Round Robin DNS Load Balancing
Published: Thursday, May 20, 2004
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.
Other considerations
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.
Considering the functionality, the round robin DNS is not a load balancing mechanism but a load distribution option. Some of these drawbacks can be overcome by implementing an advanced version of the DNS load balancer using Perl scripts. The details can be found here.
Some other variety of load balancing can be performed by using a proxy server, where one of the web servers, is solely used for re-routing of traffic to the other servers. If Apache is used as a web server, the mod_rewrite feature of Apache can be used for this purpose as detailed in this Apache website article.
|
 |
|
|
|
|
|
|
|
|
RELATED ARTICLES
|
 |
Introduction To Server Load Balancing
Load balancing applies to all types of servers (application server, database
server), however, we will be devoting this section for load balancing of web servers (HTTP server)
.
|
|
SubDomain - The Third Level Domain
A subdomain is the part of the website address before the domain name. Learn about using subdomain names.
|
|
|
|
RELATED NEWS
|
|
Teachers Follow Issues Like the College Admission Scandal Closely, Have Surprising Views on Security, and Prioritize Concerns on Issues that Impact Them
K-12 Teachers Online Research Report from global insights provider OvationMR who conducted 407 interviews from March 14-17, 2019 with U.S. K-12 teachers about topics including current events, budgets, security, pay, student advocacy, administrative and government support. This report reveals insights broken out by schools with teachers of affluent students versus those who teach in non-affluent areas and also looks at opinions through the political affiliation lens.
|
|
CancerIQ and AliveAndKickn Collaborate to Advance Lynch Syndrome Screening, Education and Research
Partnership to better help providers and patients take preventative measures against hereditary cancer
|
|
Salesforce Grants Equity Awards to Griddable Employees Under Its Inducement Equity Incentive Plan
SAN FRANCISCO, March 22, 2019 /PRNewswire/ -- Salesforce (NYSE: CRM), the global leader in CRM, today announced it has grante...
|
|
Take a Quick Contest and Win the Women's Day Scholarship
JanBask Training launched a women's only contest on the eve of International Women's day 2019 that fetches its winners' prizes worth up to 100% Scholarship on the world's most trending online training courses. All you have to do is to answer a few questions. Come on, give it a shot. Who knows you may be one of the lucky winners
|
|
Apex® HCM Announces 2019 Conference Registration: Align & S3
Registration opens for Apex HCM's S3 and Align 2019 conferences. S3 is the two-day Selling Skills Summit focusing on helping customers grow their payroll businesses. Align is the annual Apex Users' Conference.
|
|
Submit News |
View More News
|
|
|
|
|
|
|
|
|
Server Load Balancing Methods
