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Round Robin DNS

Posted 1 year, 9 months and 27 days ago

Round Robin DNS & DDNS is a technique of load distribution, load balancing, or fault-tolerance provisioning multiple, redundant Internet Protocol service hosts, like Web servers, FTP servers, by managing the Domain Name System's (DNS) responses to address requests from client computers according to an appropriate statistical model.

INTRODUCTION:

A load balancing technique in which balance power is placed in the DNS server instead of a strictly dedicated machine as other load techniques do.

Round-Robin-DNS

Round robin works on a rotating basis in that one server IP address is handed out, then moves to the back of the list; the next server IP address is handed out, and then it moves to the end of the list; and so on, depending on the number of servers being used. This works in a looping fashion.

Round robin DNS is usually used for balancing the load of geographically distributed Web servers. For example, a company has one domain name and three identical home pages 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.

SUMMARY:

How DNS load balancing works:

When the request comes to the DNS server to resolve the domain name, it gives out one of the several canonical names in a rotated order. This redirects the request to one of the several servers in a server group. Once the BIND feature of DNS resolves the domain to one of the servers, subsequent requests from the same client are sent to the same server.

DNS load balancing implementation (Multiple CNAMES)

This approach works for BIND 4 name servers, where multiple CNAMES are not considered as a configuration error. Assuming there are 4 web servers in the cluster configured with IP addresses 123.45.67.[1-4], add all of them to the DNS with Address records (A Names) as below. The srv[1-4] can be set to any name you want, such as foo[1-4], but should match the next step.

srv1 IN A 123.45.67.1
srv2 IN A 123.45.67.2
srv3 IN A 123.45.67.3
srv4 IN A 123.45.67.4

Add the following canonical names to resolve www.domain.com to one of these servers.

www IN CNAME srv1.domain.tld.
IN CNAME srv2.domain.tld.
IN CNAME srv3.domain.tld.
IN CNAME srv4.domain.tld.

The DNS server will resolve the www.domain.com to one of the listed servers in a rotated manner. That will spread the requests over the group of servers.

Note: The requests sent to http://domain.com (without 'www') should be forwarded to http://www.domain.com in this case to work. For BIND 8 name servers, the above approach will throw an error for multiple CNAMES. This can be avoided by an explicit multiple CNAME configuration option as shown below.

options {
multiple-cnames yes;
};

DNS load balancing implementation (Multiple A Records)

This above approach with multiple CNAMES for one domain name is not a valid DNS server configuration for BIND 9 and above. In this case, multiple A records are used.

www.domain.tld. 60 IN A 123.45.67.1
www.domain.tld. 60 IN A 123.45.67.2
www.domain.tld. 60 IN A 123.45.67.3
www.domain.tld. 60 IN A 123.45.67.4

The TTL value should be kept to a low value, so that the DNS cache is refreshed faster.

CONCLUSION:

The DNS based load balancing method shown above does not take care of various potential issues such as unavailable servers (if one server goes down), or DNS caching by other name servers. The DNS server does not have any knowledge of the server availability and will continue to point to an unavailable server. It can only differentiate by IP address, but not by server port. The IP address can also be cached by other nameservers, hence requests may not be sent to the load balancing DNS server.

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