Simscan/README

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Revision as of 11:49, 7 October 2005 by 66.18.218.37 (Talk)
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Contents

Overview

SimScan is a simplified scanner for qmail similar to qmail-scanner and qscand. It uses clamav, trophie, and/or spamassassin. It also supports attachment blocking by extension. Simscan is written entirely in C to ensure maximum speed. There are several options to allow simscan to scan per domain, and reject spam mail.

Requirements

  1. ripmime (If you plan on using attachment blocking)
  2. qmail with qmail-queue patch
  3. clamav (optional)
  4. spamassassin (optional)
  5. trophie (or sophie) (both optional)


How it works

  • Simscan creates a temporary working directory. You can specify the base working directory with the --enable-workdir=/path. The default location of this base is the /var/qmail/simscan directory. The temporary working directory under this base directory is named as "unix time in seconds" . "microseconds" . "process id".

The email is read into a temporary file named "msg.unixtime.micro.pid.

  • ripmime is called to break this file into separate mime parts.
  • Optionally (--enable-attach) the mime message parts are checked against the list of attachments to block.

Put the list of attachments in /var/qmail/control/ssattach or place the list in the /var/qmail/control/simcontrol file for per-domain blocking. This file gets read each time simscan is kicked off.

  • clamdscan is called to check all the files in the working directory. The return code of clamdscan is checked to see if it found a virus. If it did, simscan exits back to qmail-smtpd with a permenent error, causing the mail to be bounced back to the sender.
  • Optionally (--enable-spam) spam assassin is called via spamc.

The email returned by spamc is checked X-Spam-Flag: header. If this header contains "YES", the spam is rejected. You can reject instead on the calculated hit count value by adding the --enable-spam-hits. SimScan checks for both "hits=" and "score=" so that both old and new versions of spamassassin are supported.

  • Optionally (--enable-per-domain) enable per domain clamav and spamassassin processing. You will need to edit /var/qmail/control/simcontrol to define what domains get what scans. Further details on editing this file are below.

Then run /var/qmail/bin/simscanmk to build the cdb files that simscan uses. The changes will take effect immediately.

  • Finally, if all of the above succeeds then the msg file is passed on to qmail-queue. And the working directory and all the temporary files are deleted, unless the SIMSCAN_DEBUG environment variable is set.

Configuration Options

Following is a list of all of the configure options and defaults:

 --enable-user=<user>              Change the user for simscan.
                                   Default: simscan.
 --enable-clamav=y|n               Turn on clamav scanning. default yes.
 --enable-clamdscan=PATH           Full path to clamdscan program.
 --enable-custom-smtp-reject=y|n   Return smtp reject message with virus name.
 --enable-per-domain=y|n           Turn on per domain based checking.
 --enable-attach=y|n               Turn on attachment scanning. default no.
 --enable-spam=y|n                 Turn on spam scanning. default no.
 --enable-spam-passthru=y|n        Pass spam email thru or reject
                                   Default: disable (reject)
 --enable-spamc-user=y|n           Set user option to spamc.
 --enable-spam-hits=number         Reject spam above this hit level.
                                   Default 10.0
 --enable-spamc=PATH               Full path to spamc program.
 --enable-spamc-args=ARGS          Arguments to pass to spamc.
 --enable-dropmsg=y|n              Drop message in case of virus/spam found.
                                   Don't return error to sender.
                                   Default: disable (return error)
 --enable-quarantinedir=DIR        Directory to keep spam and/or infected emails
                                   Default: disabled
 --enable-qmaildir=DIR             Base qmail directory /var/qmail.
 --enable-workdir=DIR              Directory to unpack emails
                                   Default: /var/qmail/simscan
 --enable-qmail-queue=PATH         Full path to qmail-queue program.
 --enable-trophie-path=PATH        Full path to the trophie binary.
 --enable-trophie-socket=PATH      Full path to the trophie socket.
 --enable-ripmime=PATH             Full path to ripmime program.
 --enable-received=y|n             Simscan should add a received line showing
                                   the version of all scanners that checked
                                   the message
                                   NOTE: simscanmk -g must be run to build the                  
                                   version file.

These options are only needed when the received line option (--enabled-received=y) and the corresponding scanners are enabled as well:

 --enable-spamassassin-path=PATH   Path to the spamassassin binary
 --enable-clamavdb-path=PATH       Directory where the clamav master.cvd and
                                   daily.cvd files are saved
 --enable-sigtool-path=PATH        Path to the sigtool binary

Configuration Details

Below are more detailed descriptions of each configuration option.

--enable-user=<user>

  This option defines the user that simscan will run as.  By default, the
  user is 'simscan'.

--enable-clamav=y|n

  This option turns clamav scanning on or off.  When enabled, an incoming
  email will be rejected if a virus is detected in the email.

--enable-clamdscan=PATH

  This option defines the path to the clamdscan binary.  This is the full
  path to the binary.
  Note : This option does nothing when clamav is not enabled.

--enable-custom-smtp-reject=y|n

  This option turns custom smtp reject messages on and off.  When enabled
  simscan will place the virus name in the reject message if a virus is
  detected.
  Note : The qmail-queue-custom-error.patch is needed to make this option
         work properly.  You can find this patch in the contrib directory.
  Note 2 : Enabling dropmsg disables this option

--enable-per-domain=y|n

  This option turns per-domain scanning on and off.  Per domain scanning
  allows the administrator to explicitly state what scanning occurs for
  what domain.  In addition, attachment scanning can be enabled or disabled
  for each domain.  Details about how to set up per-domain scanning are
  below.

--enable-attach=y|n

  This option turns on attachment scanning.  Attachment scanning will block
  all attachments specified in /var/qmail/control/ssattach.  Attachment
  scanning is disabled by default.

--enable-spam=y|n

  Ths option turns spam scanning on and off.  When enabled, simscan allows
  mail over a certain spam threshold to be rejected back to the sender.

--enable-spam-passthru=y|n

  This option turns spam passthru on and off.  When enabled, email
  identified as spam via the X-Spam-Status: header will be passed on to the
  user instead of rejected.
  Note : Enabling spam-hits effectively disables this option.

--enable-spamc-user=y|n

  This option turns per-user spamassassin on or off.  When enabled, the email
  address of the first rcpt to user is sent to spamassassin.  This allows
  spamassassin to use customized rules and settings for that email.

--enable-spam-hits=number

  This option specifies the number of hits a spam must receive to be rejected
  by simscan.  This option defaults to 10 hits.
  Note : This option disables spam passthru

--enable-spamc=PATH

  This option specifies the full path to the spamc binary.

--enable-spamc-args=ARGS

  This option defines the arguments to pass to spamc.  Be sure to place quotes
  around the options you define.

--enable-dropmsg=y|n

  This option causes messaged to be dropped when a virus/spam is found, rather
  than returning a 5xx error to the sender.  This option is disabled by default.
  Note : This option overrides the Custom SMTP Reject option
  Note 2 : If SPAM Passthru is enabled, SPAM will NOT be dropped unless 
           spam-hits is enabled

--enable-quarantinedir=DIR

  This option defined a directory to keep spam and/or infected emails.  This
  option is disabled by default.

--enable-qmaildir=DIR

  This option defines the location of qmail.

--enable-workdir=DIR

  This option defines the location of the working directory.  Note : The
  default directory is /var/qmail/simscan

--enable-qmail-queue=PATH

  This option defines the full path and name of the qmail-queue program.
  Incoming mail is passed to this program after being scanned by SimScan.

--enable-trophie-path=PATH

  This option defines the full path to the trophie binary. This option is only
  necessary if the received line option (--enable-received) is chosen.

--enable-trophie-socket=PATH

  This option defines the path to the trophie socket.  Defining this option
  enables trophie scanning.

--enable-ripmime=PATH

  This option defines the path to the ripmime program.  This program is used
  to rip apart emails into files.

--enable-received=y|n

  Simscan adds a Received line, showing the runtime, and a version string of
  the scanners that checked the message:
  Received: by simscan 1.0.6 ppid: 25399, pid: 25400, t: 4.7007s
        scanners: attach: 1.0.6 clamav: 0.80/m:27/d:556 trophie: 7.000-1011/218/74141 spam: 2.63
  NOTE: You must run simscanmk -g to create the version file.

--enable-spamassassin-path=PATH

  This option defines the full path to the spamassassin binary.  This option
  is only necessary if the received line option (--enable-received) is chosen.

--enable-clamavdb-path=PATH

  This option defines the full path to clamav master.cvd and daily.cvd files.
  This option is only necessary if the received line option (--enable-received)
  is chosen.

--enable-sigtool-path=PATH

  This option defines the full path to the sigtool binary.  This option is only
  necessary if the received line option (--enable-received) is chosen.


Attachment blocking option

Attachments can be blocked. It is disabled by default. If you use the per domain scanning as well, look in that section of the documentation on how to enable the checking. With only the attachment blocker and NO per domain option it works like this:

Put the list of attachments in /var/qmail/control/ssattach. List each attachment on it's own line. For example:

 [user@mailserver user]$ cat /var/qmail/control/ssattach
 .jpg
 .mp3
 .scr
 .bat

Then configure with:

 ./configure --enable-attach (with any other options you want)
 make
 make install-strip


SpamAssassin options


There are four different ways to configure simscan with spamassassin

  • no spam processing
 This is the default. No spamassassin processing will be done.
  • Reject email at smtp level if spamassassin considers it spam. This might reject false positives (good email that looks like spam)
 --enable-spam
  • Reject email above a "score" of some number, like 15 and pass everything else through to the user.
 --enable-spam --enable-spam-hits=15
  • Do not reject anything. Pass the spamassassin processed message through to the user
 --enable-spam --enable-spam-passthru

In addtion you can enable per user preference processing for email with just one recipient. Add the following option

 --enable-spamc-user

Look at the rc.spamd sample startup script for spamd options that work with vpopmail and per user preferences. Also look at the contrib directory for patches to spamassassin to make vpopmail/per user processing work.


Trophie/Sophie option

To enable the trophie virus scanner :

 ./configure --enable-trophie-socket=/path/to/the/socket \
   --enable-trophie-path=/path/to/trophie/binary

As sophie uses the same interface it may work as well, but is not tested!


How SMTP rejection works

There are currently three options for handling SMTP rejection. SMTP rejection occurs when a virus is detected, or when the spam score is over the spam-hits level.

The default rejection option is to reject with a standard 5XX reject message. This rejection message looks as follows :

 554 mail server permanently rejected message (#5.3.0)

If --enable-dropmsg is used, messages are dropped with no rejection message. The connection is simply closed without warning.

If --enable-custom-smtp-reject is used, messages are rejected with a custom message. You will need to apply the qmail-queue-custom-error.patch patch located in the contrib directory in order to make this work.

For virus rejection, the message contains the name of the virus such as :

 Your email was rejected because it contains the Worm.Bagle.AU virus

For spam rejection, the message is more generic, merely stating that the message was rejected because it was considered spam :

 Your email is considered spam (53.5 spam-hits)

For attachment rejection, the message contains the name of the attachment :

 Your email was rejected because it contains a bad attachment: trojan.exe


Enable Per Domain processing

To enable per domain processing :

 ./configure --enable-per-domain (with any other options you want)
 make
 make install-strip

Edit the /var/qmail/control/simcontrol text file You can enable/disable clam/spam/trophie/attachments per domain and per user and set a default for the whole machine.

Here is an example:

 [user@mailserver user]$ cat /var/qmail/control/simcontrol
 postmaster@example.com:clam=yes,spam=no,attach=.txt:.com
 example.com:clam=no,spam=yes,attach=.mp3
 :clam=yes,spam=yes,trophie=yes,spam_hits=20.1

The third line sets the default for the whole machine to enable clam,trophie, spam scanning, and sets the reject level for spam hits to 20.1.

The second line sets clam off and spam on for the example.com domain and disallows .mp3 files for the attachment scanner

The first line sets clam on and spam off for postmaster@example.com and .txt and .com files for the attachment names.

The order of precedence is:

 email address (overrides all)
 domain (overrides default)
 default (only used if not overridden by domain or email address)

First the sender address will be looked up and then the recipients. Without any matches, no scans will be done.

Then run /var/qmail/bin/simscanmk to build the simcontrol.cdb file. You can rebuild this files at any time. The simscanmk program can safely update the cdb files while the system is running.

Qmail extensions are handled like this: the address is broken into their parts and are looked up. test-list-owner@test.ch looks up:

 test@test.ch
 test-list@test.ch
 test-list-owner@test.ch

Security

The simscan program is restricted to running setuid simscan to protect the rest of the system. It does all of it's work in the /var/qmail/simscan directory (default location).

The simscan program runs setuid simscan. It does all of it's work in the /var/qmail/simscan directory which is owned by simscan.


Permissions and ClamAntiVirus

To get ClamAV to play nicely with simscan's permissions you have two options:

  • run clamd as root
  • Add clamav to simscan's group.

Then clamav will have access to the working directory and it's files.

  1. The /var/qmail/simscan directory defaults to ownership to simscan.root. So change the group to 'simscan'.
  2. Set the sticky bit on the directory so when simscan creates it's temporary directories and files they are group owned simscan as well.
  3. Add the clamav user to the simscan group.

On Linux like systems:

  1. chgrp simscan /var/qmail/simscan
  2. chmod g+s /var/qmail/simscan
  3. usermod -G simscan clamav

Also make sure AllowSupplementaryGroups is set in your clamd.conf file so that the clamd daemon knows about the simscan group.

How to chain additional scanning programs with simscan

When simscan finishes it expects to call a program that reads the file descriptors like qmail-queue does. You can configure simscan to call a different program. By default the configure script picks up the path to your qmail-queue program, which is normally /var/qmail/bin/qmail-queue. Use this configure option:

 --enable-qmail-queue=PATH

where PATH is full path to qmail-queue program.

I do not know why you would want to have another scanning process happen, but you can sure configure it before compiling.


How to Disable/Enable simscan for smtp connections by IP ranges

Use the standard tcp.smtp text file to set or not set the QMAILQUEUE environment variable per IP ranges. qmails smtp server is normally run via tcpserver with the -x option to the constant database file tcp.smtp.cdb.

Example: Turn simscan off for our machines loopback address (127.*.*.*).
Disable it for hypothetical local linux users on the 192.168.1.* lan.
Enable it for hypothetical local windows users on the 192.168.2.* lan.
Finally, we enable simscan(clamav/spamassassin) for all those untrusted internet email senders.

Example tcp.smtp file contents:

 [user@mailserver user]$ cat tcp.smtp
 127.:allow,RELAYCLIENT=""
 192.168.1.:allow,RELAYCLIENT=""
 192.168.2.:allow,RELAYCLIENT="",QMAILQUEUE="/var/qmail/bin/simscan"
 :allow,QMAILQUEUE="/var/qmail/bin/simscan"

This tcp.smtp file then needs to be compiled into the tcp.smtp.cdb file using your systems method of generating it. If you only need the rules in the tcp.smtp file you can compile it with this command:

 [user@mailserver user]$ cd /to/directory/where/your/tcp.smtp.cdb file lives
 [user@mailserver user]$ /usr/local/bin/tcprules tcp.smtp.cdb tempfile < tcp.smtp

Once compiled, the rules take effect immediately. Actually it takes effect on every new smtp connection.


How to Disable/Enable simscan globally for all smtp connections

In some cases, is desirable to enable simscan for all SMTP connections, independently of the settings brought by the tcprules inside the tcp.smtp.cdb file.

To accomplish this, set the QMAILQUEUE variable directly in the script that activates the qmail-smtpd daemon, usually the /service/qmail-smtpd/run. Add the QMAILQUEUE variable to the begining of this script and export the variable. Like the example bellow:

!#/bin/bash
QMAILQUEUE="/var/qmail/bin/simscan"
export QMAILQUEUE
exec .. (normal startup here)

Temporary File Management

Simscan uses unique file names for the message, to/from headers and the optional spamassassin output. The files are created in the unique simscan work directory for this process. The files are unlinked along with the temporary work directory just before we hand/execl all the information to qmail-queue or on error exits.

        data          file name
      ____________   _______________
      message fd 0 = msg.X.Y.Z          where X = unix seconds
      to/from fd 1 = addr.X.Y.Z               Y = microseconds
      spamassassin = spamc.msg.X.Y.Z          Z = simscan process id

fd 0 and fd 1 normally come from the qmail smtp daemon.

Temporary files and directories are not deleted if the SIMSCAN_DEBUG environment variable is set.

Sample qmail startup script

rc.qmail in the contrib directory is our sample qmail startup script. It shows you how you can set the QMAILQUEUE environment variable to call simscan for all incoming SMTP email.

Notice that we run our smtp server as root. We also run clamd as root. If you are having permission problems you may want to consider running both as root.

Sample spamassassin startup script

rc.spamd in the contrib directory is a sample spamd startup script. It sets enables the vpopmail and per user perferences option. It also sets the spamd socket to /tmp/spamd.sock.

You must use the --enable-spamc-args="-U /tmp/spamd.sock" option to simscan if you use this startup script.

On the other hand if you are using Bill Shupp's toaster, and you are running qmail-smtp and therefore simscan as vpopmail:vchkpw you need to include clamav as a member of the vchkpw group:

vchkpw:!:89:clamav

and in your clamd.conf you must enable Allow Supplementary Groups in your cland.conf:

  1. Initialize supplementary group access (clamd must be started as root).
  2. Default disabled

AllowSupplementaryGroups

If clamd is running when you edit this file, you must kill it, and restart for the changes to take effect.

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