PMDF User's Guide
UNIX Edition

Aliases have multiple uses with e-mail. Individual users typically use aliases as abbreviations. For instance, rather than remember John Doe's full e-mail address, an alias jd can be created so that mail sent to the address jd is properly sent using John Doe's full address (e.g., jd573@hostc.example.com). System managers often use aliases in order to create valid mail addresses for non-existent users. For instance, an alias named postmaster might be created and equated with the username root so that incoming network mail for the user postmaster is routed to the root account. These are just two examples of the many practical uses of aliases.

The process of interpreting an alias is called "alias expansion". In the two examples above, the aliases jd and postmaster expand, respectively, to jd573@hostc.example.com and root.

In this documentation, the expansion of an alias is represented with the following notation
alias-name -> alias-value For example, John -> jd573@hostc.example.com.

PMDF alias names are "case insensitive". This means that the alias names jd, JD, jD, and Jd are all considered to be identical by PMDF; the case (upper versus lower case) of the individual characters in an alias name is irrelevant to PMDF. However, PMDF does preserve the case of alias values.

PMDF aliases may expand to:

• an address: jd -> jd573@hostc.example.com,
• a list of addresses: staff -> bob@example.com,sue@example.com,
• other aliases: jd -> johnd -> jd573@hostc.example.com,
• a list of aliases: all -> staff,admin,faculty, or
• a mixture of addresses and aliases: list -> staff,bob@example.com.

Note that in the above example, it is not clear whether or not the expanded value of an alias is another alias or not; i.e., in "jd -> johnd", johnd could have been either an alias or a legitimate username. PMDF always starts by assuming that an address without any domain part (e.g., @example.com) is an alias and attempts to expand it. When expanding an alias, PMDF first tries to look up the alias in the user's personal alias database and, if the alias is not found there, then PMDF consults system-level alias sources. After expanding an alias once, PMDF then tries to expand the result (or results in the case of a list). This expansion process is repeated until no more expansions are possible at which point the results are all assumed to be real mail addresses and not aliases.

By default, alias names may be from 1 to 80 characters long and their expansion values 0 to 252 characters. This corresponds to a "long" alias database file which is the type of file pmdf db normally creates.