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Red Hat Enterprise Linux 6 Essentials eBook now available in PDF and ePub formats for only $9.99
RHEL 6 Essentials contains 40 chapters and over 250 pages.

23.3. Mail Transport Agents

Red Hat Enterprise Linux includes two primary MTAs, Sendmail and Postfix. Sendmail is configured as the default MTA, although it is easy to switch the default MTA to Postfix.

23.3.1. Sendmail

Sendmail's core purpose, like other MTAs, is to safely transfer email among hosts, usually using the SMTP protocol. However, Sendmail is highly configurable, allowing control over almost every aspect of how email is handled, including the protocol used. Many system administrators elect to use Sendmail as their MTA due to its power and scalability.

23.3.1.1. Purpose and Limitations

It is important to be aware of what Sendmail is and what it can do, as opposed to what it is not. In these days of monolithic applications that fulfill multiple roles, Sendmail may seem like the only application needed to run an email server within an organization. Technically, this is true, as Sendmail can spool mail to each users' directory and deliver outbound mail for users. However, most users actually require much more than simple email delivery. Users usually want to interact with their email using an MUA, that uses POP or IMAP, to download their messages to their local machine. Or, they may prefer a Web interface to gain access to their mailbox. These other applications can work in conjunction with Sendmail, but they actually exist for different reasons and can operate separately from one another.

It is beyond the scope of this section to go into all that Sendmail should or could be configured to do. With literally hundreds of different options and rule sets, entire volumes have been dedicated to helping explain everything that can be done and how to fix things that go wrong. Refer to the Section 23.7, “Additional Resources” for a list of Sendmail resources.

This section reviews the files installed with Sendmail by default and reviews basic configuration changes, including how to stop unwanted email (spam) and how to extend Sendmail with the Lightweight Directory Access Protocol (LDAP).

23.3.1.2. The Default Sendmail Installation

The Sendmail executable is /usr/sbin/sendmail.

Sendmail's lengthy and detailed configuration file is /etc/mail/sendmail.cf. Avoid editing the sendmail.cf file directly. To make configuration changes to Sendmail, edit the /etc/mail/sendmail.mc file, back up the original /etc/mail/sendmail.cf, and use the following alternatives to generate a new configuration file:

  • Use the included makefile in /etc/mail (make all -C /etc/mail) to create a new /etc/mail/sendmail.cf configuration file. All other generated files in /etc/mail (db files) will be regenerated if needed. The old makemap commands are still usable. The make command will automatically be used by service sendmail start | restart | reload if the make package is installed.

  • Alternatively you may use the included m4 macro processor to create a new /etc/mail/sendmail.cf.

More information on configuring Sendmail can be found in Section 23.3.1.3, “Common Sendmail Configuration Changes”.

Various Sendmail configuration files are installed in the /etc/mail/ directory including:

  • access — Specifies which systems can use Sendmail for outbound email.

  • domaintable — Specifies domain name mapping.

  • local-host-names — Specifies aliases for the host.

  • mailertable — Specifies instructions that override routing for particular domains.

  • virtusertable — Specifies a domain-specific form of aliasing, allowing multiple virtual domains to be hosted on one machine.

Several of the configuration files in /etc/mail/, such as access, domaintable, mailertable and virtusertable, must actually store their information in database files before Sendmail can use any configuration changes. To include any changes made to these configurations in their database files, run the command

makemap hash /etc/mail/<name> < /etc/mail/<name> 

where <name> is replaced with the name of the configuration file to convert.

For example, to have all emails addressed to the example.com domain delivered to , add the following line to the virtusertable file:

            @example.com     bob@other-example.com
          

To finalize the change, the virtusertable.db file must be updated using the following command as root:

makemap hash /etc/mail/virtusertable < /etc/mail/virtusertable 

This creates an updated virtusertable.db file containing the new configuration.

23.3.1.3. Common Sendmail Configuration Changes

When altering the Sendmail configuration file, it is best not to edit an existing file, but to generate an entirely new /etc/mail/sendmail.cf file.

Caution

Before changing the sendmail.cf file, it is a good idea to create a backup copy.

To add the desired functionality to Sendmail, edit the /etc/mail/sendmail.mc file as the root user. When finished, use the m4 macro processor to generate a new sendmail.cf by executing the following command:

            m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
          

By default, the m4 macro processor is installed with Sendmail but is part of the m4 package.

After creating a new /etc/mail/sendmail.cf file, restart Sendmail for the changes to take effect. The easiest way to do this is to type the following command:

            /sbin/service sendmail restart
          

Important

The default sendmail.cf file does not allow Sendmail to accept network connections from any host other than the local computer. To configure Sendmail as a server for other clients, edit the /etc/mail/sendmail.mc file, and either change the address specified in the Addr= option of the DAEMON_OPTIONS directive from 127.0.0.1 to the IP address of an active network device or comment out the DAEMON_OPTIONS directive all together by placing dnl at the beginning of the line. When finished, regenerate /etc/mail/sendmail.cf by executing the following command:

              m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
            

The default configuration which ships with Red Hat Enterprise Linux works for most SMTP-only sites. However, it does not work for UUCP (UNIX to UNIX Copy) sites. If using UUCP mail transfers, the /etc/mail/sendmail.mc file must be reconfigured and a new /etc/mail/sendmail.cf must be generated.

Consult the /usr/share/sendmail-cf/README file before editing any files in the directories under the /usr/share/sendmail-cf directory, as they can affect the future configuration of /etc/mail/sendmail.cf files.

23.3.1.4. Masquerading

One common Sendmail configuration is to have a single machine act as a mail gateway for all machines on the network. For instance, a company may want to have a machine called mail.example.com that handles all of their email and assigns a consistent return address to all outgoing mail.

In this situation, the Sendmail server must masquerade the machine names on the company network so that their return address is user@example.com instead of user@host.example.com.

To do this, add the following lines to /etc/mail/sendmail.mc:

FEATURE(always_add_domain)dnl 
FEATURE(`masquerade_entire_domain') dnl
FEATURE(`masquerade_envelope') dnl
FEATURE(`allmasquerade') dnl
MASQUERADE_AS(`bigcorp.com.') dnl
MASQUERADE_DOMAIN(`bigcorp.com.') dnl 
MASQUERADE_AS(bigcorp.com) dnl

After generating a new sendmail.cf using m4, this configuration makes all mail from inside the network appear as if it were sent from bigcorp.com.

23.3.1.5. Stopping Spam

Email spam can be defined as unnecessary and unwanted email received by a user who never requested the communication. It is a disruptive, costly, and widespread abuse of Internet communication standards.

Sendmail makes it relatively easy to block new spamming techniques being employed to send junk email. It even blocks many of the more usual spamming methods by default. Main anti-spam features available in sendmail are header checks, relaying denial (default from version 8.9), access database and sender information checks.

For example, forwarding of SMTP messages, also called relaying, has been disabled by default since Sendmail version 8.9. Before this change occurred, Sendmail directed the mail host (x.edu) to accept messages from one party (y.com) and sent them to a different party (z.net). Now, however, Sendmail must be configured to permit any domain to relay mail through the server. To configure relay domains, edit the /etc/mail/relay-domains file and restart Sendmail.

However, many times users are bombarded with spam from other servers throughout the Internet. In these instances, Sendmail's access control features available through the /etc/mail/access file can be used to prevent connections from unwanted hosts. The following example illustrates how this file can be used to both block and specifically allow access to the Sendmail server:

            badspammer.com       ERROR:550 "Go away and do not spam us anymore" 
tux.badspammer.com   OK 
10.0                 RELAY
          

This example shows that any email sent from badspammer.com is blocked with a 550 RFC-821 compliant error code, with a message sent back to the spammer. Email sent from the tux.badspammer.com sub-domain, is accepted. The last line shows that any email sent from the 10.0.*.* network can be relayed through the mail server.

Because /etc/mail/access.db is a database, use makemap to activate any changes. Do this using the following command as root:

            makemap hash /etc/mail/access < /etc/mail/access
          

Message header analysis allows you to reject mail based on header contents. SMTP servers store information about an emails journey in the message header. As the message travels from one MTA to another, each puts in a "Received" header above all the other Received headers. It is however important to note that this information may be altered by spammers.

The above examples only represent a small part of what Sendmail can do in terms of allowing or blocking access. Refer to the /usr/share/sendmail-cf/README for more information and examples.

Since Sendmail calls the Procmail MDA when delivering mail, it is also possible to use a spam filtering program, such as SpamAssassin, to identify and file spam for users. Refer to Section 23.5.2.6, “Spam Filters” for more about using SpamAssassin.

23.3.1.6. Using Sendmail with LDAP

Using the Lightweight Directory Access Protocol (LDAP) is a very quick and powerful way to find specific information about a particular user from a much larger group. For example, an LDAP server can be used to look up a particular email address from a common corporate directory by the user's last name. In this kind of implementation, LDAP is largely separate from Sendmail, with LDAP storing the hierarchical user information and Sendmail only being given the result of LDAP queries in pre-addressed email messages.

However, Sendmail supports a much greater integration with LDAP, where it uses LDAP to replace separately maintained files, such as aliases and virtusertables, on different mail servers that work together to support a medium- to enterprise-level organization. In short, LDAP abstracts the mail routing level from Sendmail and its separate configuration files to a powerful LDAP cluster that can be leveraged by many different applications.

The current version of Sendmail contains support for LDAP. To extend the Sendmail server using LDAP, first get an LDAP server, such as OpenLDAP, running and properly configured. Then edit the /etc/mail/sendmail.mc to include the following:

LDAPROUTE_DOMAIN('yourdomain.com')dnl 
FEATURE('ldap_routing')dnl 

Note

This is only for a very basic configuration of Sendmail with LDAP. The configuration can differ greatly from this depending on the implementation of LDAP, especially when configuring several Sendmail machines to use a common LDAP server.

Consult /usr/share/sendmail-cf/README for detailed LDAP routing configuration instructions and examples.

Next, recreate the /etc/mail/sendmail.cf file by running m4 and restarting Sendmail. Refer to Section 23.3.1.3, “Common Sendmail Configuration Changes” for instructions.

For more information on LDAP, refer to Chapter 24, Lightweight Directory Access Protocol (LDAP).

23.3.2. Postfix

Originally developed at IBM by security expert and programmer Wietse Venema, Postfix is a Sendmail-compatible MTA that is designed to be secure, fast, and easy to configure.

To improve security, Postfix uses a modular design, where small processes with limited privileges are launched by a master daemon. The smaller, less privileged processes perform very specific tasks related to the various stages of mail delivery and run in a change rooted environment to limit the effects of attacks.

Configuring Postfix to accept network connections from hosts other than the local computer takes only a few minor changes in its configuration file. Yet for those with more complex needs, Postfix provides a variety of configuration options, as well as third party add ons that make it a very versatile and full-featured MTA.

The configuration files for Postfix are human readable and support upward of 250 directives. Unlike Sendmail, no macro processing is required for changes to take effect and the majority of the most commonly used options are described in the heavily commented files.

Important

Before using Postfix, the default MTA must be switched from Sendmail to Postfix.

23.3.2.1. The Default Postfix Installation

The Postfix executable is /usr/sbin/postfix. This daemon launches all related processes needed to handle mail delivery.

Postfix stores its configuration files in the /etc/postfix/ directory. The following is a list of the more commonly used files:

  • access — Used for access control, this file specifies which hosts are allowed to connect to Postfix.

  • aliases — A configurable list required by the mail protocol.

  • main.cf — The global Postfix configuration file. The majority of configuration options are specified in this file.

  • master.cf — Specifies how Postfix interacts with various processes to accomplish mail delivery.

  • transport — Maps email addresses to relay hosts.

Important

The default /etc/postfix/main.cf file does not allow Postfix to accept network connections from a host other than the local computer. For instructions on configuring Postfix as a server for other clients, refer to Section 23.3.2.2, “Basic Postfix Configuration”.

When changing some options within files in the /etc/postfix/ directory, it may be necessary to restart the postfix service for the changes to take effect. The easiest way to do this is to type the following command:

            /sbin/service postfix restart
          

23.3.2.2. Basic Postfix Configuration

By default, Postfix does not accept network connections from any host other than the local host. Perform the following steps as root to enable mail delivery for other hosts on the network:

  • Edit the /etc/postfix/main.cf file with a text editor, such as vi.

  • Uncomment the mydomain line by removing the hash mark (#), and replace domain.tld with the domain the mail server is servicing, such as example.com.

  • Uncomment the myorigin = $mydomain line.

  • Uncomment the myhostname line, and replace host.domain.tld with the hostname for the machine.

  • Uncomment the mydestination = $myhostname, localhost.$mydomain line.

  • Uncomment the mynetworks line, and replace 168.100.189.0/28 with a valid network setting for hosts that can connect to the server.

  • Uncomment the inet_interfaces = all line.

  • Restart the postfix service.

Once these steps are complete, the host accepts outside emails for delivery.

Postfix has a large assortment of configuration options. One of the best ways to learn how to configure Postfix is to read the comments within /etc/postfix/main.cf. Additional resources including information about LDAP and SpamAssassin integration are available online at http://www.postfix.org/.

23.3.3. Fetchmail

Fetchmail is an MTA which retrieves email from remote servers and delivers it to the local MTA. Many users appreciate the ability to separate the process of downloading their messages located on a remote server from the process of reading and organizing their email in an MUA. Designed with the needs of dial-up users in mind, Fetchmail connects and quickly downloads all of the email messages to the mail spool file using any number of protocols, including POP3 and IMAP. It can even forward email messages to an SMTP server, if necessary.

Fetchmail is configured for each user through the use of a .fetchmailrc file in the user's home directory.

Using preferences in the .fetchmailrc file, Fetchmail checks for email on a remote server and downloads it. It then delivers it to port 25 on the local machine, using the local MTA to place the email in the correct user's spool file. If Procmail is available, it is launched to filter the email and place it in a mailbox so that it can be read by an MUA.

23.3.3.1. Fetchmail Configuration Options

Although it is possible to pass all necessary options on the command line to check for email on a remote server when executing Fetchmail, using a .fetchmailrc file is much easier. Place any desired configuration options in the .fetchmailrc file for those options to be used each time the fetchmail command is issued. It is possible to override these at the time Fetchmail is run by specifying that option on the command line.

A user's .fetchmailrc file contains three classes of configuration options:

  • global options — Gives Fetchmail instructions that control the operation of the program or provide settings for every connection that checks for email.

  • server options — Specifies necessary information about the server being polled, such as the hostname, as well as preferences for specific email servers, such as the port to check or number of seconds to wait before timing out. These options affect every user using that server.

  • user options — Contains information, such as username and password, necessary to authenticate and check for email using a specified email server.

Global options appear at the top of the .fetchmailrc file, followed by one or more server options, each of which designate a different email server that Fetchmail should check. User options follow server options for each user account checking that email server. Like server options, multiple user options may be specified for use with a particular server as well as to check multiple email accounts on the same server.

Server options are called into service in the .fetchmailrc file by the use of a special option verb, poll or skip, that precedes any of the server information. The poll action tells Fetchmail to use this server option when it is run, which checks for email using the specified user options. Any server options after a skip action, however, are not checked unless this server's hostname is specified when Fetchmail is invoked. The skip option is useful when testing configurations in .fetchmailrc because it only checks skipped servers when specifically invoked, and does not affect any currently working configurations.

A sample .fetchmailrc file looks similar to the following example:

            set postmaster "user1" 
set bouncemail
  
 poll pop.domain.com proto pop3
 	user 'user1' there with password 'secret' is user1 here  
 
 poll mail.domain2.com      
 	user 'user5' there with password 'secret2' is user1 here
	user 'user7' there with password 'secret3' is user1 here
          

In this example, the global options specify that the user is sent email as a last resort (postmaster option) and all email errors are sent to the postmaster instead of the sender (bouncemail option). The set action tells Fetchmail that this line contains a global option. Then, two email servers are specified, one set to check using POP3, the other for trying various protocols to find one that works. Two users are checked using the second server option, but all email found for any user is sent to user1's mail spool. This allows multiple mailboxes to be checked on multiple servers, while appearing in a single MUA inbox. Each user's specific information begins with the user action.

Note

Users are not required to place their password in the .fetchmailrc file. Omitting the with password '<password>' section causes Fetchmail to ask for a password when it is launched.

Fetchmail has numerous global, server, and local options. Many of these options are rarely used or only apply to very specific situations. The fetchmail man page explains each option in detail, but the most common ones are listed here.

23.3.3.2. Global Options

Each global option should be placed on a single line after a set action.

  • daemon <seconds> — Specifies daemon-mode, where Fetchmail stays in the background. Replace <seconds> with the number of seconds Fetchmail is to wait before polling the server.

  • postmaster — Specifies a local user to send mail to in case of delivery problems.

  • syslog — Specifies the log file for errors and status messages. By default, this is /var/log/maillog.

23.3.3.3. Server Options

Server options must be placed on their own line in .fetchmailrc after a poll or skip action.

  • auth <auth-type> — Replace <auth-type> with the type of authentication to be used. By default, password authentication is used, but some protocols support other types of authentication, including kerberos_v5, kerberos_v4, and ssh. If the any authentication type is used, Fetchmail first tries methods that do not require a password, then methods that mask the password, and finally attempts to send the password unencrypted to authenticate to the server.

  • interval <number> — Polls the specified server every <number> of times that it checks for email on all configured servers. This option is generally used for email servers where the user rarely receives messages.

  • port <port-number> — Replace <port-number> with the port number. This value overrides the default port number for the specified protocol.

  • proto <protocol> — Replace <protocol> with the protocol, such as pop3 or imap, to use when checking for messages on the server.

  • timeout <seconds> — Replace <seconds> with the number of seconds of server inactivity after which Fetchmail gives up on a connection attempt. If this value is not set, a default of 300 seconds is assumed.

23.3.3.4. User Options

User options may be placed on their own lines beneath a server option or on the same line as the server option. In either case, the defined options must follow the user option (defined below).

  • fetchall — Orders Fetchmail to download all messages in the queue, including messages that have already been viewed. By default, Fetchmail only pulls down new messages.

  • fetchlimit <number> — Replace <number> with the number of messages to be retrieved before stopping.

  • flush — Deletes all previously viewed messages in the queue before retrieving new messages.

  • limit <max-number-bytes> — Replace <max-number-bytes> with the maximum size in bytes that messages are allowed to be when retrieved by Fetchmail. This option is useful with slow network links, when a large message takes too long to download.

  • password '<password>' — Replace <password> with the user's password.

  • preconnect "<command>" — Replace <command> with a command to be executed before retrieving messages for the user.

  • postconnect "<command>" — Replace <command> with a command to be executed after retrieving messages for the user.

  • ssl — Activates SSL encryption.

  • user "<username>" — Replace <username> with the username used by Fetchmail to retrieve messages. This option must precede all other user options.

23.3.3.5. Fetchmail Command Options

Most Fetchmail options used on the command line when executing the fetchmail command mirror the .fetchmailrc configuration options. In this way, Fetchmail may be used with or without a configuration file. These options are not used on the command line by most users because it is easier to leave them in the .fetchmailrc file.

There may be times when it is desirable to run the fetchmail command with other options for a particular purpose. It is possible to issue command options to temporarily override a .fetchmailrc setting that is causing an error, as any options specified at the command line override configuration file options.

23.3.3.6. Informational or Debugging Options

Certain options used after the fetchmail command can supply important information.

  • --configdump — Displays every possible option based on information from .fetchmailrc and Fetchmail defaults. No email is retrieved for any users when using this option.

  • -s — Executes Fetchmail in silent mode, preventing any messages, other than errors, from appearing after the fetchmail command.

  • -v — Executes Fetchmail in verbose mode, displaying every communication between Fetchmail and remote email servers.

  • -V — Displays detailed version information, lists its global options, and shows settings to be used with each user, including the email protocol and authentication method. No email is retrieved for any users when using this option.

23.3.3.7. Special Options

These options are occasionally useful for overriding defaults often found in the .fetchmailrc file.

  • -a — Fetchmail downloads all messages from the remote email server, whether new or previously viewed. By default, Fetchmail only downloads new messages.

  • -k — Fetchmail leaves the messages on the remote email server after downloading them. This option overrides the default behavior of deleting messages after downloading them.

  • -l <max-number-bytes> — Fetchmail does not download any messages over a particular size and leaves them on the remote email server.

  • --quit — Quits the Fetchmail daemon process.

More commands and .fetchmailrc options can be found in the fetchmail man page.


 
 
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