SPAMCONTROL is an extension for qmail. It provides the following basic features:
Enhancements for qmail-smtpd:
Enhancements for qmail-remote:
Enhancements for qmail-pop3d:
Enhancements for qmail-queue:
Enhancements for qmail-send:
SMTP Pluggable Authentication Modules
Tow additional PAMs are now delivered with SPAMCONTROL:
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
Throughout this document, I assume that qmail-smtpd is under control of supervise (out of the Daemontools package) and served by sslserver (part of the UCSPI-SSL package) or tcpserver (part of the UCSPI-TCP package).
Some skeletons for the daemontools run-scripts are provided which are useful to set up the services:
A typical - minimal - so called run script looks like follows:
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:
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!
As a convention, I will call the tcperver's cdb, which rules the behaviour of qmail-smtpd, tcp.smtp. A typical tcp.smtp would look like
The cdb is constructed on the fly:
Caution: For use with tcpserver, the value of the environment variable has to be included in quotes.
Though qmail can live happily without the knowledge of domains to be responsible for as provided by rcpthosts/morercpthosts.cdb, it is highly advisable 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.
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.
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 Pluggable Authentication Module PAM).
SPAMCONTROL provides the following features:
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:
qmail-smtpd including SMTP Authentication may be called by tcpserver/sslserver in a supervise run script. Here is an example (with some more features):
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
The environment variable SMTPAUTH can be used to specify the type of SMTP Authentication. Your choices are:
|""||Left blank to allow Authentication types "PLAIN" and "LOGIN"|
|"+cram"||Add "CRAM-MD5" support|
|"cram"||Just (secure) "CRAM-MD5" support, no other types offered|
|"!"||Enforcing SMTP Auth (of type "LOGIN" or "PLAIN")|
|"!cram"||Enforcing SMTP Auth of type "CRAM-MD5"|
|"!+cram"||Enforcing SMTP Auth of type "LOGIN", "PLAIN" or "CRAM-MD5"|
|"-"||Disabling SMTP Auth (for a particular connection)|
Unlike the Submission feature, these settings may be realized in the tcprules cdb per connection.
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:
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.
Now, the Submission feaure requires the additional declaration
of the SMTPAUTH with a leading 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. 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: If you use Sqwebmail in addition, the user is free to
set his/her own password. SPAMCONROL's STARTTLS support for qmail-smtpd is aligned
Gifford's approach and depends on the following: My STARTTLS implementation conforms with RFC 3207 but lacks
1.2.6 User Database for cmd5checkpw
1.2.7 SMTP Authentication and Vpopmail
# 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
1.3 (START)TLS support
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.
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:
If you use Sqwebmail in addition, the user is free to set his/her own password.
SPAMCONROL's STARTTLS support for qmail-smtpd is aligned with Scott Gifford's approach and depends on the following:
My STARTTLS implementation conforms with RFC 3207 but lacks support of
Most current STARTTLS/TLS solutions depend on the existence and availability of the OpenSSL libraries -- so does SPAMCONTROL. 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.
Install ucspi-ssl (version >= 0.80).
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.
Apart from the 'global' environment variable UCSPITLS sslserver are fed by several environment typically included in a "profile" /var/qmail/ssl/env.
On my system, this file includes the the following settings:
Of course it is required, to have raised the directory /var/qmail/ssl before and to generate via openssl the appropriate files 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 settings, 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.
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:
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.
Unlike the SMTP+STARTTLS the SMTPS setup facilitates a TLS connection immediately the TCP handshake is finished and thus the entire SMTP session dialoge is protected against espionage and by the same token not vulnerable against sabotage (see: VU#555316).
Setting up qmail-smtpd on the SMTPS port is easy:
Please realize also, that sslserver is called without the option "-n" thus no delay is required. In this case, sslserver and qmail-smtpd communicate over the standard file descriptors entirely.
Once UCSPITLS ist set and exported, qmail-smtpd accepts STARTTLS connections. However, it might be necessary to customize this behavior per sending MTA:
Typically, the ESMTP client connecting to the server does not provide a X.509 certificate. In this case, the ESMTP connection is still TLS encrypted and the client might use the server's certificate for verification/validiation purpose.
However the server may request a valid X.509 certificate from the client. In this case, the (ssl)server needs to have the certificate of the CA (Certificate Authority) which has signed the client's certificate in the first place (in PEM format).
sslserver will not only ask for the client certificate but will verify it by means of the CA certificate and validate it's content matching the received FQDN with the presented 'SubjectNameAltName' and perhaps the 'SlsubjectName' in the client certificate.
In case, any of the checks fail, the connection attempt will be refused.
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.
SPAMCONTROL displays the use of TLS in the Received header (according to RFC 3207). The following information is added:
Here's a sample using Thunderbird als email client:
Note: The received DN ist typically 'unknown' because the client doesn't provide a certificate.
In the above case the keyword ESMTPSA tells that the connection was established via SMTPS and additionally was authenticated A: (E)SMTPS(A).
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.
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:
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.
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:
Since this might not be helpful for the (innocent) sender, you might use the environment variable REPLYMAV to add a qualification to that message.
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.
qmail-smtpd reads serveral environment variables which allows the customization of the SMTP Reply messages:
Those (customized) variables and thir specific content can be defined in a single file smtpreplies (sample attached) to be loaded from the run script for qmail-smtpd.
For instance, qmail-smtpd will send a SMTP 554 Error Reply under the following conditions:
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 are 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 here.
The environment variable
allows to include a particular SMTP 554 Reply. Typically, an URL might be referenced: REPLY_CONTENT="[ see: http://www.fehcom.de/emailpolicy.html]" which allows to detail possible circumventions.
For special purposes, it might be necessary to have knowledge of the SMTP envelope information while qmail-smtpd is running. SPAMCONTROL (>= 2.5.26) puts the following informations into the environment -- as soon as available:
The provided qmail-queue.scan script makes use of these variables. You may also populate the DELIVERTO environment variable -- read by qmail-smtpd -- to include additional recipients.
In case of the SSL/TLS session further environment variables a la mod_ssl are available.
qmail-smtpd will include the information received by a Relay-Blacklist (i.a. Spamhaus) into the header of the message, once the environment variable $RBLSMTPD is available and filled with some information.
Here is a sample:
Note: Currently only ucspi-tcp6 supports this with it's rblsmtpd program invoked in interrogation mode.
qmail-smtpd is able to use received SPF (Sender Policy Framework) information to display those in the email header:
For this purpose, SPAMCONTROL includes an API:
Note: Except for result any of these variables may be empty/unsupplied.
Currently, I'm not aware of any SPF retrieval and feeding process, though their exists a patch for ucspi-tcp including a SPF lookup. However, considering the demand this kind of information -- which can be understood as a prototype even for the competing DKIM algorithm -- this should be understood as starting point.
The SMTP Envelope consists of three parts:
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
The following sets of wildmat control characters can be used:
Any E-Mail address, lets say <email@example.com> consists of a
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.
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:
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
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.
SPAMCONTROL allows four types of checks against the provided "Mail From:" SMTP envelope address (which I often call the "Originator"):
Invoking the environment variable MFDNSCHECK in the qmail-smtpd startup script enables globally the DNS check for the envelope's Sender.
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:
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.
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.
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 "firstname.lastname@example.org", 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 ("-"):
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.
In particular for webmailer (ie. hotmail.com, yahoo.com) the domain-part of the provide Mail From: address coincides with the provided domain name in TCPREMOTEHOST. Enforcing coincidence can be achieved for addresses appended with an equal-sign ("=") in control/badmailfrom:
Receiving a Mail From: address like "email@example.com" will only be accepted if TCPREMOTEHOST ends with "t-online.de", for instance "mailout03.t-online.de". Otherwise, the email will be rejected by badmailfrom.
Reversely to badmainfromwellknown now the MTA and it's FQDN
is considered in the first place.
Thus, if you receive emails from mx1.yahoo.com you may require that the provided Mail from: address matches yahoo.com.
In order to enforce this, within control/badmailfrom you define extended (domain) addresses with a leading similar-sign ("~"):
Caution: It should be noted, that this filter is not in accordance with RFC 2821, since it couples the SMTP envelope with the domain of the sender. For example, yahoo uses DKIM records to label allowed envelope addresses for their domain. Further, this entry does not affect the 'Mail From:' address in the first place, but rather the value of TCPREMOTEHOST.
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:
Under some conditions, it might by necessary to by-pass all reqular badmailfrom checks and in addition the MFDNSCHECK. For this purpose, individual email addresses included in badmailfrom and enhanced with a leading question mark ("?") can be defined. While recognizing this address, qmail-smtpd will skip all further tests:
<Note: The enhanced addresses don't allow wildcarding.
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).
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:
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:
Note: The evaluation of control/badrcptto is done independent from the setting of the RELAYCLIENT environment variable.
The environment variable
can be used to restrict the number of counted "Rcpt To: "s in the SMTP session. By default, no restriction is facilitated.
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:
TARPITCOUNT denotes the number of sessions before starting the TARPITDELAY, which defaults 5 seconds.
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: <firstname.lastname@example.org>").
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.
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.
The RECIPIENTS mechanism supports natively Qmail's address extensions (VERP). If a recipient address like 'email@example.com' defined, all VERP addresses like 'firstname.lastname@example.org' are accepted for SMTP reception.
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:
Compatibility Note: Due to this new syntax, the old RECIPIENTS (version 0.4x) wilddomain support (as part of the cdb) is not supported anymore.
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.
Defining a TARPITCOUNT can be 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).
However, defining a TARPITCOUNT together with a TARPITDELAY < 0 acts as 'tarpitting reloaded':
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.
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.
The checkpassword API is defined in:
and typically consists of the string:
written to file descriptor 3 (FD 3) to be read by the checkpassword compatible PAM.
For email address (recipient) verification, we replace
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.
The general syntax invoking a PAM from control/recipients follows the principals invoking a cdb. However, the triggering token is a '|' and the PAM may be accompanied by at most five (5) arguments separated by white spaces:
The ldap_pam.pl is a PERL script using the PERL library 'Net::LDAP' from http://www.cpan.org while providing simple and TLS encrypted (strong) binds to an LDAP directory server.
ldpap_pam.pl -h:host:port -d:DN -w:password -b:base -s:scope -c:certificate -l(ogging) -ll
The ldap_pam.pl shall be considered as prototype only. Due to the large variety of LDAP servers and possibilities to store the information into and to fetch it from the directory, there is the need for specific customization. Logging might be activated in two steps.
The additional module qmail-smtpam is a generic part of SPAMCONTROL and
can be considered as stripped-down version of qmail-remote providing the first
steps of the email dialoge using the standard Helo message, a
Nullsender Mail From: address, and finally the provided the Rcpt To:
address available on FD 3.
Once it receives the SMTP return code, it exits either with '0' in case of a '250' positive reply, a '1' in case of a failure, or a '111' realizing communication problems.
qmail-smtpd thus uses a standard SMTP dialoge without advanced features, though complying to RFC 821 while using the canonical name for host to connect to.
The Recipients extension needs no customization except for the following circumstances:
With the Recipients extension qmail-smtpd will act for none-RELAYCLIENTs like follows
In any other case, a SMTP temporary failure protocol error is issued to the client saying:
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.
The badmimetype filter becomes active if
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:
Adding new badmimetypes is simple:
Comments (starting with "#") are allowed in badmimetypes; the length of the signature will be truncated to nine characters.
The badloadertype filter becomes active if
The BADLOADERTYPE mechanism deals in particular with "transport stealth" worms, ie. UPX encoded Windows executables.
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:
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.
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:
The QHPSI allows to use the following environment variables:
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:
Here is a sample of Clam AV without and with the argument "--disable-summary":
Note: Even in case no virus is detected, the "SCAN SUMMARY" is provided.
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.
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
one can advice QHPSI to scan only those emails which contain a BASE64 encoded attachment.
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:
which advices qmail-smtpd to use the executable qmail-qfilter as first stage queueing program instead of qmail-queue itself.
In order to reject during the SMTP DATA phase, vanilla qmail
requires that the qmail-queue replacement returns RC=31.
However, SPAMCONTROL enhances this mechanism with the following additional return codes:
Those return codes can be extensibly used in a qmail-queue wrapper-script (see below).
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.
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:
Here, you include the recipients's domains, followed by a colon, and the spam threshold for this domain:
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).
SPAMCONTROL modifies and extends the behavior of qmail-remote in the following ways:
While qmail-smtpd can be bound (via tcpserver/sslserver) to any available IP address on the system, qmail-remote uses only the 'canonical' IP address of the system. However employing the file
you can advise qmail-remote to bind to domain-specific IP addresses:
Check the existing IP addresses qmail-remote can bind to by means of the qmail command ipmeprint.
In addition, the domain name is now used as ESMTP EHLO greeting string.
This is in particular useful hosting serveral domains and if you set up your email system in the DNS with different SPF records matching the FQDN of the IP. Further, if you use client certificates for authenticated TLS connections you are now able to match the (outgoing) IP with the Subject(Alt)Name in the X.509 client certificate.
qmail-remote uses the SSL routines provided by UCSPI-SSL and can be advised to set up a TLS session with the ESMTP peer in the following ways:
You can control the TLS connectivity behavior of qmail-remote by means of
This control file is separated into a 'destination domain' the left side of the colon and a (structured) informational part right from the colon employing '|' the bar character for separation and perhaps a colon for the port in the last position:
cafile is the full-qualified path name to the CA certificate file in PEM format. This certificate file can be build of several X.509 certs. If however, cafile ends with a slash "/", a CADIR is assumed hosting the certificates in a "hashed" format.
The expression for the accepted cipher can be constructed individually. Check the OpenSSL information for the syntax and the accepted values.
The verifydepth parameter defaults 1 and should be adjusted to the nesting of the certificates in the chain.
In case port equals 465 the SMTPS port, STARTTLS is not tried but rather a standard TLS connection is required.
Finally, an entry in control/tlsdestinations can be bound against the the senddomain in case several outgoing domains are used. Thus, an entry is only valid, if it matches the senddomain.
The file control/tlsdestinations is evaluated in the following order (for the same domain, the more specific information is taken) with additional customization samples included:
Thus, you can specify particular hosts/domain to set up a STARTTLS connection to, - or - exclude particular hosts by means of an prepended exclamation mark (!) following the hostname.
Since the ESMTP server always presents a X.509 certificate, this can used for additional interrogation:
Here, the server's X.509 certificate can be checked:
On the ESMTP's server request, qmail-remote will provide (if available) a X.509 client certificate. The server may evaluate this certificate and set credentials (ie. relaying), accordingly.
This information can be provided per sending domain by means of the control file
which is structured comparable to control/tlsdestinations. Thus the part left from the colon tells the sending domain; in case this equals "*" all outgoing connections use the same certificate/keyfile. The right parts provides the location and the certfile (in PEM format), the keyfile (if any) and perhaps the password to decrypt the keyfile; all separated by a '|':
The qmail-remote 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, LOGIN and CRAM-MD5 on a now be realized
Authentication for outgoing SMTP sessions is faciliated, if the control file
is populated accordingly. Sample:
In the first case, an email with SMTP envelope sender Mail From: 'email@example.com' tries to authenticate always with username 'test' and password 'testpass'. In the second case emails from the sender 'firstname.lastname@example.org' are tries to relay through 'smtp.example.com' on port 26 with authentication username 'other' and password 'otherpw'. The third sample shows, that any outgoing email irrespectively of the sender is relayed thru 'mailrelay.example.com' with authentication username 'e=mc2' and password 'testpass'.
Mechanism 2:Instead of binding the authentication procedure to a specific user rather you can define it per destination, defining the control file
This syntax follows the smtprules interpretation. The first lines shows how emails for 'example.com' is relayed through 'smtp.example.com' on port '587' (Submission) using the authentication usernaem 'other' and password 'otherpw'. In the second case any email is relayed by means of 'mailrelay.example.com' with authenticated username 'e=mc2' and password 'testpass'.
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:
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:
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:
In essence, the ANY query is a heavy burdon on the DNS traffic and MAY result in the famous qmail-remote error message: "CNAME temporary lookup failure". It has been suggested, to replace the ANY query with a standard CNAME query, but I hesitate:
Therefore, I added a verbose CNAME message into qmail-remote logs, thus you are able to figure out the problem using tools like dig. If you use dnscache, i suggest to raise the udpbuf (to ) in dns_transmit.c.
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.
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
qmail-remote applies the definded routing advices in the following order:
The STLS (Start TLS support) for qmail-popup follows the same scheme as qmail-smtpd.
Actually, you can 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.
In order to use SSL encryption for a POP3 connection, the following run script for qmail-pop3d is appropriate:
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. Unfortunately, checkvpw does not support APOP but rather USER authentication only.
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.
Therefore access to the POP3 accounts can be monitored now in the logfiles.The information provided is equivalent to those available for qmail-smtpd. You will realize the following line per connection:
Authentication ttempts are labled 'Accept' or 'Reject'. The type of authentication is indicated as 'User' or 'Apop' and additionally the provided authentication-id is displayed together with the protocol type.
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, generates a bounce to the Sender. While this is legitimate and necessary for normal operation, in case of SPAM attacks the bounces are meaningless:
Unless you use a 'whitelisting' of Recipient E-Mail addresses, there is not much to do about. However, SPAMCONTORL helps you in theses cases:
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
Unlike the original patch (from Frank DENIS aka Jedi/Sector One <email@example.com>), 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.
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
into control/smtproutes or control/qmtproutes and you are done.
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
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".
As a further gadget, the qmail-send control files
are re-read by means of a HUP signal (eg. svc -h /service/qmail-send).
Note: The 'silent' spawn limit has been increased to 500. Thus, in maximum qmail-send can run 500 qmail-local (and more reasonable) 500 qmail-remote processes.
The queue directories ./intd and ./todo are 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 change.
The following fixes for Qmail's sendmail wrapper have been included for compatibility reasons:
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.
|421||unable to check recipients||(#4.3.0)|
|450||sorry, mailbox currently unavailable||(#4.2.1)|
|451||DNS temporary failure||(#4.3.0)|
|452||Too many recipients||(#4.5.3)|
|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)|
|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, 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)|
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.
|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||TLS||SNDR accepted TLS connection|
|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
This scheme is easy extensible to other successful/deferred SMTP sessions. Sample:
Accept::SNDR::Relay_Client: P:orion.fehnet.de S:22.214.171.124:xdsl-81-173-229-48.netcologne.de H:mail.fehcom.de F:firstname.lastname@example.org T:email@example.com
A typical tcpserver/sslserver start script applying standard splogger:
Since splogger is now facilitated, ACCUSTAMP time information is included.
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:
Note: tcpserver/sslserver'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:
In this case, multilog adds at first a TAI64 time stamp.
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:
In case your E-Mail environment complies to the assumption in PURPOSE do the following:
to your needs.
In case of TLS support for qmail-smtpd and qmail-pop3d:
See above samples and check the included samples for ./badmailfrom and ./badrcptto.
TLS support for qmail-remote depends on:
The SPAMCONTROL patch is incompatible with the Qmail LDAP patch. It should be applied against qmail-1.03 and not against netqmail-1.0x.
SPAMCONTROL will happily run under AMD64 and compile successfully with clang under the following conditions:
Major enhancements for SPAMCONTROL are taken from these authors:
Thanks to the discussion in the Qmail Mailing List (firstname.lastname@example.org) in particular:
Erwin Hoffmann (email@example.com)