|Lesson 4||Responsibilities of sendmail |
|Objective||List the major responsibilities of the sendmail agent. |
Responsibilities of sendmail Agent
The principal task of the
program is to function as a Mail
Transfer Agent, forwarding email to its correct destination. On a typical small network, one machine is configured to act as a mail hub.
Suppose that this machine is named HUB and that another machine on the network is named CLIENT. Mail from a user on CLIENT to a remote site takes the following path:
- The user’s MUA on CLIENT contacts the
sendmail program on CLIENT and passes it the email message.
sendmail on CLIENT contacts the
sendmail on HUB using a TCP connection to the SMTP port (25), then passes the mail to HUB.
- HUB contacts a remote mail-relaying site and passes the email message on. Eventually it reaches its destination.
Similarly, incoming mail is delivered to HUB, which is responsible for routing the mail properly on the local network.
Not all email comes from people. Many UNIX services, such as
, use electronic mail to send alert messages. The daemon programs supplying such services contact
directly to forward their messages.
The process of manipulating headers plays an important role in masquerading. Suppose that a user named james is logged on to the machine CLIENT, as in the example at the beginning of this lesson. Suppose further that CLIENT is part of the domain corporation.com. Email from james on the machine CLIENT is in fact from james@CLIENT.corporation.com, but user james prefers that the mail appear to be from his “real” email address, which is email@example.com. The
sendmail program can be configured to rewrite the “FROM:” header on james’s messages so that they appear to be from firstname.lastname@example.org. This process is called masquerading.
sendmail agent and other MTAs can also maintain a database of address aliases. An alias is simply an alternative address for a specific user. Aliasing allows all users to refer to users by simple nicknames or by names other than their UNIX username. Aliasing is different from masquerading, because it works with sendmail on the local host. Unlike masquerading, aliasing does not rewrite message headers to change the machine name. Perhaps the best way to explain aliasing is to provide an example:
- Stay Organized. Principal Task tracks all of the dimensions of your todos so that you can organize things your way.
- Never Miss a Deadline. Keep track of important dates, recurring tasks, and get reminders so you always complete your todos on time
- Maintain momentum.
Each line constitutes an alias. First is the username, then the assigned email name that SMTP uses.
Sendmail Responsibilities - Quiz