211.2. Managing E-Mail Delivery
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Description: Candidates should be able to implement client e-mail management software to filter, sort and monitor incoming user e-mail.
Key Knowledge Areas:
Understanding of Sieve functionality, syntax and operators
Use Sieve to filter and sort mail with respect to sender, recipient(s), headers and size
Awareness of procmail
Terms and Utilities:
Conditions and comparison operators
keep, fileinto, redirect, reject, discard, stop
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In previous lesson we understand that the postfix store e-mails in /var/spool/mail directory. In this lesson we will get familiar with procmail software for managing e-mails and different ways of doing that.
Most of MTAs (Mail Transfer Agents / Mail Servers) work with procmail or at leased they are enabled to work with procmail.
procmail, a mail delivery agent (MDA), can sort incoming mail into various directories and filter out spam messages. Procmail is stable, but no longer maintained, and a number of security vulnerabilities have been discovered since its last release. Users who wish to use a maintained program are advised by procmail's author, Philip Guenther, to use an alternative tool(like maildrop or sieve), because it has not been updated since 2001.Any how,as we are preparing for lpic2 exam lets continue.
postfix stores users emails in /var/spool/mail/<user name> . It creates a file and append every user's e-mail to the end of that big text file! Mean while it takes space from system /var directory.
procmail lets us to store e-mails in one of specified user's directories, usually /home/<user name>/mail/ directory and sort it.
procmail can store e-mails in two different storage formats, mbox format or maildr format.
mbox is the older mail storage system. While mbox places all messages in the same file on the server, Maildir stores messages in individual files with unique names.
Mbox is the traditional email storage format. In this format there is only one regular text file which serves as the user’s mailbox.Typically, the name of this file is /var/spool/mail/<user name>. Mbox locks the mailbox when an operation is performed on the mailbox. After the operation the mailbox is unlocked.
Advantages:
Mbox format is universally supported
Appending new email is fast
Searching inside single mailbox is fast
Disadvantages:
Mbox is known for locking problems
The mbox format is prone to corruption
Maildir is the newer email storage format. A directory maildir is created for each email user, typically in the users’ home directories. Under this maildir directory by default three more directories exist: new, cur and tmp.
Advantages:
Locating, retrieving and deleting a specific email is fast, particularly when a email folder contains hundreds of messages
Minimal to no file locking needed
Can be used on a network file system
Immune to mailbox corruption (assuming the hardware will not fail)
Disadvantages:
Some filesystems may not efficiently handle a large number of small files
Searching text, which requires all email files to be opened is slow
procmail might has been installed on your distro, other wise install it:
with that procmail just install some binary files which would be used by postfix. procmail dosen't have any services! And it doesn't have any configuration files!
Procmail can be invoked in several different ways. As email is placed on your email spool file, Procmail can be configured to start up, filter the email to locations configured for use with your MUA, and quit. Or, your MUA could be configured to bring up Procmail any time a message is received so that messages are moved into their correct mailboxes(as we do here).
So what we need to do is configuring postfix to use procmail by setting up mailbox_command = procmail in main.cf configuration file.
Now postfix sends every thing which delivers localy to the procmail, how ever our procmail is not configured.
There are two layers of prcmail configuration files:
Global Configuration : /etc/procmailrc
User specified configuration (Recipes) : ~/.procmailrc
When Procmail starts, it reads the email message and separates the body from the header information. Next, Procmail looks for the /etc/procmailrc file and rc files in the /etc/procmailrcs directory for default, system-wide, Procmail environmental variables and recipes. Then, Procmail looks for a .procmailrc file in the user's home directory to find rules specific to that user. Many users also create additional rc files of their own for Procmail that are referred to by their .procmailrc file but may be turned on or off quickly if a mail filtering problem develops.
Be careful using the system-wide /etc/procmailrc. It is usually read and processed as root. This fact means that a poorly designed recipe in that file could do serious damage. For instance, a typo could cause Procmail to overwrite an important system binary rather than use that binary to process a message. For this reason, you should keep system-wide Procmail processing to a minimum and instead focus on using ~/.procmailrc to process email using individual accounts.
Okey lets start with Global Configuration. By default the Global configuration file does not exit and we should create procmailrc file under /etc directory:
Please notice that "/" changes the behaviour! Now let stick to mbox and check it:
it concatenate emails after each other.it would be much easier to use mail utility but we need to specify the folder in which mails exist by usinf -f option:
As we said Procmail allows us to filter email as it is received from a remote email server, or placed in our spool file on a local or remote email server. Filtering is done by using Recipes( user specified configurations). Recipes are configured by creating .procmailrc in the home directory of user.
The construction of recipes is the most difficult part of learning to use Procmail, beacause recipes do their message matching using regular expressions. However, regular expressions are not very difficult to construct and even less difficult to understand when read. the problem is understanding of Procmail recipe structure.
Procmail recipes take the following form:
The first two characters in a Procmail recipe are a colon and a zero. Various flags can be placed after the zero to control how Procmail processes the recipe. A colon after the flags_section specifies that a lockfile is created for this message. If a lockfile is created, the name can be specified by replacing lockfile-name_ .
A recipe can contain several conditions to match against the message. If it has no conditions, every message matches the recipe. Regular expressions are placed in some conditions to facilitate message matching. If multiple conditions are used, they must all match for the action to be performed. Conditions are checked based on the flags set in the recipe's first line. Optional special characters placed after the asterisk character ( *) can further control the condition.
The action-to-perform argument specifies the action taken when the message matches one of the conditions. There can only be one action per recipe. In many cases, the name of a mailbox is used here to direct matching messages into that file, effectively sorting the email. Special action characters may also be used before the action is specified.
For now lets take a look at simple recipe which take e-mails with "lpi" subject and throw them into "linuxcert" file:
Lets test:
And lets try another subject to make sure if our Recipe is working:
Now that we have seen mbox format lets see maldir format too lets remove inbox file which mbox format has made:
next lets edit systemwide promailrc :
Now lets send an email to user1 :
and see the maildir format for stroing mail:
good news is that mail tool works with both mbox and maildir format:
Okey that was all, surf the net and learn more about recipes if you like!
