NIS+ mail_aliases Table

The NIS+ mail_aliases table contains the names by which a system or person is known in the local domain. The sendmail program can use the NIS+ mail_aliases table, instead of the local /etc/mail/aliases files, to determine mailing addresses. Refer to the aliasadm(1M) and nsswitch.conf(4) man pages for more information.

Aliases in the NIS+ mail_aliases table adhere to the following format:

alias: expansion # ["options	" # "comments"]

Table 26-12 describes the four columns that are in an NIS+ mail_aliases table.

Table 26-12 Columns in the NIS+ mail_aliases Table

The NIS+ mail_aliases table should contain entries for all mail clients. You can list, create, modify, and delete entries in the NIS+ aliases table with the aliasadm command. To use the aliasadm command, you must be a member of the NIS+ group that owns the aliases table. For task information, refer to "How to Manage Alias Entries in an NIS+ mail_aliasesTable" in Chapter 25, Mail Services (Tasks). Alternately, you can use the AdminTool's Database Manager to administer the NIS+ mail aliases.

Note - If you are creating a new NIS+ aliases table, you must initialize the table before you create the entries. If the table exists, no initialization is needed.

.forward Files

Users can create a .forward file in their home directories that sendmail, along with other programs, can use to redirect mail or send mail. Refer to the following topics.

• "Situations to Avoid"

• "Controls for .forward files"

• ".forward.hostname File"

• ".forward+detail File"

For a task map, refer to "Administering .forward Files (Task Map)" in Chapter 25, Mail Services (Tasks).

Situations to Avoid

The following list describes some situations that you can avoid or easily fix.

• If mail is not being delivered to the expected address, check the user's .forward file. The user might have put the .forward file in the home directory of host1, which forwards mail to user@host2. When the mail arrives at host2, sendmail checks for user in the NIS or NIS+ aliases and sends the message back to user@host1, which results in a loop and more bounced mail.

• To avoid security problems, never put .forward files in the root and bin accounts. If necessary, forward the mail by using the aliases file instead.

Controls for .forward files

For the .forward files to be an effective part of mail delivery, ensure that the following controls (mostly permissions settings) are correctly applied.

• The .forward file must be writable only by the owner of the file. This restriction prevents other users from breaking security.

• The paths that lead to the home directory must be owned and writable by root only. For example, if a .forward file is in /export/home/terry, /export and /export/home must be owned and writable by root only.

• The actual home directory should be writable only by the user.

• The .forward file cannot be a symbolic link, and it cannot have more than one hard link.

.forward.hostname File

You can create a .forward.hostname file to redirect mail that is sent to a specific host. For example, if a user's alias has changed from sandy@phoenix.example.com to sandy@example.com, place a .forward.phoenix file in the home directory for sandy.

% cat .forward.phoenix
sandy@example.com
"|/usr/bin/vacation sandy"
% cat .vacation.msg
From: sandy@example.com (via the vacation program)
Subject: my alias has changed

My alias has changed to sandy@example.com.
Please use this alias in the future.
The mail that I just received from you
has been forwarded to my new address.

Sandy

In this example, mail can be forwarded to the correct place while the sender is notified of the alias change. Because the vacation program permits only one message file, you can forward only one message at a time. However, if the message is not host specific, one vacation message file can be used by .forward files for many hosts.

.forward+detail File

Another extension to the forwarding mechanism is the .forward+detail file. The detail string can be any sequence of characters except operator characters. The operator characters are .:%&!^[]+. By using this type of file, you can determine if someone else is using your email address without your knowledge. For instance, if a user tells someone to use the email address sandy+test1@example.com, the user would be able to identify any future mail that was delivered to this alias. By default, any mail that is sent to the sandy+test1@example.com alias is checked against the alias and the .forward+detail files. If no matches are made, the mail falls back to delivery to sandy@example.com, but the user is able to see a change in the To: mail header.

/etc/default/sendmail File

This file is used to store startup options for sendmail so that they are not removed when a host is upgraded. The following variables can be used.

CLIENTOPTIONS="string"

Selects additional options to be used with the client daemon, which looks in the client-only queue (/var/spool/clientmqueue) and acts as a client queue runner. No syntax checking is done, so be careful when making changes to this variable.

CLIENTQUEUEINTERVAL=#

Similar to the QUEUEINTERVAL option, CLIENTQUEUEINTERVAL sets the time interval for mail queue runs. However, the CLIENTQUEUEINTERVAL option controls the functions of the client daemon, instead of the master daemon. Typically, the master daemon is able to deliver all messages to the SMTP port. However, if the message load is too high or the master daemon is not running, then messages go into the client-only queue, /var/spool/clientmqueue. The client daemon, which checks in the client-only queue, then acts as a client queue processor.

ETRN_HOSTS="string"

Enables an SMTP client and server to interact immediately without waiting for the periodic queue run intervals. The server can immediately deliver the portion of its queue that goes to the specified hosts. For more information, refer to the etrn(1M) man page.

MODE=-bd

Selects the mode to start sendmail with. Use the -bd option or leave it undefined.

OPTIONS=string

Selects additional options to be used with the master daemon. No syntax checking is done, so be careful when making changes to this variable.

QUEUEINTERVAL=#

Sets the interval for mail queue runs on the master daemon. # can be a positive integer that is followed by either s for seconds, m for minutes, h for hours, d for days, or w for weeks. The syntax is checked before sendmail is started. If the interval is negative or if the entry does not end with an appropriate letter, the interval is ignored and sendmail starts with a queue interval of 15 minutes.

QUEUEOPTIONS=p

Enables one persistent queue runner that sleeps between queue run intervals, instead of a new queue runner for each queue run interval. You can set this option to p, which is the only setting available. Otherwise, this option is not set.

Mail Addresses and Mail Routing

The path a mail message follows during delivery depends on the setup of the client system and the topology of the mail domain. Each additional level of mail hosts or mail domains can add another alias resolution, but the routing process is basically the same on most hosts.

You can set up a client system to receive mail locally. Receiving mail locally is known as running sendmail in local mode. Local mode is the default for all mail servers and some clients. On a mail server or a mail client in local mode, a mail message is routed the following way.

Note - The following example assumes that you are using the default rule set in the sendmail.cf file.

1. Expand the mail alias, if possible, and restart the local routing process.

   The mail address is expanded by checking for the mail alias in the name service and substituting the new value, if one is found. This new alias is then checked again.

2. If the mail is local, deliver it to /usr/lib/mail.local.

   The mail is delivered to a local mailbox.

3. If the mail address includes a host in this mail domain, deliver the mail to that host.

4. If the address does not include a host in this domain, forward the mail to the mail host.

   The mail host uses the same routing process as the mail server, but the mail host can receive mail that is addressed to the domain name as well as to the host name.