E-Mail, Advanced Topics

This tutorial discusses advanced topics concerning e-mail on SDF: spam filtering, automatic processing, forwarding. It is probably only useful for ARPA members.

Contents

Introduction, and what you will not find here

Here we discuss advanced topics for e-mail processing on SDF, mainly automatic processing of incoming messages with “procmail”, and forwarding issues. As “procmail” is only available to ARPA members, the contents of this tutorial will be mostly useless to user members.
For basic info about e-mail (addresses, reader programs, file size, etc), please read the corresponding FAQ entry - and if you still have problems with basic sending and receiving of messages via SDF, this tutorial is most probably not for you!

Forwarding E-Mail

The “sendmail” system allows for automatic forwarding of incoming mail to other addresses or even programs (filters).
The file “$HOME/.forward” may contain the following types of lines:

somebody@somewhere.com: external e-mail address, where a copy of each mail will be sent |filterprogram: will pipe the incoming messages through ”/bin/sh” with the argument “filterprogram”

However, if you want to do more than forwarding to one address, use of “procmail” is highly recommended! In fact, it is best to set up nospam -e, which will generate a .forward file with a line calling procmail, and then to modify the resulting ”.procmailrc”.

Fetching E-Mail

Use “fetchmail” to collect messages from accounts you might have elsewhere. The messages are given to “procmail” (see Forwarding E-Mail above) for further processing.

Sample ".fetchmailrc"

  poll pop.provider.net
       proto pop3
       user "john.doe"
       pass "secret"
       is "jdoe" here
       mda "/path/to/procmail -f- ~/.procmailrc"
       ssl;

Fighting Spam

To fight unwanted bulk e-mail (spam), there are several possibilities available on SDF:

  1. As a general remark, think before you publish or tell somebody your e-mail address! And try to make clear to people that you don't want your address on a visible list of 500 recipients from somebody's address book; one should use Bcc: for sending to a lot of people in parallel, not To:!
  2. SDF also supports plus-addressing; you can use this to join mailing lists and do filters. This can be used by adding +something before the @ in your email address.
  3. The FAQ has more information about nospam, available on SDF.
  4. The automatic mail processor “procmail” (see “man procmail” for more information) allows to filter incoming mail following user defined rules; in particular, you can generate a white list of sender addresses you know as good, and put messages from other addresses into a special folder which you browse through now and then, to check for good messages and adding the corresponding addresses to your white list.

nospam

“nospam” is a script which can set up .forward and .procmailrc files so that certain e-mails will be stored in “bulk folders”.
“nospam -i” gives some additional information, and “nospam -e” will enable this filtering. We will suppose that this has been done also for the following discussion of white lists.

Personal White List Against Spam

White lists contain e-mail addresses which are always to be accepted as good. It is easy to implement this with “procmail”: Supposing that “nospam -e” has been executed, so that basic procmail processing is set up, one can add the following fragment to ”.procmailrc”.

Note: For this to be working, the following must have been set up:

  • a ”.forward” file which pipes incoming e-mail through “procmail”
  • a standard ”.procmailrc” file, where this fragment is attached or inserted

The easiest method to install these files is by launching “nospam -e” as mentioned above, then adding the fragment to the generated ”.procmailrc”.
(Of course, it is possible to deactivate or delete all the initial procmailrc lines generated by nospam, which move messages with attachments to the bulk folder, if such messages should pass through the whitelist filter.)

  # procmail spamfilter using whitelist (2006 Yargo Bonetti)
  # :: use at your own risk and any way you want! ::
  # whitelist: file containing one valid e-mail address per line
  # (only generic xxx@yyy.zz form, without "Name.." <*> parts)
  WHITELIST=$MAILDIR/.whitelist
  # spamfilter
  FROM=`formail -c -x 'From:'|sed -e 's/.*<\(.*@[^>]*\)>.*/\1/'`
  :0 hb:
  * ! ? grep -i -F -e "$FROM" $WHITELIST >/dev/null
  $MAILDIR/quarantine

This will compare the address in the From: field of incoming messages to the ones saved in the file $MAILDIR/.whitelist (of course, name and location can be changed), and when an address is not found in this file, the message will be saved in the folder $MAILDIR/quarantine (which can be modified as well) and not show up in the normal Inbox (and not be processed further). Now and then, one can look through the quarantine folder for “good messages”, and add the corresponding addresses to the .whitelist file.

If you want to use the mail directories proposed by nospam, you may prepend the above noted fragment by

MAILDIR=$HOME/mail

which will result in storing the “possibly bad” messages in ”~/mail/quarantine” and expect the whitelist in ”~/mail/.whitelist”.

A sample .whitelist may look like this:

  my.friend@his.domain.com
  president@whitehouse.gov
  myself@gmail.com

All messages not coming from these three addresses would end up in the “quarantine” folder defined in the .procmailrc fragment.

Using Procmail With a DNSBL

DNSBL stands for “DNS blacklist”, and it is essentially a collection of IP addresses known to send out spam and other bad stuff. Two popular DNSBLs are SpamCop and Spamhaus. Typically the mail server will check the source of incoming mail with the DNSBL before even accepting the message. SDF does not do this, but using Procmail to check a DNSBL at the user level is easy. In this tutorial we will use Spamhaus.

(Thank you Benya for the original instructions.)

First make sure that ”~/.forward” is set-up to forward incoming mail to Procmail. Then we'll edit ”~/.procmailrc”. The beginning of this file should declare basic variables such as “SHELL=/bin/sh”, “LOGFILE=$HOME/.procmaillog”, “ORGMAIL”, “DEFAULT”, and “MAILDIR”. “ORGMAIL” and “DEFAULT” should point to your inbox, and “MAILDIR” should point to the directory that contains your saved mail.

Next Procmail should extract the IP address from which the message was sent. This is done using “formail” to get the headers, “grep” to find the correct line, and “sed” to find the actual IP address, which is then saved as “SENDERIP”.

  SENDERIP = `formail -c -XReceived | grep "by sdf.lonestar.org" | \
  grep -v "from sdf.lonestar.org" | \
  sed "s/^Received: from .*\[\([0-9]*\.[0-9]*\.[0-9]*\.[0-9]*\)\].*by sdf.lonestar.org.*$/\1/"`

Then we'll begin the actual Procmail recipe, which will only be executed after checking to make sure that the “SENDERIP” variable exists and is in the correct format. Anything written to the “LOG” variable will be inserted into the log file. (New lines must be explicitly stated.)

  :0
  * SENDERIP ?? ^^[0-9]*\.[0-9]*\.[0-9]*\.[0-9]*^^
  {
  	LOG = "The sender's IP address is "
  	LOG = $SENDERIP
  	LOG = "
  "

The next step is to reverse the IP address because Spamhaus wants it that way. Procmail will then use “host” to determine the IP address that [reversed IP address].zen.spamhaus.org resolves to.

  	SENDER_REVERSED = `expr "$SENDERIP" | \
  	sed "s/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)/\4.\3.\2.\1/"`

  	KNOWNOFFENDER = `host "$SENDER_REVERSED".zen.spamhaus.org | \
  	sed "s/^.*\(127\.0\.0\.[0-9]*\)$/\1/"`

If Spamhaus returns an IP address between 127.0.0.0 and 127.0.0.9, then we know that this particular email is of dubious origin. In that case we can put it in the folder “$MAILDIR/Spam”. Otherwise, the message will get written to your inbox by default.

  	:0
  	* KNOWNOFFENDER ?? ^^127.0.0.[0-9]*^^
  	{
  		LOG = "This sender is a known source of spam.
  "

  		:0:
  		Spam
  	}

  	LOG = "This sender is not a known source of spam.
  "
  }

This completes the Procmail recipe.

Use SDF SMTP Server Remotely via SSH

Introduction

Using SSH's port-forwarding capabilities, MetaARPA members can create an encrypted tunnel which makes the SDF[-EU] SMTP mail server appear local, avoiding need for authentication or additional encryption.

Prerequisites:

  • SDF[-EU] MetaARPA membership
  • SSH plus suitable MUA (email program)
  • public key authentication (optional)

Tunnel Construction

Port-forwarding can occur on ports 53, 110, 143, 443, and 993 on most SDF hosts. On SDF-EU, use these ports: 465, 587, 995, 996 1025, 8080 and 7070.

ex: Use port 465 on SDF-EU host “odin”, tunnel local port 2525 to port 25 on SDF-EU host “mail” (SMTP server); restrict ssh to IPv4, no remote commands, run in background after connecting:

  %  ssh -4 -fN -p 465 -L 2525:mail.sdf-eu.org:25 sdf_user@odin.sdf-eu.org

Note if public key authentication isn't setup a password prompt occurs.

Tunnel Test

telnet(1) can be used; type “quit” to exit session:

  %  telnet localhost 2525
  Trying 127.0.0.1...
  Connected to localhost.
  Escape character is '^]'.
  220 sdf-eu.org ESMTP Sendmail 8.14.4/8.14.3; Sun, 8 Jan 2012 20:19:59 GMT
  ^]
  telnet> quit
  Connection closed

Tunnel Usage

Configure MUA to use “127.0.0.1” (localhost), port 2525 for out-going messages. Also ensure the From: and ReplyTo: addresses are routeable. Handling of rejected email is MUA-dependent - some do queuing, others write to $HOME/dead.letter or /dev/null:

ex: Configure and test the Heirloom Mailx MUA for user “frog” on localhost “mud.bog” to use “localhost:2525” for SMTP (off-site email):

  # $HOME/.nailrc
  set smtp="localhost:2525"
  set from="You <someone@sdf-eu.org>"
  set replyto="You <someone@sdf-eu.org>" 
  %  hmail -v someone@sdf.org
  Subject: tunneled SMTP test
  test 123
  .
  Resolving host localhost . . . done.
  Connecting to 127.0.0.1:2525 . . . connected.
  220 sdf-eu.org ESMTP Sendmail 8.14.4/8.14.3; Sun, 8 Jan 2012 20:29:22 GMT
  >>> HELO mud.bog
  250 sdf-eu.org Hello odin [178.63.35.194], pleased to meet you
  >>> MAIL FROM:<someone@sdf-eu.org>
  250 2.1.0 <someone@sdf-eu.org>... Sender ok
  >>> RCPT TO:<someone@sdf.org>
  250 2.1.5 <someone@sdf.org>... Recipient ok
  >>> DATA
  354 Enter mail, end with "." on a line by itself
  >>> .
  250 2.0.0 q08KTMoc023275 Message accepted for delivery
  >>> QUIT
  221 2.0.0 sdf-eu.org closing connection

Tunnel Teardown

The SSH tunnel can be torn down by identifying the appropriate PID (process ID) and sending it a kill signal, via the kill(1) command or similar tool. The following illustrates the teardown process using sockstat(1) to determine the PID on the NetBSD operating system:

  %  sockstat -np 2525
  USER     COMMAND    PID   FD PROTO  LOCAL ADDRESS         FOREIGN ADDRESS
  frog      ssh       24362  7 tcp    127.0.0.1.2525        *.*
  %  kill -HUP 24362
  Killed by signal 1.

Tunnel Maintenance

Network connections can fail for various reasons; a keep-alive script can help: (script contributed by SDF user nullogic - thanks!)

#!/bin/bash
# sdftun.sh: Keep alive script for local ssh tunnel from home to SDF SMTP
 
SSH_USER=
SSH_HOST='faeroes.freeshell.org'
SSH_PORT=443
SSH_ARGS='-N -f -L'
TAG_HOST=mx
TAG_PORT=25
LIS_PORT=8080
LOG_FILE=~/l0g/sdftun.log
SLEEP="30m"
 
while [ 1 ]; do
SSH_PID=`ps aux | sed -n -e /sed/d -e /ssh\ \-N/p | wc -m`
        if [ $SSH_PID -eq 0 ]; then
                ssh $SSH_ARGS $LIS_PORT:$TAG_HOST:$TAG_PORT $SSH_USER@$SSH_HOST \
                        -p$SSH_PORT &>/dev/null
                DATE_TIME=`date '+%y.%m.%d %T'`
                echo "$DATE_TIME SSH Tunnel restarted.">>$LOG_FILE
        fi
        sleep $SLEEP
done

Writing the PID to a file at the time of tunnel creation can simplify the ID process.

References

$Id: e-mail-advanced.html,v 1.17 2012/01/06 21:53:01 jgw Exp $