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Article ID : # Last review : 2005-12-24 14:30:17

Load balancing with and without AUX value

  You can perform load balancing in several ways on the CloudfloorDNS servers using the 'AUX' record. The AUX DNS value is most commonly used with MX records and is designed for resillience purposes since most people have more than one mail server. This AUX value enabled mail to be received at an alternate location if you main mail server fails for some reason and it acts as a priority or importance value. The AUX value will tell other servers which order mail should be tried in. The remote server will use the MX records with the lowest value first. The remote server would attempt to connect to the server with value of 10 first, then 20, then 30, etc.

Load Balancing The AUX value also has another purpose on the CloudfloorDNS servers, and this is load balancing. If your zone file has more than one IP address for the same A record we will weight the reponses using the aux value. A low aux value makes an address record more likely to be listed first. The balancing algorithm causes servers with a lower aux to be selected more frequently than those with higher values, although all servers will still be listed first occasionally, as the algorithm is partially random. See the example below on how this is used in a zone file. You can see that there are four WWW records, all at different IP's and has the lowest AUX value of 10, so it will get the most hits.

CloudfloorDNS DNS Load Balancing

Records where aux is 0 (zero) will be listed first almost every time. Records where aux is 50,000 or greater will always be listed last. Here is an example of how hosts were distributed on a 100,000 query test against ten hosts with aux values 10-100. The number shown is the number of times that host was listed first:

aux 10 - 51,211 queries

aux 20 - 21,881 queries

aux 30 - 10.983 queries

aux 40 - 6,209 queries

aux 50 - 3,661 queries

aux 60 - 2,311 queries

aux 70 - 1,526 queries

aux 80 - 1,032 queries

aux 90 - 675 queries

aux 100 - 511 queries

Round Robin If your zone file contains more than one address record for the same A record name we will serve them up in a random order each time. Round robin is used only if all the address records found have an aux value of zero. If any of the records have an aux value that is non-zero, load balancing will be used instead. Note that we will also return multiple same-preference MX records in random order, to help equalize the load among same-preference MX hosts.


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