I'm trying to set up an auto-forwarding system, in which the following happens:
a) user A and user B have actual email addresses A and B and forwarding addresses C and D. This info is stored in the mySQL database of my site.
b) users A and B know each others forwarding addresses through the site, but no one knows anyone's actual email address.
c) user A, from his actual email account, sends a message to user B's forwarding address.
d) site's mail server receives the email. a program alters the email's header line so that the recipient address is changed to user B's actual email address B, and the sender's address is changed to user A's forwarding email address C. Anonymity is therefore preserved.
e) changed email is now put in the mailing queue and sent off.

I'm a linux beginner, and have just managed to make a test server which successfully receives and sends email from the root account. Using advice from other sites, I've done the following:

a. changed /etc/mail/virtusertable and added the following line:

admin@mydomain.com root
@mydomain.com remailer-alias

b. changed /etc/aliases and added the following line:

remailer-alias: |remailer

The idea is all non admin@mydomain.com emails should be piped into the remailer program. The remailer program should reedit the message and place it in the queue.

The problem is, I'm not sure how to write the remailer program or where to place it. I have written only extremely basic shell scripts or Perl programs, and in particular have not written a program that processed piped information, having used pipes only on the command line. I have tried to make remailer programs just to test the above functionality, for example:

#! /usr/bin/perl -w
# test remailer program in perl
print "hello!\n";

or using a shell script:

#! /usr/bin/sh
# test remailer program, shell script
echo "hello"

I changed the /etc/aliases line to:

remailer-alias: |/etc/mail/remailer

I then placed the remailer file into /etc/mail, chmod-ed it a+rx, and send myself an external email. The email either was bounced or was eaten up. I was hoping that a "hello" would appear to indicate the program was triggered when an email arrived.

My main question is: how do I go about writing the remailer program? Can I write it in Perl, or must I do it in a different language? Do I have to change config files somewhere to make this work? What is a good resource, either online or a book, that would help me become an expert in writing the kind of program that I want, particularly a program that can processed piped in information?

I think if I could get the email to trigger, say, a Perl program, I could probably figure out much of the rest. Its the initial triggering in and of itself that causing me a lot of problems.

Lastly, I do wonder: is this system robust enough to handle, for example, 100-500 forwarded emails a day?

Thanks so much for your help!

Yes, you can do it with perl. Go to CPAN and look at Mail::Mailer or Mail::Header to manipulate headers. You can either then pipe the message directly to sendmail or send it using Mail::Mailer. Perl also allows you to easily access the mySQL db using the DBI module.

Alternatively, you could do it with bash. Pass the message to formail which allows you to manipulate headers and then pipe them back to sendmail.

I appreciate your comments. I went to cpan.org and downloaded MailTools -- it seems to be the most reliable and popular Perl module for processing email. And I followed the instructions and installed it successfully.

I think I'm still having a problem, though, in getting an incoming email to pipe directly it any program that I am writing. Currently here is my setup:

/etc/mail/virtusertable:

[added] @mydomain.com remailer-alias

/etc/aliases:

[added] remailer-alias: |/etc/mail/remailer.pl

/etc/mail/remailer.pl

[contents of file]
#! /usr/bin/perl -w
use Mail::Internet
$message=new Mail::\*STDIN;
print $message;

The file runs fine, say, with a command line ls | remailer.pl.

However, when I send an email to my site, it just gets bounced. I was hoping that it would print the message. Somehow, the piping isn't working, but I can't figure out why. Do you have an idea?

When I alter the /etc/aliases file to reroute remailer-alias mail into root, the root account safely receives the mail. So it's not a problem with the mail getting through the firewall or something.

I really appreciate your help. Thanks!

What's the bounce/error message that sendmail is giving you?

Here's the text I get in the bounced email. The email was sent from Yahoo Mail to my home server. The virtusertable directs all domain mail to the remailer-alias, and the remailer-alias mail is directed using the aliases file line:

remailer-alias: |/usr/local/bin/mailstore.sh

/usr/local/bin/mailstore.sh is composed of the following:

#! /bin/bash
while read var_str;
do echo $var_str >> /tmp/test_out.txt
done;

I would expect, when outside mail comes in, that I would get a growing test_out.txt file in /tmp. Instead, the email is bounced, with the following error message embedded:

The original message was received at Wed, 23 Jun 2004 19:54:27 -0700
from web41704.mail.yahoo.com [66.218.93.121]

----- The following addresses had permanent fatal errors -----
|/usr/local/bin/mailstore.sh
(reason: Service unavailable)
(expanded from: <blabla@64.36.78.186>)

----- Transcript of session follows -----
smrsh: "mailstore.sh" not available for sendmail programs (stat failed)
554 5.0.0 Service unavailable

Thanks so much for inquiring, I'd appreciate any help you can give.

I figured it out, thank you for inquiring! I had to put a link file in /etc/smrsh or the file wouldn't run. I hope I can now run a Perl file, as that is my main goal, rather than just a bash script .... thanks, on to further adventures in Linux...