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KB00060101 Using Multi-Homed Services

Mailtraq allows you to assign each service to a specific IP address. This allows your server to be multi-homed, running a different configuration depending on the IP address your client connects to (if you have more than one NIC [Network Interface Card]).

For example, if you have a network card identified as and another as then you can run the SMTP Mail server on both, but with different configurations. You might only allow relaying for the service. It is more common to multi-home the web services, giving completely different web sites depending on which IP address (and thus which domain name) the client requests.


In Mailtraq, each service can be assigned to a specific port. For example, web servers are usually run on port 80. In the service configuration dialogue, Mailtraq allows you to specify this port. You can now prefix this with the IP address, as in this example :-

This format can be used in any of Mailtraq's services.

Technical information

Multi-homing enables multiple service instances to be run on a single port by binding each service instance to a particular interface. Multi-homing is an alternative method of binding ports and services to interfaces and cannot be implemented on a port if a non-multi-homed service is already bound to that port.


Example Usage: SMTP Service


Example Usage: SMTP Service
In this example, the Mailtraq server is accessible via three separate interfaces – via from the Mailtraq machine itself, via from a machine connected to the Local Area Network (LAN) and via an Internet Service Provider (ISP) assigned fixed IP address of 
By creating a multi-homed configuration, separate instances of the SMTP Service can be presented to the users on each of those interfaces using the standard port for the SMTP protocol, port 25. Note that the ISP provided IP address here is only an example and only IP addresses which have been explicitly assigned to you should be used in this way. Depending on your Windows network setup, the ISP assigned IP address may need to be bound to the dial-up adaptor (Control Panel | Networks) before it becomes available for multi-homing otherwise it may only be active while connected to the Internet.
After checking that it is idle, remove the current SMTP Service instance on port 25 by selecting Services in the Console Tree View and choosing Stop from its context menu (via right-click). In Services, remove the existing SMTP Service definition and create three new instances – the first with a port specification of, that's the IP address followed by colon (ASCII 58) followed by the port number, the second with and the third with Start each of the new instances via its context menu (via right-click). Under Services, each instance of the SMTP service now announces the interface on which it is listening, for example "<accepting connections on>".
Each of the multi-homed SMTP instances may now be configured appropriately for their role. For example, the and instances might be configured to permit all relaying with no message size limits, whilst the only instance which can be accessed by external hosts,, might be configured to permit no relaying with a restrictive message size limit. Note that LAN users may still be able to access the instance unless it is firewalled appropriately via its Access Control tab.

Keywords: kb advanced
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