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CareerQuill Group

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Tenishat Cosset
Tenishat Cosset

Smtp Load Balancing


Load balancing SMTP traffic is something that makes sense for a lot of organizations. They have an investment in load balancers for their CAS array, web server farm, etc and so SMTP seems like another logical protocol to run through the load balancers and get all the benefits that it delivers.

However it is also quite easy to create a situation where SMTP traffic is not being load balanced as intended, and worse still there are scenarios where the use of some load balanced configurations may actually diminish SMTP high availability, or even undermine security.

Consider the following scenario where incoming internet email is passed through an email security server/appliance, which is configured to then send the traffic to a load balancer for distribution to the Hub Transport servers. Various internal applications and systems also use the load balancer as their SMTP target.

In most load balancer configurations you can configure a priority or weight for the servers that are the targets of the traffic. Different vendors use their own terminology for this, but the general idea is that it provides the option to have preferred servers that will be considered first for a new connection if they are available.

Here is a traffic graph of a typical day for two servers that were configured with different weightings/priorities in the load balancer. You can see that SERVER1 handled a higher volume of traffic than SERVER2.

Depending on your server resources and traffic load this may not be an issue for you, but in some environments it could lead to load issues that interrupt mail flow. So if your actual intention is evenly distribute traffic across multiple Hub Transport servers then you would consider adjusting the server weight/priority accordingly.

Along similar lines to the previous issue, a load balancer will usually have multiple methods for deciding which server should be used for a connection. For example, the Kemp load balancers have quite a few scheduling options available.

As one specific example, if the load balancing is based on source IP it may inadvertently lead to traffic imbalances. In the example environment shown at the beginning of this article, source IP-based load balancing would generally result in well balanced traffic from the internal applications and systems, assuming each internal IP is sending roughly equal volumes of email, otherwise some imbalances can still occur.

The obvious reaction here may be to choose a different load balancing algorithm, however my recommendation for environments where incoming internet email all traverses a single host like that is to consider not using the load balancer for distribution of that incoming internet traffic.

With all internal and incoming SMTP traffic going via the load balancer, which is source NATing the connections, the protocol logs only recorded traffic from the load balancer (IP below) and no other IP addresses.

In the example scenario used in this article the email security server has its own load balancing capability for incoming email because you can specify multiple internal hosts to deliver email to. This would also apply to hosted email security services.

By configuring each Hub Transport as an internal delivery target instead of just using the load balancer, the protocol logs now log incoming internet email as coming from the IP addresses for the email security system, rather than the load balancer.

If you do not have an email security server/appliance or other hosted solution, and SMTP connections go directly from the internet to the load balancer, then you could look at using multiple MX records instead, although this would require the availability of multiple public IP addresses.

In addition, any traffic imbalance being caused by the use of source IP-based load balancing should no longer be present. This graph represents incoming internet SMTP connections per server, which began imbalanced and then evened out almost precisely once the load bal


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