How to configure SMTP on a Load Balancer service

Objective

This guide outlines how to configure the OVHcloud Load Balancer to distribute traffic across multiple SMTP servers.

Requirements

• An OVHcloud Load balancer service in your OVHcloud account.
• Access to the OVHcloud Control Panel.
• A Postfix-type SMTP service installed and configured on your servers.

Instructions

As a reminder, each protocol (HTTP, TCP and UDP) in the OVHcloud Load Balancer service has its own associated front-ends, farms and servers.

In the Control Panel, you will see the features detailed below:

Via the OVHcloud API, use the following call:

For more information on the API features, you can refer to the following guide.

Add a server farm

A TCP server farm must be added to the service; this component manages traffic balancing across the servers.

Via the OVHcloud Control Panel

In the Farms tab for servers, click on the +TCP/TLS button.

Fill in the fields. The mandatory fields for a basic configuration are Port and Zone. For SMTP, port 25 is used. If no ports are specified, your OVHcloud Load Balancer will automatically use the same port as the corresponding front-end.

Optionally, you can add an SMTP probe on your farm.

Click on Add once you have filled in the fields.

Your server farm should appear in the list, in the Farms tab.

Via the OVHcloud API

• List of TCP server farms:

• Details of a specific TCP server farm:

• Add a new TCP server farm:

• Modify a specific server farm:

• Delete a specific server farm:

Add a server

A server must now be added to the server farm.

Via the OVHcloud Control Panel

In the Farms tab, select the farm you want to add a server to by clicking on the corresponding line. The list of servers already configured in the farm will appear beneath the list of farms, along with the Add a server button. Click this button to add a new server.

The mandatory fields are the IPv4 address, Status, and ProxyProtocol version. If a server port is configured, it will override the port defined in the farm. To keep the configuration as standard as possible and easy to maintain, we recommend only using this parameter in advanced cases.

Click on Add once you have filled in the fields.

Your server should appear in the server list, in the Farms tab, just below the list of farms.

Via the OVHcloud API

• List of servers in the farm:

• Details of a specific server:

• Add a new server:

• Modify a specific server:

• Delete a specific server:

Add a front-end

A front-end must now be added to the service and connected to the server farm. The front-end is the component of your OVHcloud Load Balancer that exposes your service to the internet.

Via the OVHcloud Control Panel

Go to the + Front-ends tab, and click +TCP/TLS.

Fill in the fields. The mandatory fields for a basic configuration are Port (25 for a standard SMTP service), Zone, Default farm and Probe (if a probe was configured on your farm). If you want your service to be available across several ports at once, you can specify a list of ports, separated by commas, or a port range, in the format "-".

If you have routed Additional IPs to your OVHcloud Load Balancer service, you can also attach a front-end to one or more specific Additional IPs.

Ensure the farm you created earlier is specified as the “default farm”.

Click Add once the fields are filled.

Your front-end must appear in the list, in the Front-ends tab.

Via the OVHcloud API

• List of TCP front-ends:

• Details of a specific front-end:

• Add a new front-end:

• Modify a specific front-end:

• Delete a specific front-end:

Apply the modifications

Modifications made to your OVHcloud Load Balancer must be explicitly applied in each of the zones configured for your service. They will only become visible to your users at this point. This process allows for complex configuration changes to be prepared and applied only when fully ready.

If you have several zones, the same configuration must be applied to each.

Via the OVHcloud Control Panel

Go to the homepage for the OVHcloud Load Balancer, and click Apply:Zone for each of the zones concerned.

Via the OVHcloud API

• Refresh a zone:

Postfix configuration

To make Postfix compatible with HAProxy ProxyProtocol, an option is required in the postfix main.cf configuration file:

smtp_upstream_proxy_protocol = haproxy

The Postfix daemon must then be restarted.

Confirmation

Upon completing these steps, you will have a functional Load Balancer service for your SMTP servers. The service status can now be checked by requesting your IP Load Balancer as an SMTP server.

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