11.8 Things to Try

• All machines can be known by two names: the normal hostname (as defined by $w) and the generic name localhost. Design and test a rule for Hubset that tests for and rewrites a sender address like you@localhost.

• When we speak of the sender, we have lumped together the envelope and header senders into one. Try to think of examples in which the sender header address should be rewritten in a way that is different from the sender envelope address.

• At a site that has more than one hub, it would be necessary to route mail from some clients to one hub and mail from other clients to another. Think of a basis for such a division (such as subnets or NIS domains) and design a set of rules (in a single configuration file) to rewrite the sender address appropriately for a selected hub.

• Consider the need to match subsets of a subdomain. For example, assume that the local hostname is here and the local domain is my.sub.domain. At such a site, all of the following are legal addresses: you@here, you@here.my, and you@here.my.sub.domain. But the following is illegal: you@here.my.sub. Modify rule set Hubset in the client.cf file so that it recognizes all the legal forms of addressing in a subdomain.