Configuring SMTP on a Load Balancer Service – Support Guides

Learn how to configure an OVHcloud Load Balancer service to balance load across several servers that respond in SMTP.

In this guide, we will configure a basic TCP load balancing service, for one or more SMTP servers. A TCP front-end will listen to TCP traffic on port 25. It is configured to direct traffic to a TCP server cluster with one or more TCP servers, depending on how you configure it.

NOTE: Questions on what an SMTP service is and how it works will not be covered here. This guide is aimed at readers who have a general understanding of SMTP, or practical experience with it.

NOTE: We will assume that you already have a Postfix SMTP service set up and configured your servers.

NOTE: This guide will take you through the steps required. Depending on the way you have designed your architecture, some configurations may vary.

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

NOTE: The order in which elements are created is important. In particular, the server clusters must be configured before we can attach servers to them.

Instructions

Access your OVHcloud Load Balancer service

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From the OVHcloud Control Panel:

Select Bare Metal Cloud from the top navigation bar.
Expand the Network section in the left-hand menu.
Click Load Balancer and select your service.

On the Home screen, you will see the following:

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Navigate to the GET /ipLoadbalancing section of the OVHcloud API.

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

Add a server cluster

We will add a server cluster of TCP servers to our service, which is the part that balances traffic on the servers.

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From the OVHcloud Control Panel select the Server clusters tab and click the Add a server cluster button.

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On the next screen, fill in the fields:

Name (optional)
Protocol: SSL TCP (TLS)
Port: 25
Datacenter

If no ports are specified, your OVHcloud Load Balancer will automatically use the same port as the corresponding front-end. Also, in the Advanced settings, you can add an SMTP probe on your server cluster.

Click Add once you have filled in the fields.

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Your server cluster should appear in the list in the Server clusters tab.

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From the OVHcloud API you can:

List TCP server clusters: GET /ipLoadbalancing/{serviceName}/tcp/farm

Find details of a specific TCP server: GET /ipLoadbalancing/{serviceName}/tcp/farm/{farmId}

Add a new TCP server cluster: POST /ipLoadbalancing/{serviceName}/tcp/farm

Modify a specific server cluster: PUT /ipLoadbalancing/{serviceName}/tcp/farm/{farmId}

Delete a specific server cluster: DELETE /ipLoadbalancing/{serviceName}/tcp/farm/{farmId}

Add a server

We will now add a server to our server.

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From the OVHcloud Control Panel, from the Server clusters tab, expand the server cluster you just created.

Click Add a server.

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On the next screen, fill in the fields:

Name (optional)
IPv4 address
Port: 25

If a server does not use the same port as the one defined earlier in the server cluster, you may overload it by configuring a server. However, we recommend only using this parameter in advanced cases to keep the configuration as standard and easy to maintain as possible.

NOTE: It is important to configure ProxyProtocol in version v1 to get the real source IP on your SMTP service. Postfix is compatible with this protocol.

Click Add once you have filled in the fields.

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Your server should appear in the Server clusters tab, just below the server cluster created in the previous step.

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From the OVHcloud API you can:

List servers in the server cluster: GET /ipLoadbalancing/{serviceName}/tcp/farm/{farmId}/server

Find details of a specific server: GET /ipLoadbalancing/{serviceName}/tcp/farm/{farmId}/server/{serverId}

Add a new server: POST /ipLoadbalancing/{serviceName}/tcp/farm/{farmId}/server

Modify a specific server: PUT /ipLoadbalancing/{serviceName}/tcp/farm/{farmId}/server/{serverId}

Delete a specific server: DELETE /ipLoadbalancing/{serviceName}/tcp/farm/{farmId}/server

Add a front-end

We will now add a front-end to our service, and connect it to our server cluster. The front-end is the part of your OVHcloud Load Balancer that exposes your service on the internet.

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In the OVHcloud Control Panel, from the Front-ends tab, click Add a front-end.

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On the next screen, fill in the fields:

Name (optional)
Protocol: SSL TCP (TLS)
Port: 25
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 range of ports, in the format "start_port-end_port".

Datacenter
Default server cluster: the cluster you created at the beginning of this guide.

NOTE: If you configured a probe on your cluster, you need to open the Advanced settings to configure a probe here as well.

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.

Click Add once you have filled in the fields.

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Your front-end will appear in the Front-ends tab.

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From the OVHcloud API you can:

List TCP front-ends: GET /ipLoadbalancing/{serviceName}/tcp/frontend

Find details of a specific front-end: GET /ipLoadbalancing/{serviceName}/tcp/frontend/{frontendId}

Add a new front-end: POST /ipLoadbalancing/{serviceName}/tcp/frontend

Modify a specific front-end: PUT /ipLoadbalancing/{serviceName}/tcp/frontend/{frontendId}

Delete a specific front-end: DELETE /ipLoadbalancing/{serviceName}/tcp/frontend/{frontendId}

Apply the modifications

The modifications made to your OVHcloud Load Balancer must be explicitly applied in each of the zones configured for your service. Only at this point will they be visible to your website visitors. This way, you can make complex configuration changes several times, and only apply them once the configuration is ready.

If you have several zones, you must apply the same configuration for each of them.

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In the OVHcloud Control Panel, click Apply configuration in the yellow banner.

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On the next screen, select ☑ your Datacenter and click Apply configuration again.

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From the OVHcloud API:

Refresh a zone: POST /ipLoadbalancing/{serviceName}/refresh

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

You then need to restart your Postfix daemon.

Confirmation

After you have completed all of these steps, you should now have a functional Load Balancer service for your SMTP servers. You can now check the service status by requesting your IP Load Balancer as an SMTP server.

network_LB_cases_SMTP14* 2.png

Go further

For more information and tutorials, please see our other OVHcloud Load Balancer guides or explore the guides for other OVHcloud products and services.

Instructions

Access your OVHcloud Load Balancer service

Add a server cluster

Add a server

Add a front-end

Apply the modifications

Postfix configuration

Confirmation

Go further

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