sendmail Cookbook

Chapter 4. Masquerading

Introduction

Masquerading is sendmail-speak for rewriting the hostname in the address of outbound mail. The reasons for masquerading fall into two general categories:

Mail routing

Many networks are designed to route all inbound mail through a central mail hub. When a host sends out mail using its own hostname in the sender address, replies to that mail may well come back to the host. Replacing the sending system’s hostname with the mail hub’s hostname in the sender address guarantees that replies come back to the hub. Masquerading is not the only way to do this. An MX record can also route mail to the hub. However, maintenance of the DNS zone file is under the control of the domain administrator. The sendmail administrator maintains masquerading, and most sendmail administrators prefer to be in charge of their own fate. Also, hostnames change over time. Masquerading can provide more consistent email addresses and can simplify maintenance.

Organizational requirements

Some organizations simply have a policy of hiding hostnames. Management may think that “busy” hostnames project an image of disorganization. Marketing may think that “frivolous” hostnames project the wrong image to customers. Naive security people may even believe that hiding hostnames increases security. For whatever reason, management requires masquerading, which, in turn, creates the need for systems configured to receive replies to the masqueraded mail.

The MASQUERADE_AS macro enables masquerading. ...