Round robin DNS
Encyclopedia
Round robin DNS
is a technique of load distribution, load balancing, or fault-tolerance provisioning multiple, redundant Internet Protocol
service hosts, e.g., Web server
s, FTP servers, by managing the Domain Name System
's (DNS) responses to address requests from client computers according to an appropriate statistical model.
In its simplest implementation Round-robin DNS works by responding to DNS requests not only with a single IP address
, but a list of IP addresses of several servers that host identical services. The order in which IP addresses from the list are returned is the basis for the term round robin. With each DNS response, the IP address sequence in the list is permuted
. Usually, basic IP clients attempt connections with the first address returned from a DNS query so that on different connection attempts clients would receive service from different providers, thus distributing the overall load among servers.
There is no standard
procedure for deciding which address will be used by the requesting application - a few resolvers attempt to re list to give priority to numerically "closer" networks. Some desktop clients do try alternate addresses after a connection timeout of 30-45 seconds.
Round robin DNS is often used for balancing the load of geographically distributed Web server
s. For example, a company has one domain name and three identical web sites residing on three servers with three different IP addresses. When one user accesses the home page it will be sent to the first IP address. The second user who accesses the home page will be sent to the next IP address, and the third user will be sent to the third IP address. In each case, once the IP address is given out, it goes to the end of the list. The fourth user, therefore, will be sent to the first IP address, and so forth.
Also, it may not be the best choice for load balancing
on its own since it merely alternates the order of the address records each time a name server is queried. There is no consideration for matching the user IP address and its geographical location, transaction time, server load, network congestion, etc. Round robin DNS load balancing works best for services with a large number of uniformly distributed connections to servers of equivalent capacity. Otherwise it just does load distribution.
Methods exist to overcome such limitations. For example, modified DNS servers (such as lbnamed) can routinely poll mirrored servers for availability and load factor. If a server does not reply as required, the server can be temporarily removed from the DNS pool, until it reports that it is once again operating within specifications.
is a technique of load distribution, load balancing, or fault-tolerance provisioning multiple, redundant Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
service hosts, e.g., Web server
Web server
Web server can refer to either the hardware or the software that helps to deliver content that can be accessed through the Internet....
s, FTP servers, by managing the Domain Name System
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
's (DNS) responses to address requests from client computers according to an appropriate statistical model.
In its simplest implementation Round-robin DNS works by responding to DNS requests not only with a single IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
, but a list of IP addresses of several servers that host identical services. The order in which IP addresses from the list are returned is the basis for the term round robin. With each DNS response, the IP address sequence in the list is permuted
Permutation
In mathematics, the notion of permutation is used with several slightly different meanings, all related to the act of permuting objects or values. Informally, a permutation of a set of objects is an arrangement of those objects into a particular order...
. Usually, basic IP clients attempt connections with the first address returned from a DNS query so that on different connection attempts clients would receive service from different providers, thus distributing the overall load among servers.
There is no standard
Internet standard
In computer network engineering, an Internet Standard is a normative specification of a technology or methodology applicable to the Internet. Internet Standards are created and published by the Internet Engineering Task Force .-Overview:...
procedure for deciding which address will be used by the requesting application - a few resolvers attempt to re list to give priority to numerically "closer" networks. Some desktop clients do try alternate addresses after a connection timeout of 30-45 seconds.
Round robin DNS is often used for balancing the load of geographically distributed Web server
Web server
Web server can refer to either the hardware or the software that helps to deliver content that can be accessed through the Internet....
s. For example, a company has one domain name and three identical web sites residing on three servers with three different IP addresses. When one user accesses the home page it will be sent to the first IP address. The second user who accesses the home page will be sent to the next IP address, and the third user will be sent to the third IP address. In each case, once the IP address is given out, it goes to the end of the list. The fourth user, therefore, will be sent to the first IP address, and so forth.
Drawbacks
Although easy to implement, round robin DNS has problematic drawbacks, such as those arising from record caching in the DNS hierarchy itself, as well as client-side address caching and reuse, the combination of which can be difficult to manage. Round robin DNS should not solely be relied upon for service availability. If a service at one of the addresses in the list fails, the DNS will continue to hand out that address and clients will still attempt to reach the inoperable service.Also, it may not be the best choice for load balancing
Load balancing (computing)
Load balancing is a computer networking methodology to distribute workload across multiple computers or a computer cluster, network links, central processing units, disk drives, or other resources, to achieve optimal resource utilization, maximize throughput, minimize response time, and avoid...
on its own since it merely alternates the order of the address records each time a name server is queried. There is no consideration for matching the user IP address and its geographical location, transaction time, server load, network congestion, etc. Round robin DNS load balancing works best for services with a large number of uniformly distributed connections to servers of equivalent capacity. Otherwise it just does load distribution.
Methods exist to overcome such limitations. For example, modified DNS servers (such as lbnamed) can routinely poll mirrored servers for availability and load factor. If a server does not reply as required, the server can be temporarily removed from the DNS pool, until it reports that it is once again operating within specifications.