E-mail as a System Console, Part II

Editors' Note: The following is a chapter from the book
Multitool Linux, written by Michael Schwarz, Jeremy Anderson, Peter
Curtis and Steven Murphy. Consult the
book's web site for
links, updates and errata.In Part I of this article,
I described the impetus for this project, devising a way to
remotely access my home system so I could control internet
connection times. Now let's get into what this system
involves.FetchmailFetchmail is one of those handy tools that makes using an
operating system like Linux so much fun. It's easy to configure,
has lots of functionality, and it does what you want, when you
want, without much complaining. You can run it from cron or from
the command line. If life could only be so easy.For the e-mail console, you use fetchmail to retrieve e-mail
from your ISP. This means you have to switch your current e-mail
client from using POP/IMAP to picking up e-mail locally. Why do you
have to do this? Well, when the MDA on your system wants to send
e-mail to your account, it first checks to see if a file named
.forward is in your home folder. If it is, all your mail is
forwarded to the e-mail address in the .forward file. But wait! We
don't want that to happen. What we want is the e-mail to be sent to
procmail for processing. You'll learn how to do this in the next
section. For now, remember the .forward file is the key.Fetchmail ConfigurationConfiguring fetchmail is a breeze. All you need to do is
create a .fetchmailrc file in your home folder. You can read the
man page for fetchmail and write this file yourself, or you can use
fetchmailconf and generate the .fetchmailrc file. I wrote my
original configuration file by hand, but the fetchmailconf
application is a much faster way to get things going.Here's a screenshot where I'm adding a new remote e-mail
server.
Figure 1. Fetchmail Config, New E-mail Server
Here's a screenshot where I'm setting up the server protocol
and adding a new e-mail account.
Figure 2. Fetchmail Config, Protocol Setup
Finally, here's a screenshot where I'm configuring the e-mail
account password and setting up the processing options.
Figure 3. Fetchmail Config, Account Setup
Once fetchmail is configured and working, you can run it from
the command line or via cron.
Figure 4. Fetchmail Being Executed
This screen shows that fetchmail got a single e-mail from my
ISP and then flushed (removed) it from my ISP. When you want to
automate the fetchmail process, use cron to schedule a fetchmail
call and make sure you use the silent (-s) command-line parameter;
otherwise, you'll be getting lots of messages from cron each time
fetchmail is run.CrontabIn order to make the e-mail console work, you must run
fetchmail at regular intervals. Cron is usually a good way to go
about doing this. Here's a sample crontab entry:

  0,15,30,45 * * * * /usr/bin/fetchmail

ProcmailHere's where it really starts to gets interesting. Procmail
is the tool that makes the e-mail console possible. It scans a text
file (your e-mail file, which is one giant file when it comes from
your MDA) looking for patterns you specify, and then it performs
some kind of action that you specify on a particular e-mail within
the file. You see, procmail understands how to read this giant
e-mail file and can perform an action either on the text itself or
an action in general. For instance, you might have procmail scan
incoming e-mail, and for each subject line have it execute a
text-to-speech program so it reads each subject to you when new
e-mail arrives. Kind of cool, right?Our use of procmail involves scanning the subject lines of
each e-mail looking for a given pattern that indicates the body of
the e-mail holds a command sequence to be executed and the output
captured and sent back to the sender of the e-mail. You also could
have procmail react to the sender's e-mail address, but this is not
as flexible. There are better ways to lock things down, as you will
see in a while. So the big question you should have now is, "How do
I tell procmail to scan my e-mail and do something?" Easy! You
create a recipe for procmail.RecipesA recipe is how you tell procmail what to search for and what
to do when it finds something. A recipe is the heart of procmail.
You create a recipe for each type of thing you want to search for
or an action you want to perform. There are many good resources on
the Internet for creating recipes, and your best bet is to start at
the procmail web site.So where do you put your recipes once they're created? You
put them in your home folder in a file named .procmailrc, of
course. Here is a beginning .procmailrc with two recipes:

     01 #---------------
     02 # my .procmailrc file
     03 #----------------
     04 SHELL=/bin/bash  # path to your sh (try "which sh" to find it)
     05 MAILDIR=${HOME}/Mail # e-mail goes here after your client gets it
     06 LOGFILE=${MAILDIR}/procmail.log
     07 LOG="--- Logging ${LOGFILE} for ${LOGNAME}, "
     08
     09 #-------------------------------------
     10 # Your recipes (the order is extremely important!!!!
     11 #-------------------------------------
     12 :0
     13 * ^Subject: test test test
     14 /dev/null
     15
     16 #------------------------------
     17 # Accept all the rest to your default mailbox
     18 #------------------------------
     19 :0:
     20 ${DEFAULT}

I stuck the line numbers in so I can reference each important
line for this example. You should not put line numbers in your
file, because they would really hose things up. Lines 04 through 07
are fairly obvious, so just follow the comments. Lines 12 through
14 make up the first recipe, which searches for the string
"Subject: test test test" at the beginning of a line.Remember, all your e-mails get sent to procmail as one giant
file, but procmail knows how to logically break up the file into
each message and check for whatever pattern you provide. In this
case, line 13 is the pattern you want to search for. The ^ at the
beginning means the string of text you are looking for must be
anchored at the beginning of the line. Line 14 directs procmail to
send the matching e-mail to the digital garbage can, black hole or
bit bucket, if you prefer. Each e-mail is passed from the first
recipe to the next until a match is made.If you haven't already figured it out, the search criteria is
a regular expression. If you know regular expressions, then you
already know how to enter a search string for any text you want to
search for. If you don't know how to write regular expressions,
then I highly recommend reading up on them. They are powerful and
are used in many of the best tools available for Linux.Line 14 instructs procmail to take the e-mail where the
regular expression matches and sends it to the null device. In
other words, it copies the text to null device and removes it from
the e-mail file. Can you say, "spam filter"? I can think of a few
rules to filter out unwanted e-mails. Of course, there are numerous
examples on the Internet of spam filters for procmail--do a search
on Google.com for them.Another good use for procmail is to direct e-mail from
various e-mail lists into their own e-mail subfolder in your e-mail
folder. This is beyond the scope of this article, but you can
experiment and read some FAQs to get the answers on how to achieve
this functionality.The next recipe is probably the most important recipe of them
all. The second recipe on lines 19 and 20 is the catch all-recipe
for any e-mail not already processed. When I wrote my first
.procmailrc file, I neglected to include this recipe and my e-mail
simply went away. Don't make this mistake.This last recipe tells procmail to put all e-mail not caught
by previous recipes into your normal mailbox location
(/var/mail/your_account). Wait. I thought it was in my Mail folder
in my home folder? Nope. You see, when fetchmail picks up the
e-mail from your ISP and delivers it to your MDA. Your MDA looks
for a .forward file before writing the e-mail to the
/var/mail/your_account file. The .forward file tells your MDA to
send the incoming e-mail to the address within. The neat thing is
that you can also pipe the e-mail to a program such as
procmail.Procmail acts as a filter and has the ability to remove
e-mail from the giant e-mail file. Once procmail is done with it,
the e-mail not filtered out goes to /var/mail/your_account, where
your normal e-mail client can pick it up.I'll talk more about the contents of the .forward file in
[Part III]. So what's all that other stuff, like :0, *, :0:>,
and ${DEFAULT} all about? Here's a table that should sort it all
out for you:

      __________________________________________________________________
     |__________________________________________________________________|
     |          |Begin a recipe and use a lock file. Use a lock file to |
     |          |prevent multiple procmails from trying to write to your|
     |:0:       |mail file at the same time. (From the procmail FAQ:    |
     |          |"Rule of thumb: Use file locking when delivering to a  |
     |          |file. Don't use file locking when delivering to /dev/  |
     |__________________________________________________________________|
     |__________________________________________________________________|
     |${DEFAULT}|This is your default e-mail location (/var/mail/        |
     |__________________________________________________________________|
     |__________________________________________________________________|

So filtering e-mail is great and all, but what about
executing commands? No problem! Procmail has support for that too,
and it even can execute scripts. The recipe I use for the e-mail
console is as follows:

     :0
     * ^Subject: <console/>
     |/usr/bin/perl /home/stmurphy/bin/e-mail_console.pl

For our e-mail console recipe, we want to look for a pattern
in the subject line that indicates the e-mail contains a command to
be executed and its output returned to the sender. The best way to
parse this e-mail and execute those commands is to write a Perl
script and have procmail execute the Perl script, passing the
entire e-mail to the script. It's important to do for security
reasons. You should notice that I am putting the e-mail_console.pl
script in a folder named "bin" in my home folder. This is a
convenient place to put stuff I want executed by my account
only.In Part III, we'll take a look at the Perl script.Copyright 2002 by Addison-Wesley. All rights reserved.
Reproduced by permission of Pearson Education, Inc.

email: mtl@multitool.net

email: mtl@multitool.net