README_SPAMCONTROL (2.5)

Objective

SPAMCONTROL is an extension for qmail. It provides the following features:

Enhancements for qmail-smtpd:

Enhancements for qmail-remote:

Enhancements for qmail-pop3d:

Enhancements for qmail-queue:

Enhancements for qmail-send:

Qmail Multiple Queue

This version of SPAMCONTROL provides support for the Qmail Multiple Queue (QMQ):

With SPAMCONTROL, qmail-smtpd can stand the two most common threats:

Additionally, qmail-smtpd allows

0. Conventions

0.1 Definitions

Sender = SMTP envelope sender (Mail From: <Return-Path>)
Recipient = SMTP envelope recipient (Rcpt To: <Forwarding-Path>)
Nullsender Mail = E-Mail with empty Sender (Mail From: <>)

Full qualified SMTP E-Mail address: "user@domain.com" (for Sender/Recipient)

0.2 qmail-smtpd run Script

Throughout this document, I assume that qmail-smtpd is under control of supervise (out of the Daemontools package) and served by tcpserver (part of the UCSPI package) or a patched version of sslserver (part of the UCSPI-SSL package).

A typical - minimal - so called run script looks like follows:

#!/bin/sh
# qmail-smtpd startup
QMAILDUID=`id -u qmaild`
QMAILDGID=`id -g qmaild`
HOSTNAME=`hostname`
exec tcpserver -vR \
      -u $QMAILDUID -g $QMAILDGID \
      -l $HOSTNAME 0 smtp \
      /var/qmail/bin/qmail-smtpd 2>&1

0.3 Environment Variables

Qmail - and SPAMCONTROL - relies on the concept of environment variables which are available for a task (sharing the same environment). qmail-smtpd may be fed by environment variables in three different fashions:

1. (Gobal) Exported variables in the run script; eg. export RELAYCLIENT="".
2. (Gobal) By means of the envdir facility as part of the Daemontools package.
3. (Gobal) Using a "profile" configuration file called in the run script by means of the "dot" syntax (. pofile).
4. (Individual) Setting the appropriate variable in the tcpserver cdb database.

While the first three cases define static and "global" environments variables, the last case makes the environment variables client-dependent and - by means of tcprules - dynamically changeable. Any mixture is possible, though only the "last" setting of an environment variable is effective!

0.4 tcpservers' cdb

As a convention, I will call the tcperver's cdb, which rules the behavior of qmail-smtpd, tcp.smtp. A typical tcp.smtp would look like

=mydomain.com:allow,RELAYCLIENT="",LOCALMFCHECK=""
1.2.3.4:allow,RELAYCLIENT="",LOCALMFCHECK="otherdomain.com"
5.6.7.8:allow,REQUIREAUTH=""
:allow,MFDNSCHECK="",BADMIMETYPE="",BADLOADERTYPE="M"

The cdb is constructed on the fly:

tcprules tcp.smtp.cdb tcp.smtp.tmp < tcp.smtp

Caution: For use with tcpserver, the value of the environment variable has to be included in quotes.

0.5 rcpthosts/morercpthosts.cdb

Though qmail can live happily without the knowledge of domains to be responsible for as provided by rcpthosts/morercpthosts.cdb, it is highly adviceable to include all domains to receive emails for (as per DNS MX Records) into those control files. Otherwise, qmail-smtpd may act as an Open Relay. Further, some LOCALMFCHECKs will fail, as discussed below.

1. qmail-smtpd Protocol Extensions

1.1 Size Extension

qmail-smtpd's "Mail From:" parameter parser is used to detect and evaluate the SIZE parameter and to eventually reject messages which initially exceed the databytes limit.

Nevertheless, qmail-smtpd checks the size of the incoming message anyway.

For incoming E-Mails which exceed the message size values (in Bytes) defined in

or via the $DATABYTES environment variable.

1.2 SMTP Authentication

SMTP Authentication requires a Client to authenticate and a Server to honor the authentication procedure. In this version of SPAMCONTROL, Qmail acts as an Authentication Server for qmail-smtpd and as an Authentication Client for qmail-remote.

Usually, a MTA (such as Qmail) will accept transmissions of E-Mails anyway as long as the "Rcpt To: <forwarding-path>" is targeted to a local Recipient (according to control/rcpthosts). However, with SMTP Authentication you may allow an authenticated User to relay E-Mails. In this respect, SMTP Authentication copes with the deficiencies of the POP3/IMAP4 protocol and is applied as an alternative to SMTP-after-POP, which is ugly as well.

I have taken the SMTP-Auth Patch from Krzysztof Dabrowski and included this into SPAMCONTROL. However, SPAMCONTROL's implementation is compliant with the checkpassword API designed by Dan Bernstein (the Plugable Authentication Module PAM).

SPAMCONTROL provides the following features:

1.2.1 Pluggable Authentication Module PAM

While SASL is a generic concept, the information flow for authentication between e.g. qmail-smtpd and the PAM is defined by Dan Bernstein's checkpassword API. SPAMCONTROL provides the PAM on file descriptor 3 as an informational string composed of:

You are free to choose or even write your own PAM program, but in any case, the SASL Procedure of the client and the server has to match and the procedure has to be advertised. Compliant PAMs:

1.2.3 qmail-smtpd Setup for SMTP Authentication

qmail-smtpd including SMTP Authentication may be called by tcpserver/sslserver in a supervise run script. Here is an example (with some more features):

#!/bin/sh
# qmail-smtpd startup with SMTP Authentication
QMAILDUID=`id -u qmaild`
QMAILDGID=`id -g qmaild`
HOSTNAME=`hostname`

export SMTPAUTH=""
exec softlimit -m 2000000 \
tcpserver -vR -l $HOSTNAME \
 -u $QMAILDUID -g $QMAILDGID 0 smtp \
   /var/qmail/bin/qmail-smtpd /bin/cmd5checkpw true 2>&1

Beware! Unlike the original implementation, I omitted the inclusion of the Hostname as argument for qmail-smtpd.

Unlike the standard qmail-smtpd, now you have

1. define the environment variable SMTPAUTH to allow SMTP authentication and
2. to provide in addition a PAM program, here cmd5checkpw, which itself calls a shell named 'true' (/bin/true or /usr/bin/true) exiting simply with "0".

The environment variable SMTPAUTH may be left blank to allow Authentication types "PLAIN" and "LOGIN" or may be currently set to "cram..." (lower or upper case) to enable CRAM-MD5 authentication in addition.

1.2.4 Submission Port

SMTP clients requiering SMTP authentication expect the SMTP server to listen to the Submission port 587 instead of the standard SMTP port 25. To use this feature, you need to set up a second qmail-smtpd instance bound to port 587:

#!/bin/sh
# qmail-smtpd startup with SMTP Authentication
QMAILDUID=`id -u qmaild`
QMAILDGID=`id -g qmaild`
HOSTNAME=`hostname`

export SMTPAUTH="CRAM"
exec softlimit -m 2000000 \
tcpserver -vR -l $HOSTNAME \
 -u $QMAILDUID -g $QMAILDGID 0 submission \
   /var/qmail/bin/qmail-smtpd /bin/cmd5checkpw true 2>&1

If used in conjuntion with the previous run-script [1.2.3], qmail-smtpd will accept SMTP authentication sessions on port 25 and 587; however demanding a successful authentication on the submission port, while providing a fall-back to none-authentication on the standard SMTP port.

Populating the environment variable SUBMISSIONPORT allows to set-up an submission enabled qmail-smtpd instance on any port defined here.

1.2.5 User Database for cmd5checkpw

For SMTP Authentication, a User Database has to be generated and maintained. The SMTP Authentication User may exist independently of any System Users, Qmail Users, or E-Mail Accounts. In case of the modified cmd5checkpw I decided to keep the User in the Qmail directory as

There exist other flavors, in particular the saslpasswd scheme or the Cyrus SASL Library you may want to use. Further, for users with POP3/IMAP4 Accounts on the system it is advisable to use a common User Database. For Vpopmail you may use vchkpw.

However, since you are free to use any other checkpassword compliant PAM, it's up to you whatever you apply. Please remember: In order to access the Unix /etc/passwd the respective program has to run as root.

1.2.6 Requiring SMTP Authentication

You can use the environment variable REQUIREAUTH to enforce authentication for particular clients. A typical run script to require SMTP Authentication for particular SMTP clients looks like:

12.34.56:allow,RELAYCLIENT="",SMTPAUTH="CRAM-MD5",REQUIREAUTH=""
:allow

1.2.7 SMTP Authentication and Vpopmail

SMTP Authentication works well with vpopmail, however, you have to use a checkpassword compatible PAM. Older versions of vchkpw have to be patched accordingly (see http://www.fehcom.de/qmail/smtpauth.html).

vchpkw offers a lot of authentication capabilities; it supports login, plain, and CRAM-MD5 and may authenticate the user against a mysql database and others. In start-up script for qmail-smtpd you have to make sure to access the user database with the correct user access rights:

#!/bin/sh
# qmail-smtpd startup with SMTP Authentication + vpopmail
QMAILDUID=`id -u vpopmail`
QMAILDGID=`id -g vpopmail`
HOSTNAME=`hostname`

export SMTPAUTH="crammd5"
exec softlimit -m 2000000 \
tcpserver -vR -l $HOSTNAME \
 -u $QMAILDUID -g $QMAILDGID 0 smtp \
   /var/qmail/bin/qmail-smtpd /home/vpopmail/bin/vchkpw true 2>&1

If you use Sqwebmail in addition, the user is free to set his/her own password.

1.3 STARTTLS support

SPAMCONROL's STARTTLS support for qmail-smtpd is aligned with Scott Gifford's approach and depends on the following:

1. Superscript's sslserver (0.70) instead of tcpserver
2. the enclosed patch against sslserver to allow STARTTLS in addition with standard TLS,
3. the availability of a valid X.509 cerificate, an appropiate key, and additionally a Diffi-Hellman parameter file,
4. the correct feeding (via environment variables) of those settings to sslserver to allow encryption.

My STARTTLS implemention conforms with RFC 3207.

1.3.1 STARTTLS implementation

Most current STARTTLS/TLS solutions depend on the existance and availability of the OpenSSL libraries -- so does SPAMCONROL. However, unlike other implementations, qmail-smtpd is insulated against OpenSSL by means of sslserver. In fact, all encryption and certificate verification is facilitated by sslserver. In this respect, Scott's and my STARTTLS implementation is very much OSI-like. The communication and presentation happens at a well defined environment, typically assigned to the user and group ssl. Any potential attacks or bugs are kept away from the application and don't harm.

The reading and response to client cerificates and the actual encryption happens in the assigned user spaces; which should never be root.

1.3.2 Prereqs

Install Superscript's ucspi-ssl (version 0.70). Apply the attached patch "ucspi-ssl-0.70-ucspitls-0.4.patch_" against the source, typically found at /package/host/superscript.com/net/ucspi-ssl-0.70/src and execute package/install base. This patch includes (delayed) STARTTLS support to sslserver and will allow to substitude tcpserver completely, even if no SSL/TLS communication is required.

Further, it is helpful to create a low privileged user and group ssl, which will be used by sslserver for SSL/TLS communication purposes. Please follow Scott Giffords' advices.

1.3.3 STARTTLS Settings

Substitute tcpserver with sslserver in the run script for qmail-smtpd. If you use softlimits, it might be necessary to raise those settings significantly due to the increased memory requirements. Here is my run script:

#!/bin/sh
QMAILDUID=`id -u qmaild`
QMAILDGID=`id -g qmaild`
HOSTNAME=`hostname`
export SMTPAUTH="crammd5"
export UCSPITLS=""
MAXCONCURRENCY=`cat /var/qmail/control/concurrencyincoming`
. /var/qmail/ssl/env
exec softlimit -m 180000000 \
sslserver -sevn -w 5 -R -l $HOSTNAME -c $MAXCONCURRENCY \
-x /var/qmail/etc/tcp.smtpd.cdb \
-u $QMAILDUID -g $QMAILDGID 0 smtp \
/var/qmail/bin/qmail-smtpd cmd5checkpw true 2>&1

It is absolutely necessary to use the "-n" flag for sslserver, since this will trigger the availability of encrypted communications channels between sslserver and qmail-smtpd.

The environment variables needed to feed sslserver are included in the "profile" /var/qmail/ssl/env. In my system, they have the following settings:

# Set
SSL_USER=ssl
SSL_GROUP=ssl
SSL_DIR="/var/qmail/ssl"
# Rest
SSL_CHROOT="$SSL_DIR"
CERTFILE="/var/qmail/ssl/cert"
KEYFILE="/var/qmail/ssl/key"
DHFILE="/var/qmail/ssl/dhparam"
SSL_UID=`id -u $SSL_USER`
if [ $? -ne 0 ] ; then echo "No such user '$SSL_USER'" >&2 ; exit; fi
SSL_GID=`id -g $SSL_USER`
if [ $? -ne 0 ] ; then echo "No such group '$SSL_GROUP'" >&2 ; exit; fi
export SSL_UID SSL_GID SSL_CHROOT
export CERTFILE KEYFILE DHFILE

Of course it is required, to have raised the directory /var/qmail/ssl before and generate via openssl the appropriate keys before.

Note: These settings are "global"; however, by means of sslserver and the settings in your tcp.smtpd file it is possible to use different certificates per connection.

Comment: Please read the documentation of UCSPI-SSL carefully w.r.t. the "mod-ssl" variables. It might in addition be necessary to define CAFILE, CADIR and other SSL options to your needs.

After you verified your stettings, restart qmail-smtpd. Whether qmail-smtpd will present "STARTTLS" in the EHLO dialogue, depends on the presence of the UCSPITLS environment variable. These can be set i.e. per IP in the tcp.smtpd control file.

In addition, you can enforce a TLS encrypted sessions defining REQUIRETLS="" for particular connections.

1.3.4 STARTTLS and SMTPAUTH

In case qmail-smtpd is instructed to use STARTTLS and SMTPAUTH, SMTP Authentication always takes place after the TLS session is active, but never reverse. Thus, all SMTP parameters like username and password are already encrypted. Of course, SMTP Authentication is still available for unencrypted SMTP connections, and STARTTLS does not require per se SMTP Authentication. However, STARTTLS and SMTP Authentication is a strong and powerful couple to secure the SMTP communication

1.3.5 STARTTLS Received Header Extensions

SPAMCONTROL displays the use of STARTTLS in the Received header (according to RFC 3207). The following information is added:

• The TLS protocol in use.
• The Cipher in use.
• The presented encryption key length the and key length in place.
• The client's "Subject" Destinguished Name (DN).

Here's a sample using Thunderbird als email client:

Received: (qmail 35450 invoked from network); 15 Mar 2006 20:22:09 -0000
Received: from dornroeschen.fehnet.de (HELO ?192.168.192.19?) (erwin@192.168.192.19)
encrypted via TLSv1: RC4-MD5 [128/128] DN=unknown
by orion.fehnet.de with ESMTPSA; 15 Mar 2006 20:22:09 -0000

1.4 MAV: "Mail From: Address Verification"

Mail From: Adress Verification (MAV) is a mean to enforce the use of the SMTP "Mail From:" address for particular Relayclients. Former versions of SPAMCONTROL used a "LOCALMF" check which allowed only a very limited granularity. However, with MAV you can control/enforce

MAV is in particluar very useful, if emails from your domains have to be undoubtly "officially" send.

1.4.1 Requiring MAV

Mail From: Address Verification is only be done if the flag 'relayclient' is set. This flag is set if

For these circumstances MAV can be enforced by means of the environment variable LOCALMFCHECK:

1.4.2 Control files mfrules and mfrules.cdb

The file control/mfrules follows roughly the same syntax as the common file tcp.smtpd used for tcpserver/sslserver. It assigns either a complete SMTP address, a FQDN, an IP adress or a domain to a set of allowed Originator addresses. In practice control/mfrules allows

Once you have populated control/mfrules run qmail-mfrules to derive control/mfrules.cdb from the input file. Additionally, define LOCALMFCHECK="!" either gobally or in the tcp.smtpd file.

1.4.2. Reply to the relayclients

Mail Clients may be setup wrongly or a user may want to use the relaying MTA to send emails for a different name. In case, MAV is in place and well configured, the particular user will not be allowed to send the mail over the gateway receiving the following SMTP reply:

553 sorry, invalid address specified (#5.7.1)

Since this might not be helpful for the (innocent) sender, you might use the environment variable REPLYMAV to add a qualification to that message.

1.4.3 MAV SMTP protocol extension

MAV puts the burden of SMTP Originator address verification ot the relaying MTA; that is the reverse scheme compared to SPF and others. Emails qualified through MAV are labeled with "ESMTPM" in the Received header, which is generated by qmail-smtpd.

2. qmail-smtpd SMTP Envelope Filters

The SMTP Envelope consists of three parts:

• The HELO/EHLO greeting from the SMTP Client.
• The Originator: Mail From: <Return-Path>.
• The Recipient: Rcpt To: <Forwarding-Path>.

SPAMCONTROL allows to filter E-Mails according to the bad* criteria with a so-called wildmat search, which is a subset of the known Regualar Expressions (RegEx). The wildmat search works in order least significant to most significant and includes

1. a case-insensitive straight search - and -
2. a case-sensitive wildmat search

The following sets of wildmat control characters can be used:

2.1 SMTP Envelope Addresses

Any E-Mail address, lets say <user@host.domain.com> consists of a

local part: user (left from the "@") and a

host part: host.domain.com (right hand side from the "@").

 

E-Mail addresses for local accounts are considered case-insensitive and delivered irrespective of their case.

Lets say - if the local account is "admin" and the RCPT to: tells <AdMin> or <adMIN> the delivery will be successful.

2.2 Filtering HELO/EHLO Greetings (badhelo)

RFC 2821 says: "These commands (HELO/EHLO) are used to identify the SMTP client to the SMTP server. The argument field contains the fully-qualified domain name of the SMTP client if one is available. In situations in which the SMTP client system does not have a meaningful domain name (e.g., when its address is dynamically allocated and no reverse mapping record is available), the client SHOULD send an address literal (see section 4.1.3), optionally followed by information that will help to identify the client system. y The SMTP server identifies itself to the SMTP client in the connection greeting reply and in the response to this command."

Qmail records the HELO/EHLO greeting string for every received message in the E-Mail "Received:" header in case the provided HELO/EHLO string is different from the connecting hosts FQDN:

Received: from foo.bar.de (HELO foo) (192.168.192.11)
by heaven.bar.de with SMTP; 25 Apr 2003 15:01:42 -0000

The HELO/EHLO string is included as "(HELO foo)". The HELO/EHLO string is usually generated by the sending MTA without much control (MUAs often use their generic hostname).

SPAMCONTROL allows a flexible filtering of the clients HELO/EHLO greeting string, which depends on the setting of the environment variable HELOCHECK:

The HELOCHECKs are only done, in case RELAYCLIENT is not set (split-horizon fashion). In my current setup, a useful setting is HELOCHECK="." and with the following input in control/badhelo

myIPAddress

myFQDN

localhost

localhost.localdomain

These settings exclude the spoofing of the MTA's own address, which is typical for spam senders, since they determine the EHLO/EHLO greeting from the initial IP/SMTP session parameter.

2.3 Filtering Mail From: SMTP Addresses (badmailfrom)

SPAMCONTROL allows four types of checks against the provided "Mail From:" SMTP envelope address (which I often call the "Originator"):

2.3.1 DNS MX Checks of the Envelope Sender

Invoking the environment variable MFDNSCHECK in the qmail-smtpd startup script enables globally the DNS check for the envelope's Sender.

Example:

#!/bin/sh
export MFDNSCHECK=""
.....

Additionally, the environment variable may be defined individually within a cdb of tcpserver/sslserver. Typically, this is done for "non-trusted" hosts within a tcpservers cdb:

127.0.01:allow,RELAYCLIENT=""

:allow,MFDNSCHECK=""

If environment variable MFDNSCHECK is not set, qmail-smtpd does not perform this DNS MX check.

Note: All DNS checks are either done by means of the libresolv library which comes with BIND, or my means of DJBDNS's routines, which can be included installing DJBDNS and using Nikola Vladov's enhancements for DJBDNS in addition with the modified Makefile.djbdns.

2.3.2 Standard badmailfrom Checks

control/badmailfrom was the only SMTP envelope filter Dan Bernstein originally implemented for qmail-smtpd. Here, only particular names or perhaps domains were listeted to be rejected in the SMTP dialogue. Since then, various flavours of badmailfrom have been brought out. However, the approach to reduce spam emails feeding control/badmailfrom with known spammer addresses is comparable trying to hit a moving target. Almost all Originator addresses spammer use today are fake and in this sense are meaningless.

2.3.3 badmailfromunknown Checks

There exist a special case, where you expect an email with a specific Originator address to be send via particular MTAs. For instance, if you see an email with Originator address "support@microsoft.com", it has to be send from a Microsoft MTA. qmail-smtpd has the knowledge of the sender's IP and FQDN (by means of the environment variables TCPREMOTEIP and TCPREMOTEHOST) in case you use tcp-env, tcpserver, or sslserver with the appropriate argument, i.e. tcpserver -h.

MTAs for which the FQDN can't be resolved are unqualified. In particular, emails from the large webmail providers (aol, hotmail, yahoo, gmx, t-online ...) have always to be send from qualified MTAs. Reversely, you can safely reject emails with those Originator hostparts, which can not be resolved tcpserver/sslserver records them as "unkonwn".

With SPAMCONTROL's badmailfrom implementation, you simply include the Originator addresses for which you enforce a qualified TCPREMOTEINFO into control/badmailfrom in the following way appending a dash ("-"):

@aol.com-

@hotmail.com-

@yahoo.com-

@gmx.de-

Note 1: Since tcp-env/tcpserver/sslserver relies on a qualified DNS lookup, it is certainly helpful to use DJBDNS' dnscache as frontend.
Note 2: Wildmat support is not provided; thus an entry "@*.yahoo.com-" won't work.

2.3.4 badmailfromwellknown Checks

In particular for webmailer (ie. hotmail.com, yahoo.com) the domain-part of the provide Mail From: address coincide with the provided domain name in TCPREMOTEHOST. Enforcing coincidence can be achieved for addresses appended with an equal-sign ("=") in control/badmailfrom:

@t-online.de=

Receiving a Mail From: address like "max.mustermann@t-online.de" will only be accepted if TCPREMOTEHOST name ends with "t-online.de", for instance "mailout03.t-online.de". Otherwise, the email will be rejected by badmailfrom.

2.3.6 badmailfrom Anti-Spoofing

Another special case is given, rejecting none-Relayclient emails with Originator addreses spoofing your domain name or email addresses. Email can be rejected if the "responsible domains" are included with a trailing plus ("+") in the following way into control/badmailfrom:

god@mydomain1.com+

@mydomain2.net+

@mydomain3.org+

Note: Wildcard support is not provided in this case.

2.4 Controlling the Rcpt To: SMTP dialogue and Filtering the Recipient Address

Apart for the RECIPIENTS mechanism, which is detailed later, you can reject SMTP Recipient addresses (Rcpt To: <Recipient>) by means of control/badrcptto. However, qmail-smtpd lets you effectively

Note: The provided Rcpt To: <Recipient> information by the SMTP client is (apart from it's IP/FQDN) the only information which can not be faked, though these addresses are today often randomly generated by means of lexical/dictionary attacks by spammers or gathered by address harvesting. Standard qmail will accept any addresses which matches an entry in control/rcpthosts or control/morercpthosts.cdb and in case the Recipient does not exist tires to bounce the email to the Originator after control/qeueulifetime has exceeded (default one week).

2.4.1 badrcptto

By populating control/badrcptto you reject emails to Recipients listed there in already in the SMTP session. Wildcards are allowed. If you don't wont to receive emails for root (from the Internet) include in control/badrcptto:

root@*

2.4.2 badrcptto Whitelisteing

Alternatively to the Recipients mechanism, as a side-effect of the wildmat filtering, you can use the control/badrcptto file as an effective whitelisting mechanism. The trick is, to initially reject everything while later to allow specific Recipients:

*
!*@otherdomain.com
!user1@mydomain.com
!user2@mydomain.com

Note: The evaluation of control/badrcptto is done independent from the setting of the RELAYCLIENT environment variable.

2.4.2 Restricting the number of Recipients

The environment variable

MAXRECIPIENTS

can be used to restrict the number of counted "Rcpt To: "s in the SMTP session. By default, no restriction is facilitated.

2.4.3 Tarpitting

I have included Chris Johnson's TARPITTING patch into SPAMCONTROL:

"What is tarpitting? It's the practice of inserting a small sleep in a SMTP session for each "Rcpt To:" after some set number of "Rcpt To:"s. The idea is to that spammers who would hand your SMTP server a single message with a long list of RCPT TOs. If a spammer were to attempt to use your server to relay a message, say, 10,000 Recipients, and you inserted a five-second delay for each Recipient after the fiftieth, the spammer would be 'tarpitted', and would most likely assume that the connection had stalled and give up."

Typically, the environment variables TARPITCOUNT and TARPITDELAY are set by menas of tcpserver's default-allow rules:

:allow,TARPITCOUNT=5,TARPITDELAY=20.

TARPITCOUNT denotes the number of sessions before starting the TARPITDELAY, which defaults 5 seconds.

Note: In combination with the Recipients extension, the TARPITCOUNT is used to terminate the SMTP session if the number of invalid Recipients ("Rcpt to:") exceeds the TARPITCOUNT. Unlike the typical tarpitting mechanism, this is a hard limit (Smart Rejection).

3. Recipients Extension

3.1 Scope

qmail-smtpd accepts messages if the SMTP domain part of Recipient address ("Rcpt to: <recip@domain>") matches an entry in control/rcpthosts or control/morercpthosts.cdb. The existence of a mailbox/maildir for the corresponding SMTP Recipient is checked later in the delivery chain. In case no Mailbox/Maildir exists, the message is bounced back to the SMTP Sender ("Mail From: <send@example.com>").

For normal SMTP mail traffic that's fine as long as the rate of undeliverable messages don't exceed 10% and the Sender is 'legitmate'; ie. exists. Today's situation is different: Spam and Virus attacks with forged/faked Sender addresses to a bunch of random Recipient addresses yield an undeliverable rate up to 90%. Worse, the generated bounces will never reach the Sender and a double-bounce is eventually send to the postmaster.

3.2 qmail-smtpd Recipients

The RECIPIENTS extension makes qmail-smtpd aware of acceptable recipients, which are fetched from an external source.
Which source to query depends on the domain-part of the recipient address.

The RECIPIENTS check is done only in a none-RELAYCLIENT case and after control/rcpthosts, control/morercpthosts.cdb has been successfully consulted.

3.2.1 VERP Support

The RECIPIENTS mechanism supports natively Qmail's address extensions (VERP). If a recipient address like 'foo@mydomain.com' defined, all VERP addresses like 'foo-bar@mydomain.com' are accepted for SMTP reception.

3.2.2 Domain recognition and Wilddomain Support

The RECIPIENTS lookup is triggered by the recipient domain, thus is domain-specific. You can specify which lookup is performed
per domain within control/recipients. Consider the following:

• An entry 'example.com' is used to match 'example.com' and in addition all subdomain addresses '*.example.com';
  depending or course on control/rcpthosts.
• An entry '@example.com' serves as exact match for the domain address.
• The entry '*' will match all domains for the respective lookup.
• Reversely, domains flagged as '!domain.com' are not queried and all recipients for this domain are accepted.

Compatibility Note: Due to this new syntax, the old RECIPIENTS (version 0.4x) wilddomain support (as part of the cdb) is not supported anymore.

3.2.3 Fail-Open Operation

The RECIPIENTS extension can be used in a 'fail-closed' or 'fail-open' mode for the domains included in control/recipients.
Typically the recipient check is done 'fail-closed', thus if all queries are negative, the incoming email with this recipient address will be rejected.

A 'fail-open' behaviour can be achieved adding '!*' as last statement in control/recipients.
Thus, emails for domains not listed in control/recipients will finally be accepted.

3.3. Setting up the recipients control file

Release 0.5 the RECIPIENTS extension provides a flexible new syntax to interprete control/recipients on a domain base, as part of the RCPT TO: envelope address.

Lines in control/recipients starting with a '#' are not evaluated, thus are treated as comment lines.

Read 'man qmail-smtpd' and 'man qmail-recipients.' Some additional scripts can be found in doc.

3.4. Generating a cdb with recipient addresses

Verify that list to be found under users/recipients.

If you have a different Qmail home directory, modify the above scripts.

You may need to change "localhost" in the above scripts to the real hostname.

3.5. Using a checkpassword compatible PAM

The checkpassword API is defined in:

http://cr.yp.to/checkpwd/interface.html

and typically consists of the string:

username\0password\0timestamp\0otherdata\0

written to file descriptor 3 (FD 3) to be read by the checkpassword compatible PAM.

For email address (recipient) verification, we replace

username\0

with

email-address\0

ie.

recipient@domain.tld\0

The attached PERL ldap_mail.pl serves as a sample.

Note: The PAM has to be either in the Unix $PATH or explicitely defined in control/recipients.

3.6 Customization

The Recipients extension needs no customization except for the following circumstances:

3.7 Results

With the Recipients extension qmail-smtpd will act for none-RELAYCLIENTs like follows

a) The domain part of the Recipient address is checked against rcpthosts/morercpthosts.cdb and accepted if either it matches or the domain is not provided.

b) the Recipient address is checked against all readable sources as listed in control/recipients and accepted if

• either the entire address matches,
• a VERP-part of the address matches,
• the domain part of address is wildcarded or,
• for the unqualified address an entry exist in the form 'user@localhost', or
• perhaps, the fall-through statement '!*' is finally provided.

In any other case, a SMTP temporary failure protocol error is issued to the client saying:

• "550 sorry, mailbox currently unavailable (#5.7.1)" unless
• the environment variable RECIPIENTS450 is specified and qmails-smtpd issues the rejection message "450, sorry no mailbox by that name (#4.2.1)"

4. Filtering E-Mails on behalf of the Message Content

Based on the "qmail-smtp-viruscan-1.1.patch" by Russell Nelson (and Charles Cazabon), SPAMCONTROL includes my WARLORD extension, which is a much robuster and efficient filter for BASE64 encoded MIME attachments and bundled with the Qmail High Performance Scanner Interface (QHPSI):

In case a badmimetype or badloadertype filter condition is met or a virus is detected, qmail-smtpd sends a SMTP 554 reply to the sender "554 sorry, invalid message content (#5.3.2)". Populating the REPLY554 environment variable, allows to include additional information (typically an URL), which can be used to deal with potential false-positives.

4.1 The BADMIMETYPE-Filter

The badmimetype filter becomes active if

4.2 Control file badmimetypes and badmimetypes.cdb

The control file control/badmimetypes.cdb is populated by the additional program qmail-badmimetypes which takes the input of control/badmimetypes. New MIME signatures can be added/removed on-the-fly. Bad MIME Type signatures have to have the length of at least 9 significant characters.

The currently included MIME signatures are:

TVqQAAMAA
TVpQAAIAA
TVpAALQAc
TVpyAXkAX
TVrmAU4AA
TVrhARwAk
TVoFAQUAA
TVoAAAQAA
TVoIARMAA
TVouARsAA
TVrQAT8AA

# *.zip
# UEsDBAkAA
# *.z (gnu-zip)
# H4sIADWWb
# double Base 64 Windows Executable
VFZxUUFBT
# triple Base 64 Windows Executable
VkZaeFVVR

# Pif File
TVoAAAEAA
# Bagle Virus
ZGltIGZpb

Adding new badmimetypes is simple:

1. Send an E-Mail to a Unix account with corresponding attachment (i.e. *.zip).
2. Use an editor to view the E-Mail and spot the corresponding BASE64 encoded content-type.
3. Take the first nine significant characters (for type "zip" its "UEsDBAkAA") and include them into control/badmimetypes.
4. Run qmail-badmimetypes.

Comments (starting with "#") are allowed in badmimetypes; the length of the signature will be truncated to nine characters.

4.3 The BADLOADERTYPE-Filter

The badloadertype filter becomes active if

The BADLOADERTYPE mechanism deals in particular with "transport stealth" worms, ie. UPX encoded Windows executables.

4.4 Control file badloadertypes and badloadertypes.cdb

badloadertypes.cdb is populated by the additional program qmail-badloadertypes which takes the input of control/badloadertypes The badloadertype mechanism looks for five significant strings in the BASE64 encoded data-stream which is matched against an entry in control/badloadertypes.cdb. badloadertype signatures can be added/removed on-the-fly.

The currently included Windows OS badloadertype signatures are:

Mi5kb
MzIuZ
MyLmR
MyLkR

Comments (starting with "#") are allowed in badloadertypes; the length of the signature will be truncated to five characters.

Caution: Unlike the badmimetype, the badloadertype signatures are placed anywhere in the BASE64 encoded datastream and are difficult to find out. In order to make the search efficient, a common character has to be providen in the environment variable BADLOADERTYPE. The provided pattern look basically for a string like "32.dll" as a subpart of "Kernel32.dll" which is an indication for an executable for the Windows OS. However, there is a small chance for false positives. Some - lets say - Word document attached as BASE64 MIME part in the message containing the buzz words "kernel32.dll" might become flagged and finally rejected as well.

4.5 Employing an AV Scanner with QHPSI

Unlike all other AV Scanners currently in use for Qmail, with Qmail High Performance Scanner Interface (QHPSI) there is no need for any other umbrella program, neither qmail-scanner, AMAViS, qscanq or whatsoever. Further, no additional MIME analyzing program like reformime, metamail, or ripmime is required. Even better, no "staging" area for temporary files are needed, except the one, the AV Scanners requires for itself.

Today's AV Scanners - and in particular Clam AV - are able to read the BASE64 encoded message and eventually dig out the files in archives, ie. in zip format. In order to use an AV Scanner with QHPSI, the AV Scanner has to have the following qualifications:

• Correct interpretation of the BASE64 and perhaps the uudecoded data in order to detect the virii/worms therein.
• Results have to be made available on STDERR/STDOUT.
• (And perhaps) Suppression of 'negative' scan results.

The QHPSI allows to use the following environment variables:

• QHPSI="av_scanner" - this is the path to the involved AV Scanner.
• QHPSIARG1, QHAPSIARG2, QHPSIARG3 - to call the AV Scanner with three arbitrary arguments.
• QHPSIRC="RC" - if the AV Scanner does not return with RC=1 in case of a detected infection, you can specify one here.
• QHPSIMINSIZE="size-in-bytes" - the minimum size of the message to scan.
• QHPSIMAXSIZE="size-in-bytes" - the maximum size of the message to scan.

The AV Scanner is directly called in the start scripts of Qmail (i.e. the run script for qmail-smtpd) or by means of tcpserver's capabilities. Here is a typical example, how to customize QHPSI together with Clam AV (clamd/clamdscan) for a tcpserver tcp.smtpd file:

:allow,QHPSI='clamdscan',QHPSIARG1='--disable-summary'

Comments:

Note:

• qmail-start does not pass environment variables to qmail-queue.
• If you need to scan in addition outgoing emails, you have to write a wrapper for e.g. qmail-inject:

#!/bin/sh
export QHPSI='clamdscan'
export QHPSIARG1='--disable-summary'
exec /var/qmail/bin/qmail-queue

Results:

• In case a client sends an infected email by means of SMTP, it receives the SMTP return code and error message: "554 mail server permanently rejected message (#5.3.0)"
• In case the virus scanner can not be called properly or has an internal problem, the following SMTP message is given: "454 mail server temporarily rejected message (#4.3.0)"
• Eventually, the scan results show up in the logs.

Here is a sample of Clam AV without and with the argument "--disable-summary":

@... tcpserver: pid 49943 from 192.168.192.11
@... tcpserver: ok 49943 qmailer.fehnet.de:192.168.192.2:25 arkon.fehnet.de:192.168.192.11::1074
@... /var/qmail/queue/mess/4/89439: Worm.Klez.H FOUND
@...
@... ----------- SCAN SUMMARY -----------
@... Infected files: 1
@... Time: 0.099 sec (0 m 0 s)

@ Reject::DATA::Virus_Infected: S:192.168.192.11:arkon.fehnet.de H:mail.fehnet.de F:me T:erwin 'clamdscan'

Note: Even in case no virus is detected, the "SCAN SUMMARY" is provided.

@... tcpserver: pid 49989 from 192.168.192.11
@... tcpserver: ok 49989 qmailer.fehnet.de:192.168.192.2:25 arkon.fehnet.de:192.168.192.11::1077
@... /var/qmail/queue/mess/4/89543: Worm.Klez.H FOUND

@ Reject::DATA::Virus_Infected: S:192.168.192.11:arkon.fehnet.de H:mail.fehnet.de F:me T:erwin 'clamdscan'
@... tcpserver: end 49989 status 256

Attention:

• The virus scanner has to be able to read the messages in the Qmail queue:
  # ls -ld /var/qmail/queue/mess/
  drwxr-x--- 25 qmailq qmail 4096 Jun 13 00:19 /var/qmail/queue/mess/
• If the virus scanner drops privileges during execution, it might be necessary to make the executable sticky (ie. clamscan).

Note: As with this writing, clamav 0.8x is broken, since it writes all logs to STDOUT instead of STDERR; thus no scanning messages will apear in the qmail-smtpd log.

4.6 QHPSI performance improvements for Virus Scanning

The badmimtypes and badloadertypes mechanism provides a wire-speed filtering of incoming emails. However, typically all not-filtered emails are subject of the AV Scannner as defined via the QHPSI. Almost all worms and virii are transported as BASE64 encoded attachments (except some trojans, encapsulated as HTML files). By means of the environment variable

• BASE64=""

one can advice QHPSI to scan only those emails which contain a BASE64 encoded attachment.

4.7 Particular SMTP 554 Replies

qmail-smtpd will send a SMTP 554 Error Reply under the following conditions:

• a badmimetype was encountered,
• a badloadertype was encountered,
• a Virus was found via an external AV Scanner,
• the email exceeds the hop count.

The SMTP Reply code for the first three conditions is always "554 sorry, invalid message content (#5.3.2)". The rejection of email because of the message content is due to some internal policy. For those users, which a subject of this policy innocently (and did not send ie. a virus mail on purpose), it might be advisable to explain the company's email policy.

The environment variable

• REPLY554

allows to include a particular SMTP 554 Reply. Typically, an URL is referenced: REPLY554="[ see: http://www.fehcom.de/emailpolicy.html ]" which allows to detail possible circumventions.

4.8 Qmail QUEUE_EXTRA

Bruce Guenter's Qmail QUEUE_EXTRA patch has almost the rank of a recommended patch, because it's used by many Qmail extensions like the Qmail-Scanner and qmail-qfilter.

The actual use is controlled via the content of environment variable "QMAILQUEUE", which usually set in a tcpserver's cdb ie. tcp.smtp or globally defined in the qmail-smtpd's run script. A typical use is:

12.34.56:allow,RELAYCLIENT=""
:allow,QMAILQUEUE="bin/qmail-qfilter"

which advices qmail-smtpd to use the executable qmail-qfilter as first stage queueing program instead of qmail-queue itself.

Note: The QUEUE_EXTRA patch is not applied against qmail-smtpd but rather against the module qmail.c itself, since it is just an extension to the general queue call-mechanism.

4.9 qmail-queue.scan Replacement

The qmail-queue.scan script can be used by the QUEUE_EXTRA mechanism to allow a per (recipient) domain

For high volume/high performance scanning, the incoming message is copied to a tmp directory which typically should be raised on a ramdisk. All scanning actions can be realized now in memory which significantly reduces disk I/0.

You can define individual SpamAssassin detection thresholds per domain using the additional control file:

• control/spamdomains

Include the domain name and the spam level separated by a colon here (ie.: mydomain.com:10). In order to use this facility, the environment variable DELIVERTO has to be set (but empty) which is used to tell qmail-queue.scan script the SMTP recipient(s).

SPAMCONTROL's qmail-smtpd is able to understand the (tagged) messages identified as infected or as spam and will issue a useful SMTP return code (see below).

4.9.1 DELIVERTO Facility

Apart from the rejection at SMTP level, emails identified to be spam  can be 'steered out' for special treatment.

For this purpose, qmail-smtpd can additionally be compiled with DELIVERTO support (conf-spamcontrol).

If the variable DELIVERTO is set (or perhaps populated and enhanced with a special email address) the original Sender is available within the qmail-queue.scan scriptt as environment variable NEWSENDER and

• a copy of the identified email will be forwarded to the forseen Recipient(s) separated by white spaces,
• while in addition a 550 SMTP return code is issued.

5. qmail-remote Extensions

SPAMCONTROL modifies and extends the behavior of qmail-remote in the following ways:

5.1 qmail-remote Authentication

The qmail-remote (qmail-smtp-auth-send) authentication from Bjoern Kalkbrenner has been included in a modular and RFC-complient version. qmail-remote sessions can be SMTP authenticated with the types PLAIN and LOGIN on a per-sender (Reverse-Path) base. Thus for each sender you can advice qmail-remote to use SMTP authentication with a particular username and password connecting to relay at port.

The qmail-remote authentication follows in this respect the smtproutes mechanism. Authentication for outgoing SMTP sessions is faciliated, if the control file

• control/authsenders

is populated accordingly. Sample:

mail@example.com|test|testpass
info@example.com:smtp.example.com:26|other|otherpw
:mailrelay.example.com|e=mc2|testpass

5.2 qmail-remote MX connectivity

Typically sites/domains on the Internet are reachable over serveral MTA listed and deployed in the DNS MX records (o.k. qmail.org is an exception). By theory, the MX with the smallest weight is the primary MX for that domain;though often sites have redundant MTA with equal weights:

10 mailc.microsoft.com
10 maila.microsoft.com
10 mailb.microsoft.com

In order to deliver emails, qmail-remote follows two strategies:

In case this MTA is exhausted and rejects the connection during the EHELO/HELO greeting, qmail-remote exists and retries the very same MTA again with it's quadratic queue schedule mechanism.

Running EZMLM with many messages to the vary same domain but different Recipients, email delivery may become throttled, which particularly happens for t-online.de sites which don't allow too many connections from the same client MTA (a policy which is actually not covered by any SMTP RFC).

Back 10 years ago, when Dan was designing qmail he already was aware of that problem:

"If I successfully connect to an MX host but it temporarily refuses to accept the message, I give up and put the message back into the queue.
But several documents seem to suggest that I should try further MX records. What are they thinking? My approach deals properly with downed hosts, hosts that are unreachable through a firewall, and load balancing; what else do people use multiple MX records for? " (THOUGHTS)

and included already the code base into qmail-remote which I simply activated. Thus, in case qmail-remote receives a rejection during the EHLO/HELO greeting it will simply try the next MTA for the DNS MX list.

After the MX lookup qmail-remote needs to fetch the CNAME from the DNS. Here, Dan decided (probably due to a bug in BIND 4.x) to employ a ANY query. This has some consquences:

• The ANY query returns ALL Resource Records for that CNAME.
• Misconfigured DNS content server will send oversized DNS records including additional AAAA and TXT information.
• The ANY query is not cached, but rather the resolver needs to contact the authoritive DNS content server.

• Anybody running a SMTP MTA on the Internet shall be able to set up DNS resources reliably.
• In case the ANY query fails, certainly something is wrong with the target's DNS configuration.
• In times, where we demand strict SMTP conformance (and in particluar a PTR record for the MTAs) why not ask for correct DNS settings ?

5.2 Fast delivery for qmail-remote

qmail-remote incorporates two performance critical steps for the delivery:

Bruce Guenter recognized the last fact and patched qmail-remote accordingly ("fastremote"), thus qmail-remotes processes the input data in chunks of 4 Kbyte. This patch has been included into SPAMCONTROL.

5.3 qmail-remote as QMTP client

qmail-remote includes now a QMTP client. This extends the 'mini-qmail' scheme and allows to setup the internal email system on QMTP rather then SMTP.

QMTP delivery is triggered by means of the file control/qmtproutes which follows the same symtay as control/smtproutes and obeys the syntax

domain:relay:port

where the typial QMTP service port is 209.

5.4 Defining bounceroutes

qmail-remote is now able to recognize bounces by means of the missing SMTP Mail From: address "<>".

To support efficient bounce handling, all bounces can be redirected to a particular 'bounce host'. Simply include

!@:bouncehost.af.mil:27

into control/smtproutes or control/qmtproutes and you are done.

6. qmail-pop3d

6.1 STLS support

The STLS (Start TLS support) for qmail-popup follows the same scheme as for qmail-smtpd

Requirements:

Actually, you use the same "env" file as for qmail-smtpd. In this case qmail-popup announces in the capability list "STLS" and the following POP3 dialogue is encrypted as is the transmission of the received emails as well.

6.2 qmail-pop3d run Script

In order to use SSL encryption for a POP3 connection, the following run script for qmail-pop3d is appropriate:

#!/bin/sh
HOSTNAME=`hostname`
export UCSPITLS=""
. /var/qmail/ssl/env
exec /usr/local/bin/sslserver -sevn -R -c100 0 pop3 \
/var/qmail/bin/qmail-popup $HOSTNAME /usr/local/bin/checkvpw \
/var/qmail/bin/qmail-pop3d Maildir 2>&1

Note: In this run script I use Bruce Guenter's checkvpw as PAM (for vmailmgr), which requires the additional presence of the "Maildir" argument after the call of qmail-pop3d.

The profile /var/qmail/ssl/env is the same as for qmail-smtpd. Defining the environment variable UCSPITLS directly in the run script instead of the profiles, allows a flexible use of the STARTTLS/STLS option for qmail-pop3d and qmail-smtpd without modifying the common profile.

7. qmail-send, qmail-queue, and the qmail's sendmail

7.1 qmail-send: Bounces

Bounces have generally a Null-Sender address (Mail From: <>) and are out-of-band error-messages to indicate a failure in the delivery process. In fact, RFC 2821/821 requires that all notification E-Mails have to have a Null-Sender address!

For every undeliverable message, qmail-send generates a bounce to the Sender.While this is legitimate and necessary for normal operation, in case of SPAM attacks the bounces are meaningless:

• The SPAMMER uses a lexical approach to generate random addresses, thus most of the E-Mails are undeliverable per se.
• The SPAMMER's Sender address is forged or is only limited available (during the SPAM attack), thus the bounce fail; and eventually generate a double-bounce. One undeliverable SPAM message may result in two additional bounces.

Unless you use a 'whitelisting' of Recipient E-Mail addresses, there is not much to do about. However, SPAMCONTORL helps you in three cases:

7.1.1 Limiting the Size of Bounces

By definition, a bounce is a SMTP notification for a failure situation. It is common practice, to include the original message in the bounce. Qmail uses a specific format, introduced by Dan Bernstein and called "QSBMF" (qmail-send Bounce Message Format); other MTA encapsulate the original message as MIME attachment in rfc822/message format.

Anyway, for a legitimate bounce reaching the Sender the original message is usually of no interest, except for identification purposes. In order to save bandwidth, you can limit the size of bounces using the control file

• control/bouncemaxbytes

Unlike the original patch (from Frank DENIS aka Jedi/Sector One <j@4u.net>), the default value is '0' byte, meaning no limits. A useful limit would by 2000 (byte), which covers the header and some body part information. The average size of a SPAM E-Mail is 5 Kbyte.

The original message included in the bounce will be limited to the defined bouncemaxbytes and truncated, which is displayed in the bounce with "--- Rest of message truncated." at the end of the bounce.

7.2.1 Dumping Double Bounces

Double bounces are generated, if the bounce can not be delivered to the Sender.

Double bounces are usually delivered to the 'Postmaster' account. It is convenient that this account is local and eventual double bounces are stored in a mbox/Maildir for later inspection. However, Qmail allows you to forward double bounces to some other account defined in

• control/doublebounceto

However, due to the forged Sender address in SPAM E-Mails, practically all bounces become double bounces eventually. In this case any storage and inspection is fruitless. Taken from Russell Nelson and Charles Cazabon, you can optionally dump all double bounces immediately. This is facilitated if doublebounceto contains a '@' in the first line.

Those dumped double bounces show up in the qmail-send log as: "double bounce: discarding".

7.3 qmail-send: IPME- and MOREIPME-Patch

Scott Gifford's "ipme.c" patch (or qmail-0.0.0.0-patch) is included by default. According to RFC 1122, Sec. 3.2.1.3 the IP address "0.0.0.0" is a special address which always refers to "this host, this network". You may wish to tell Qmail about arbitrary IP addresses employing the moreipme patch and include the following control files:

• control/moreipme
• control/notipme

See the enhanced man page qmail-smtpd and or consult the README.moreipme.

7.4 qmail-send Gadgets

As a further gadget, the qmail-send control files

• control/concurrencylocal
• control/concurrencyremote

are re-read by means of a HUP signal (eg. svc -h /service/qmail-send).

7.5 qmail-queue: BigToDo enhancement

The queue directories ./intd and ./todo may be splitted (as per conf-split) into subdirectories to allow a more efficient treatment of many incoming messages.

Caution: Make sure, that the directories ./queue/todo and ./queue/intd are empty before applying the patch; otherwise qmail-send will not be able to process those messages anymore!

Note: The shell script qmail-qstat and in addition some qmail-mrtg analyses are affected by this patch.

Hint: Consider raising the value in conf-split in the first place !

7.6 sendmail Fixes

The following fixes for Qmail's sendmail wrapper have been included for compatibility reasons:

• The 'sendmail -f' patch by David Philips, thus sendmail sets the default for the username.
• The 'sendmail -N dsn' patch by Matthias Andree, thus sendmail ignores the -N option.

8. SMTP Protocol Return Codes

SMTP allows to reject Sessions based on some technical and/or political criteria, which are not well expressed in the RFCs (2821, 2554, 2505, 1122).

The SMTP protocol mechanism between the client and the server are defined as Commands and Replies. SMTP uses a three-letter Reply Code. The first digit tells whether a command was accepted and completed (2), transaction begin (3), or whether there was as transient (4) or permanent failure (5). In addition, an explanatory description may be given.

RFC 1893 introduces a concept of "Enhanced Mail System Status Codes" (EMSSC) which should provide easily parseable SMTP server conditions and transaction statuses, usually at the end of the SMTP reply and included in parenthesis, eg. (#5.5.1).

The SMTP Reply Codes and the EMSSC are detailed in the corresponding RFCs, but don't fit well to each other, thus either providing redundant information or almost no additional information at all. In short, the EMSSC is nowadays almost meaningless.

Here's a breakdown of SPAMCONTROL's SMTP Reply Codes, informational texts, and the used EMSSC.

Reply	Informational text	EMSSC
421	unable to check recipients	(#4.3.0)
450	sorry, mailbox currently unavailable	(#4.2.1)
451	DNS temporary failure	(#4.3.0)
454	TLS not available due to temporary reason	(#5.7.3)
501	auth exchange canceled	(#5.0.0)
501	malformed auth input	(#5.5.4)
503	you're already authenticated	(#5.5.0)
503	no auth during mail transaction	(#5.5.0)
503	sorry, SMTP Authentication not available	(#5.7.3)
504	auth type unimplemented	(#5.5.1)
535	authorization failed	(#5.7.1)
535	authentication required	(#5.7.1)
535	STARTTLS required	(#5.7.1)
550	sorry, invalid HELO/EHLO greeting	(#5.7.1)
550	sorry, your envelope recipient is in my badrcptto list	(#5.7.1)
550	sorry, invalid sender address specified	(#5.7.1)
550	sorry, too many recipients	(#5.5.3)
550	sorry, bounce messages should have a single envelope recipient	(#5.7.1)
552	sorry, that message size exceeds my databytes limit	(#5.3.4)
553	sorry, your envelope sender is in my badmailfrom list	(#5.7.1)
550	sorry, that domain isn't in my list of allowed rcpthosts	(#5.7.1)
553	sorry, your envelope sender domain must exist	(#5.7.1)
554	too many hops, this message is looping	(#5.4.6)
554	sorry, invalid message content (optional text)	(#5.3.2)

---

9. qmail-smtpd Logging

Normally, qmail-smtpd doesn't log anything.With SPAMCONTROL qmail-smtpd logs accepted and some (important) rejected SMTP session attempts. The logging is done

Note: Including the PID in every line makes it possible to follow the current. transaction on port SMTP/ESMTP: From initialisation over tcpserver/sslserver, thru rblsmtpd and eventually to the (missing, rejected, accepted) (E)SMTP session in qmail-smtpd.

9.1 Extensible logging scheme

Action	Type	Condition	Explanation
Reject	SMTP	Toomany_Hops	Message Hop count exceeded
Reject	SMTP	Syntax_Error	Malformed SMTP address (e.g. missing brackets)
Reject	DATA	Invalid_Size	DATA exceeds sizelimit
Reject	DATA	Bad_MIME	DATA includes BASE 64 MIME type listed in badmimetypes
Reject	DATA	Bad_Loader	DATA includes BASE64 loader type listed in badmimetypes
Reject	DATA	Virus_Infected	DATA includes virus infected message ('<scanner>' | 'AV scanner')
Reject	DATA	Spam_Message	DATA includes an identfied Spam message
Reject	SNDR	Bad_Helo	SNDR's HELO is in the badhelo
Reject	SNDR	DNS_HELO	SNDR's HELO has no DNS A/MX RR
Reject	SNDR	Invalid_Relay	SNDR's tries relaying; but not allowd
Reject	SNDR	Missing_TLS	STARTTLS was required but not granted by client
Accept	SNDR	Relay_Client	SNDR was identfied as relay client
Info	SNDR	Start_TLS	SNDR succesfully started TLS
Reject	ORIG	Bad_Mailfrom	ORIG is in badmailfrom
Reject	ORIG	DNS_MF	Domain part of ORIG has no DNS MX RR
Reject	ORIG	Failed_Auth	ORIG tried SMTP Authentication; but failed
Reject	ORIG	Invalid_Sender	ORIG not allowed to send
Reject	ORIG	Missing_Auth	SMTP Authentication required, but not granted
Info	ORIG	Valid_Auth	ORIG was successful authenticated
Accept	ORIG	Local_Sender	ORIG was identified as local sender address
Accept	ORIG	Relay_Mailfrom	ORIG was accepted als Relaymailfrom
Reject	RCPT	Bad_Rcptto	RCPT is in badrcptto
Reject	RCPT	Toomany_Rcptto	Too many RCPTs
Reject	RCPT	Failed_Rcptto	RCPT could not acceptd as per recipients/cdb.
Accept	RCPT	Recipients_Cdb	RCPT was accepted as per recipients/cdb.
Accept	RCPT	Recpients_Pam	RCPT was accepted per PAM lookup.
Accept	RCPT	Recpients_Wild	RCPT was accepted per Domain wildlisting in recipients.
Accept	RCPT	Rcpthosts_Rcptto	RCPT was accepted as per rcpthosts/morercpthosts

The Information includes typically the following

• PROTOCOL: SMTP/ESMTP/EMSTPA/ESMTPS/EMSTPM
• SNDR: IP/FQDN (from tcpserver/sslserver)
• HELO/EHLO: The provided greeting string
• ORIG: The content of the Mail From: (if applicable).
• RCPT. The intended Rcpt To:

This scheme is easy extensible to other successful/deferred SMTP sessions. Sample:

Accept::SNDR::Relay_Client: P:orion.fehnet.de S:81.173.229.48:xdsl-81-173-229-48.netcologne.de H:mail.fehcom.de F:feh@fehcom.de T:erwin@example.com

9.2 qmail-smtpd Start-Up Script Using tcpserver And splogger

A typical tcpserver start script applying standard splogger:

#!/bin/sh

QMAILDUID=`id -u qmaild`
QMAILDGID=`id -g qmaild`
/usr/local/bin/tcpserver -v -R -H -u $QMAILDUID -g $QMAILDGID QMAIL_IP_ADDRESS smtp \

  /var/qmail/bin/qmail-smtpd /bin/cmd5checkpw true 2>&1 | \
  /var/qmail/bin/splogger smtpd &

Since splogger is now facilitated, ACCUSTAMP time information is included.

9.3 qmail-smtpd Start-Up Script Using Daemontools, tcpserver, And multilog

A better choice would be multilog. multilog allows you to write separate filtered logs; to individual directories, and/or files, STDERR respectively.A typical Daemontools qmail-smtpd run script would look like:

#!/bin/sh

QMAILDUID=`id -u qmaild`
QMAILDGID=`id -g qmaild`
/usr/local/bin/tcpserver -R -u $QMAILDUID -g $QMAILDGID QMAIL_IP_ADDRESS smtp \

   /var/qmail/bin/qmail-smtpd /bin/cmd5checkpw true 2>&1

Note: tcpserver's logging via the '-v' flag can be omitted to get mostly a full comprehensive and terse one-line logging of the SMTP session.

The corresponding multilog run script allows not only to filter the log information and write them to the file "current" in a specific directory but in addition to feed a file with specific information; here's a sample:

#!/bin/sh

exec setuidgid qmaill multilog t \
   '-*tcpserver: status:*' /var/log/qmail-smtpd \

   '-*' '+*Reject::*' =/var/log/qmail-smtpd/rejected \

   '-*' '+*DNS*' =/var/log/qmail-smtpd/nodnsmx

In this case, multilog adds at first a TAI64 time stamp.

1. In a second step, the log information - except those line including the string "tcpserver: status" - are written into the log file /var/log/qmail-smtpd/current.
2. On the third line the filter "-*" removes any unwanted lines and "+*Reject::*" picks up from qmail-smtpd's log all rejected sessions and placing the last rejected condition in the file 'rejected'!
3. On the fourth line the last failing DNS MX lookups for Mail From: <address> is logged in a file named /var/log/qmail-smtpd/nodnsmx.

9. SPAMCONTROL compile Options

In this version of SPAMCONTROL I have substantially reduced the number of compile-time options:

In order to consistently change all relevant binaries, use the file conf-spamcontrol which is evaluated by the installation routine install_spamcontrol.sh and passes the changes to the Qmail c-files:

# Configuration for SPAMCONTROL (no tabs allowed)
#
# Additional DELIVERING
#
deliverto=no # qmail-smtpd can be adviced to deliver identified spam emails
#

# Additional CONTROLLING
#
reqbrackets=yes # qmail-smtpd requires brackets "<address>" in SMTP addresses

verp=yes # allow VERP addresses for RECIPIENTS

newline=no # otherwise qmail-smtpd will accept bare Line Feeds

pam111421=yes # otherwise qmail-smtpd will allow the email in case of PAM failures.
#
# LOADSHARING enhancements
#
moreipme=no # Scott Gifford's additional control files moreipme and notipme
#
# PERFORMCANCE enhancements
#
bigtodo=no # Bruce Guenter's BigToDo patch

10. Howto

In case your E-Mail environment complies to the assumption in PURPOSE do the following:

1. Stop your Qmail system (receive and send).
2. Remove SMTP/POP3 connection to be serviced by INETD/XINETD.
   Employ tcpserver instead.
3. Follow the INSTALL.spamcontrol instructions.
4. Edit the files
   • ./control/badhelo
   • ./control/badmailfrom
   • ./control/badrcptto
   • ./control/moreipme
   • ./control/notipme
   • ./control/bouncemaxbytes
   • ./control/recipients

   to your needs.

See above samples and check the included samples for ./badmailfrom and ./badrcptto.

1. Restart Qmail.
2. Check your configuration by means of qmail-showctl. It is a good idea to pipe the output of that command with timestamp information to a file.
3. Test your filters locally (see TESTING.spamcontrol).
4. If you are already blacklisted, inform those sites that you don't act as an OPEN RELAY anymore.
5. Watch the Qmail behavior by means of the log file information.

*) Not useful, if tcpserver in employed.

Good luck!

10.1 Tested Environment

Attention: For 64 bit OS'es you may need in addition the qmail-isoc patch from James Craig Burley (http://www.jcb-sc.com/qmail/patches/qmail-isoc.patch).

10.2 Incompatibilities

The SPAMCONTROL patch is incompatible with the Qmail LDAP patch. It should be applied against qmail-1.03 and not against netqmail-1.0x.

11. Further Information

• RFC 1321 MD5 Message-Digest Algorithm
• RFC 1869 SMTP Service Extensions
• RFC 1870 SMTP Size Declaration
• RFC 1891 SMTP Service Extensions for Delivery Status Notification
• RFC 1893 Enhanced Mail System Status Codes
• RFC 2222 Simple Authentication and Security Layer (SASL)
• RFC 2505 Anti-Spam Recommendations for SMTP MTAs
• RFC 2554 SMTP Service Extension for Authentication
• RFC 2635 DON'T SPEW - A Set of Guidelines for Mass Unsolicited Mailings and Postings (spam*)
• RFC 2821 Simple Mail Transfer Protocol
• RFC 2595 Using TLS with IMAP, POP3 and ACAP
• RFC 3207 SMTP Service Extension for Secure SMTP over Transport Layer Security
• RFC 4409 Message Submission for Mail

12. Authors

13. Thanks

Thanks to the discussion in the Qmail Mailing List (qmail@list.cr.yp.to) in particular:

Erwin Hoffmann (feh@fehcom.de)
Cologne, 2009-04-05