(→Attachment Blocking: - Ops, moving it back to where it was. This section has to be inside the "per domain" section)
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How does this work? If simscan detects a virus it exits with a special error number: 82. simscan will also write a special reject message to file descriptor 4. If qmail-smtpd receives that return code then it will attempt to read the reject message from file descriptor 4 and report that message back to the sender. This is a pretty nifty feature. What I like about it is, if the email is being sent by a real user, as opposed to a zombie pc sending out viruses, then the real user should receive back a bounce message telling them which virus they have on their machine. I know I would like to know if my machine had a virus that was going out in my email. But then again I do not use Microsoft products..
= SpamAssassin Processing =
= SpamAssassin Processing =
Revision as of 08:13, 6 July 2005
simscan was written for people who need a fast and efficent way to do virus scanning or spamassassin processing or attachment blocking
If you need more features than this you should consider using qmail-scanner. It is a perl program with many excellent features. However it places a higher load on the system. This is mostly due to using perl. The system has to envoke the perl process which is fairly large. simscan is less than 100K and uses the standard system libraries so it loads and runs much faster.
To enable attachment processing in simscan use the following configure option
You also need to install ripmime to extract the mime parts of the e-mail. If you installed ripmime in a place other than /usr/local/bin/ripmime, specify it's PATH in the following configure option:
Place your list of attachments to be blocked in the /var/qmail/control/ssattach file. The list should look something like this:
.scr .pif .bat
Each time an email comes in and simscan is started by qmail-smtpd, this file is read into an array. After simscan calls ripmime to break the email MIME parts into separate files in the tmp directory /var/qmail/simscan, and then simscan will check the list of attachments against the list of file names in the email. If the attachments are set to be blocked, Simscan will remove the temporary files and tell qmail-smtpd to reject the message.
In order to make file names and attachment names case insensitive, simscan forces everything to lower case. Then it does a reverse string comparision for each file name against each attachment name. If there is a match then the email is rejected and control passes back up to qmail-smtpd for the final email rejection.
Simscan will then report to the smtp log a message similar to:
simscan: IP-of-sender pid pid-of-simscan: invalid attachment: FileName from: FromAddress to: ToAddress
If custom rejection messages are enabled then qmail-smtpd will report a failure message similar to:
Your email was rejected because it contains a bad attachment: FileName
The main advantages of using attachment blocking are:
- You can block files regardless of whether they are virus infected or not. This becomes handy when new undetectable viruses appear on the Net and start spreading through, for example, .bat .scr and .pif files. Block these extensions permanently, users never need these files. If you are not using attachment blocking, these viruses will sneak in untill your antivirus software updated its virus database.
- attachment blocking is *very* lightweight. You don't have to spawn clamdscan, it doesn't have to scan stuff; It's just simply rejected. If you have a large volume of email going through your server this can make the difference between being fine with your current hardware, or needing to buy another server.
- Its an opportunity to reduce bandwidth and employees "play time" if you block the funny .mp3, .avi, .mpeg, etc, files.
If you are not interested in attachment blocking, just don't use '--enable-attach' configure option. Attachment blocking is disabled by default. You should then enable one antivirus scanner to scan your messages for viruses.
ClamAV processing is one of the simplist parts of simscan. The simscan configure script defaults to doing just clamav processing. If you are testing simscan for the first time you might want to start with just clamav processing. In the source code directory run just: ./configure ; make ; make install-strip. That should be enough to just enable clamav processing.
- A temporary working directory is created and simscan changes it's working directory there.
- The envelope (to and from addresses) are written to a file
- The email is written to another file
Clamav can break up mime encoded parts of the email and scan inside them to find viruses. Check your clamav.conf (pre 0.80 version) or your clamd.conf file to see if Clamav e-mail scanner is enabled. Look for the following section:
# Enable internal e-mail scanner. # Default: enabled #ScanMail
This is from a 0.80 installation. As you can see, email scanning is enabled by default. If you have disabled email scanning you will need ripmime processing. However, if email scanning is enabled and you're not interested in performing attachment blocking, you can save a little processing time by telling simscan to NOT run ripmime. Reconfigure your simscan with the --disable-ripmime option. Go back into your simscan source directory and run these commands:
- make distclean
- ./configure --disable-ripmime
- make install-strip
If everything goes well, you will now have disabled ripmime processing and saved yourself some system load.
If a virus is found then simscan removes the temporary working directory and all the files and reports a permenent failure back to qmail-smtpd. qmail-smtpd will then report the pemement error back to the sender and clean up all of the data that came in for that email. The result is the data with the virus is removed from the machine before it can cause any problems.
Optional smtp reject message
How does this work? If simscan detects a virus it exits with a special error number: 82. simscan will also write a special reject message to file descriptor 4. If qmail-smtpd receives that return code then it will attempt to read the reject message from file descriptor 4 and report that message back to the sender. This is a pretty nifty feature. What I like about it is, if the email is being sent by a real user, as opposed to a zombie pc sending out viruses, then the real user should receive back a bounce message telling them which virus they have on their machine. I know I would like to know if my machine had a virus that was going out in my email. But then again I do not use Microsoft products...
There are four ways to use spamassassin with simscan.
Do not run spamassassin
The simplist way to use spamassassin and simscan is not to use it at all. Some sites prefer to have spamassassin run on local email delivery. vpopmail has an --enable-spamassassin option that tells vdelivermail, the local vpopmail delivery agent, to send the email through spamassassin with vpopmail user options so individual users can set their own perferences. maildrop can do this as well (I think).
By having your local delivery agent run spamc you avoid the multiple recipient problem.
Multiple Recipient Problem with Per User simscan option
You can enable the use of spamassassin's per user option with this option:
If there is a single recipient in the email then simscan adds the -u user@domain option to spamc. This tells spamassassin to look up the users preferences to over ride the system wide spamassassin preferences.
But if there is more than one recipient then which user perference should simscan set? The solution simscan uses is if there is more than one recipient and the --enable-spamc-user option is set, then it skips adding the -u user@domain option to spamc.
This problem is avoided by having your local delivery agent do the spamassassin processing.
Scan and pass through to users
You can also set simscan to pass the email through spamassassin and send the result on to qmail. This is done with two options:
Do not use this option:
The --enable-spam-passthru option is ignored if you use:
Reject anything marked as spam
Another option is to have simscan reject email (smtp failure 500 response) when spamassassin detects anything marked as spam. Use this option
and do not use these options
With this setup, any email designated as spam with the X-Spam flag set to Yes, will be rejected at the smtp level.
Reject really bad spam and pass through anything else
Perhaps the most popular method is a mix of the others. Which is to reject email that scores very high and to pass through other email to the user even if it is marked as spam. This seems to help with the "false positive" issue; email marked as spam but not really spam. Use these options:
--enable-spam --enable-spam-hits=number where number is the spamassassin score at which you want to reject the email
Do not use this option:
Per domain processing
The per domain processing feature allows you to fine-tune scanning parameters on a per domain or per account (email address) base.
Enable this using --enable-per-domain when compiling simscan.
When this option is enabled, simscan is controlled by the simcontrol.cdb file, usually stored in /var/qmail/control. This file can be built, using simscanmk, from /var/qmail/control/simcontrol
Here is an example simcontrol file:
firstname.lastname@example.org: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 options that are available (in version 1.1) are:
clam=(yes|no) spam=(yes|no) trophie=(yes|no)
The clam, spam, and trophie options toggle whether or not to use those scanners for scanning - the binaries must have been available and their locations must have been configured when simscan was compiled.
Processing order of options
The order of precedence is:
email address (overrides all) domain (overrides default) default (only used if not overridden by domain or email address.
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.
email@example.com firstname.lastname@example.org email@example.com
To enable regex matching you have to configure simscan using --enable-regex. You can have only one regular expression per "perdomain" match line. You can however seperate multiple regexes using a semicolon (:).
If you want to match on a semicolon you have to use \072 or \x3a in the regex.
The regex engine is libpcre, so refer to the pcre documentation for the patterns. Basically it takes perl regexes.
The regex option tries to match every line of the mail with all the regular expressions. If it matches, it refuses the mail. If you enabled the custom reject option, it tells you the regex number it matched.
A single regex can be 500 chars at most, and you can have 100 regexes per run applied.
- Creates three regexes to match against
- The (?i) at the beginning of the regex makes the regex a case insensitive match
- If one of these three regexes matches against any line of the mail, the mail is refused
- This regex was created to block some sober variant cheaper (system resource wise) than with a virus scanner.
The attach option is for banning specific attachment types, and is only available if --enable-attach was used at compile-time. The string must be a colon-separated list of extensions, like in the first line of the example above.
The spam_hits option changes the threshold spamassassin spam value. If a mail gets a spam score that number or above, it is rejected, otherwise it is accepted.
The spam_passthru option is equivalent to the --enable-spam-passthru configure option.
IMPORTANT: if you use the --enable-per-domain option, the --enable-spam-passthru option is IGNORED. You MUST add spam_passthru=yes to any and all domains you wish to use it with.
Drop Message option
Some sites have security policies in place which require them to accept every email. For these sites there is an option to do all the normal simscan processing, but if a virus or spam is detected the message is not handed to qmail-queue for local delivery. Instead it is silently dropped.
Use this option when configuring simscan
Custom Reject Option