PH-Based Email Redirection
- Backround: What is PH?
- How PH Interacts with Email
- Example: The uiuc.edu Domain
- Configuring PH-Based Email Redirection
Backround: What is PH?
Many sites maintain a PH
directory server which contains information about all people affiliated
with their organization. The PH database contains one entry for each
person, where each entry contains multiple fields which describe
various attributes of that person. The mandatory alias field
contains a unique identifier for each entry which acts as a primary key.
How PH Interacts with Email
One of the benefits of maintaining a PH server is that it defines the
email address namespace for the domain it represents. This has two main
advantages:
- Provides all users with a "domain-level" email address, such as
alias@uiuc.edu or alias@bigcompany.com.
- Provides transparent email redirection from the domain-level email
address to the actual host where the user reads mail, so that users
can change which machine their email is delivered to without
notifying all of their contacts of a new email address.
This is accomplished by allowing users to set the email field
of their PH entry to contain the email address which they want their
email to be delivered to. The DNS MX records for the organization's
top-level domain are then pointed at a set of mail relay systems which
look up the user's email field from the PH server and forward
the email to that address.
Example: The uiuc.edu Domain
Everyone affiliated with UIUC has an entry on the UIUC PH server, which
is referred to locally as the UIUC Electronic Directory.
Each user's alias field is set to their Network ID, which is
a domain-wide unique login.
To illustrate how PH-based email redirection works, let's examine a
hypothetical user named John Smith, who has the following PH entry:
alias : jsmith
name : smith john
email : jsmith@students.uiuc.edu
John advertises his email address as jsmith@uiuc.edu. The MX
records for uiuc.edu point to a set of Mail Relay machines
operated by CITES (the central academic computing department). The mail
relays receive email sent to jsmith@uiuc.edu, look up the PH
entry whose alias is set to jsmith, and forward the
message to the address listed in the email field of the matching
PH entry. This is illustrated in the diagram below.
The CITES Mail Relays do an excellent job handling incoming email
traffic, which makes sense since MTAs outside of UIUC have no way of
knowing about the local PH setup. However, since it is not
unreasonable for local machines to take advantage of their knowledge of
the local PH setup, many machines operated in departments on campus
perform their own PH-based email redirection for @uiuc.edu
email addresses, as shown on the right in the diagram above. By doing
so, many messages from those machines can be delivered directly to
their destination without an extra hop in between.
Configuring PH-Based Email Redirection
There are two ways of configuring sendmail to perform this
redirection:
Mark D. Roth <roth@feep.net>