' record. Good practice usually means that only the primary server name is mapped to an IP address, and all other names use that primary server name as an alias. This means you only have to change it's IP address in one place. |
||
Because of the way DNS works, a root domain can also have an IP address, which means that you can enter a web site address without using ''www'' in front of it (e.g. http://bbc.co.uk/ which then redirects you to http://www.bbc.co.uk/) |
Because of the way DNS works, a root domain can also have an IP address (but not a CNAME), which means that you can enter a web site address without using ''www'' in front of it (e.g. http://bbc.co.uk/ which then redirects you to http://www.bbc.co.uk/). |
||
You can also have a ''wildcard'' entry which is used whenever a specific host name is not found. This can cause considerable problems when trying to trace DNS problems (e.g. caused by a typo somewhere) so we do not recommend using wildcard entries. |
|||
There is a method called ''round robin'' on many DNS servers, where you can have many IP addresses (or aliases) for a single host name. This is usually used to load balance, using multiple servers to cope with the workload. |
|||
DNS also gives out the information required for email to be delivered to your mail server. These records are called '''MX''' and each record is given a priority. The mail server with the lowest priority is tried first, and if this doesn't work, the next lowest on the list is tried. It is very important that all MX entries are A records and not CNAME. |
|||
Although you can point multiple host names at a single IP address, the reverse is not true. A reverse lookup on an IP address is unique, and a badly set up reverse lookup is one of the biggest reasons for email delivery failure, as many receiving servers (including ours) will perform reverse lookups as part of their anti-spam strategy, and then do a forward lookup and check that it is the same IP address. Reverse DNS (rDNS) records are known as '''PTR''' records. |
|||
[[Category:KB]] |
|||
Latest revision as of 10:56, 29 May 2015
DNS (Domain Names Services) is the system that maps a host name (such as a web server) to an IP address.
An IP address is a unique address on a network. In the case of a publicly accessible server, the IP address is referred to as a public IP address and must be issued by an ISP. There are also a number of ranges that are reserved for private networks.
Every server must have at least one IP address, but a server can be known by multiple names (also known as host names). These names can be mapped to an alias, known as a CNAME, or to a specific IP address, called an A record. Good practice usually means that only the primary server name is mapped to an IP address, and all other names use that primary server name as an alias. This means you only have to change it's IP address in one place.
Because of the way DNS works, a root domain can also have an IP address (but not a CNAME), which means that you can enter a web site address without using www in front of it (e.g. http://bbc.co.uk/ which then redirects you to http://www.bbc.co.uk/).
You can also have a wildcard entry which is used whenever a specific host name is not found. This can cause considerable problems when trying to trace DNS problems (e.g. caused by a typo somewhere) so we do not recommend using wildcard entries.
There is a method called round robin on many DNS servers, where you can have many IP addresses (or aliases) for a single host name. This is usually used to load balance, using multiple servers to cope with the workload.
DNS also gives out the information required for email to be delivered to your mail server. These records are called MX and each record is given a priority. The mail server with the lowest priority is tried first, and if this doesn't work, the next lowest on the list is tried. It is very important that all MX entries are A records and not CNAME.
Although you can point multiple host names at a single IP address, the reverse is not true. A reverse lookup on an IP address is unique, and a badly set up reverse lookup is one of the biggest reasons for email delivery failure, as many receiving servers (including ours) will perform reverse lookups as part of their anti-spam strategy, and then do a forward lookup and check that it is the same IP address. Reverse DNS (rDNS) records are known as PTR records.
