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postfix反垃圾邮件

postfix反垃圾邮件

本文摘抄自:
https://www.linuxbabe.com/mail-server/block-email-spam-check-header-body-with-postfix-spamassassin
I’m going to share more tips and tricks to block email spam in this article. Specifically, we will see how to check email header and body with Postfix and SpamAssassin (SA) to detect spam. SpamAssassin is a free, open-source, flexible and powerful spam-fighting tool.
Email Header and Body Checks with Postfix SMTP Server

Postfix provides 4 simple content checking parameters.

header_checks
mime_header_checks
nested_header_checks
body_checks

Postfix will check all inbound emails when any of the above parameters is being used. Each parameter points to a lookup table containing regular expression patterns and actions. The patterns are compared to strings within email messages (header and body). If Postfix finds a match, the specified action is executed. Header and body checks are done by the Postfix cleanup daemon.

There are mainly two types of regular expressions that can be used by Postfix.

regexp: POSIX regular expression
PCRE: Perl compatible regular expression

Postfix comes with POSIX regular expression support, but PCRE is way faster. To use PCRE in Postfix, you need to install the postfix-pcre package.

sudo apt install postfix-pcre

Run the following command and you will see pcre is now supported.

postconf -m

postfix pcre perl compatible regular expression
Header Checks

To enable header_checks in Postfix, open the main configuration file.

sudo nano /etc/postfix/main.cf

Add the following line at the end of the file.

header_checks = pcre:/etc/postfix/header_checks

Save and close the file. Then you need to create the /etc/postfix/header_checks lookup file with a command line text editor such as Nano.

sudo nano /etc/postfix/header_checks

You can add regular expression checking like below.

/free mortgage quote/ REJECT
/repair your credit/ REJECT

The lefthand key is a regular expression enclosed by two forward slashes. If any of the strings on the leftland appear in any of the headers of an email message (these would most likely show up in the Subject: header), the message is rejected during the SMTP dialog. By default regular expression checking is not case-sensitive.

You can also use DISCARD, instead of REJECT.

/free mortgage quote/ DISCARD
/repair your credit/ DISCARD

This will cause Postfix to claim successful delivery and silently discard the message. DISCARD makes it look as if the message was delivered even though it was simply thrown away. I often use DISCARD when I don’t want the spammer to know I have blocked a certain phrase for incoming email. DISCARD can also be useful to minimize the backscatter problem. If an innocent user’s email address is used as the sender address, you can claim successful delivery, so that the innocent user does not receive bounce messages.

Some stupid spammers use multiple email addresses in the To: header, instead of using Blind Carbon Copy (BCC). If you are sure an email address won’t be accepting emails with multiple recipients in the To: header, you can add the following lines to discard such email.

/To:.(gmail.com|yahoo.com|outlook|hotmail.com).you@yourdomain.com/ DISCARD
/To:.you@yourdomain.com.(gmail.com|yahoo.com|outlook|hotmail.com)/ DISCARD

The above lines will check if an Gmail/Yahoo/Outlook/Hotmail address and your domain email address are in the To: header at the same time. If true, the email will be discarded. The two characters .* are a wild card in regular expressions that can be matched to any characters.

Some spammers use blank email address in the From: or To: header, you can add the following checks.

/To:.<>/ DISCARD
/From:.
<>/ DISCARD

Once you finish editing the header_checks lookup file, you need to build the index file.

sudo postmap /etc/postfix/header_checks

Then restart Postfix for the changes to take effect.

sudo systemctl restart postfix

Body Checks

In addition to header checks, Postfix can check the body of an email message. To enable body_checks in Postfix, open the main configuration file.

sudo nano /etc/postfix/main.cf

Add the following line at the end of the file.

body_checks = pcre:/etc/postfix/body_checks

Save and close the file. Then you need to create the /etc/postfix/body_checks lookup file.

sudo nano /etc/postfix/body_checks

You can add regular expression checking like below.

/free mortgage quote/ REJECT
/repair your credit/ REJECT

You can use DISCARD, instead REJECT.

/free mortgage quote/ DISCARD
/repair your credit/ DISCARD

The patterns indicated by the body_checks parameter are checked against each line of the body of the message. If any of the strings on the leftland appear in the body of an email message, the message is rejected or discarded. Once you finish editing the body_checks lookup file, you need to build the index file.

sudo postmap /etc/postfix/body_checks

Then restart Postfix for the changes to take effect.

sudo systemctl restart postfix

SpamAssassin Content Filter

The built-in content checking in Postfix is very simple. However, there is no way to whitelist individual messages that you might want to receive despite their containing phrases that trigger a rejection and you might not want to reject or discard an email message based on a single rule. For more sophisticated analysis, we need to use a dedicated content filter (such as SpamAssassin) specifically designed to detect spam.

SpamAssassin is a score-based system. It will check email message against a large set of rules, in contrast to a single rule in Postfix. Each rule adds or removes points in the message’s score. If the score is high enough (by default 5.0), the message is considered spam.
Install SpamAssassin

Note: If you used iRedMail to set up your mail server, then SpamAssassin is already installed alongside Amavis, which can read SpamAssassin rules. You don’t need to follow the instructions in this section.

Run the following command to install SpamAssassin from the default Ubuntu software repository. Spamc is the client for SpamAssassin spam filtering daemon.

sudo apt install spamassassin spamc

During the installation, the debian-spamd user and group will be automatically created. The binary installed by the spamassassin package is called spamd, which will be listening on TCP port 783 on local host.

spamassassin content filter

By default, the spamassassin systemd service is disabled, you can enable auto start at boot time with:

sudo systemctl enable spamassassin

Then start SpamAssassin.

sudo systemctl start spamassassin

Integrate SpamAssassin with Postfix SMTP Server as a Milter

There are several ways you can use to integrate SpamAssassin with Postfix. I prefer to use SpamAssassin via the sendmail milter interface, because it allows me to reject an email when it gets a very high score such as 8, so it will never be seen by the recipient.

Install the spamass-filter packages from Ubuntu default software repository.

sudo apt install spamass-milter

Next, edit /etc/postfix/main.cf file and add the following lines at the end of the file.

milter_default_action = accept
milter_protocol = 6
smtpd_milters = local:spamass/spamass.sock
non_smtpd_milters = $smtpd_milters

If you have configured OpenDKIM and OpenDMARC, then theses lines should look like below. The order matters.

milter_default_action = accept
milter_protocol = 6
smtpd_milters = local:opendkim/opendkim.sock,local:opendmarc/opendmarc.sock,local:spamass/spamass.sock
non_smtpd_milters = $smtpd_milters

Save and close the file. Now open the /etc/default/spamass-milter file and find the following line.

#OPTIONS=”${OPTIONS} -r 15″

Uncomment this line and change 15 to your preferred reject score such as 8.

OPTIONS=”${OPTIONS} -r 8″

If the score of a particular email is over 8, Spamassassin would reject it and you would find a message like below in the /var/log/mail.log file, indicating it’s rejected.

milter-reject: END-OF-MESSAGE 5.7.1 Blocked by SpamAssassin

If you want the sender to see a different reject text, then add the -R (reject text) option like below.

OPTIONS=”-u spamass-milter -i 127.0.0.1 -R SPAM_ARE_NOT_ALLOWED_HERE”

Save and close the file. Restart Postfix and Spamass Milter for the changes to take effect.

sudo systemctl restart postfix spamass-milter

Note: iRedMail users need to start the SpamAssassin service in order to use spamass-milter.

sudo systemctl start spamassassin
sudo systemctl enable spamassassin

Checking Email Header and Body with SpamAssassin

SpamAssassin ships with many spam detection rules in /usr/share/spamassassin/ directory. Allow me to explain some of the rules.

In the /usr/share/spamassassin/20_head_tests.cf file, you can find the following two lines.

header MISSING_HEADERS eval:check_for_missing_to_header()
describe MISSING_HEADERS Missing To: header

The first line tests if the To: header exists in an email message. The second line, which is optional, explains what the first line does. The uppercase letters is the name of this test.

The following 3 lines are for testing if there’s a Date: header in the email message.

header __HAS_DATE exists:Date
meta MISSING_DATE !__HAS_DATE
describe MISSING_DATE Missing Date: header

And these 3 lines are for testing if there’s a From: header in the email message.

header __HAS_FROM exists:From
meta MISSING_FROM !__HAS_FROM
describe MISSING_FROM Missing From: header

You might want to use the Cron job shipped with SpamAssassin to automatically update SpamAssassin’s rules on a daily basis. If so, open the /etc/default/spamassassin file and change CRON=0 to CRON=1.
Set Custom Score for Existing Rules

In the 50_scores.cf and 72_scores.cf file, you can see the default scores for various tests. If you think the default score is too low or too high for a certain test, you can set custom score in /etc/spamassassin/local.cf file.

sudo nano /etc/spamassassin/local.cf

For example, RFC 5322 requires that every email message must have From: and Date: header fields, so I can set a very high score if either of them is missing in an email message by appending the following two lines in local.cf file.

score MISSING_FROM 5.0
score MISSING_DATE 5.0

Although the To: header field is not mandatory in RFC 5322, I prefer to set a high score if it’s missing in an email message because I have never seen a legitimate email missing this header field.

score MISSING_HEADERS 3.0

Some spammers uses two email addresses in the From: header field like below.

From: “tonywei_darefly_mold@aliyun.com” sales10@darefly-mould.com

I think the default score for this kind of email is low, I prefer to set it to 3.0.

score PDS_FROM_2_EMAILS 3.0

There are spammers who send empty message with no subject and no textual parts in the body. I set the score for this kind of email to 5.0, so it will be placed to spam folder. Why read it if it’s empty?

score EMPTY_MESSAGE 5.0

And other spammers often ask you to send a read receipt, I set the score to 2.0 for this kind of email.

score FREEMAIL_DISPTO 2.0

There are some spammers use different domain names in the From: and Reply-To: header, I give them a 3.5 score.

score FREEMAIL_FORGED_REPLYTO 3.5

I also have seen some spammers using non-existent domain name in the From: header field. I set a 5.0 score for this type of email.

score DKIM_ADSP_NXDOMAIN 5.0

Last but not least, many spammers spoof the gmail.com domain in the From: header field. I set a 2.5 score this kind of email.

score FORGED_GMAIL_RCVD 2.5

Adding Your Own Rules

You can add custom SpamAssassin rules in /etc/spamassassin/local.cf file.

sudo nano /etc/spamassassin/local.cf

Header Rules

For example, some spammers use the same email address in the From: and To: header, you can add the following lines at the end of the file to add scores to such emails.

header FROM_SAME_AS_TO ALL=~/\nFrom: ([^\n]+)\nTo: \1/sm
describe FROM_SAME_AS_TO From address is the same as To address.
score FROM_SAME_AS_TO 2.0

Some spammers use an empty address for the Envelope From address (aka the Return Path header). Although this is legitimate for sending bounce messages, I prefer to give this kind of email a score.

header EMPTY_RETURN_PATH ALL =~ /<>/i
describe EMPTY_RETURN_PATH empty address in the Return Path header.
score EMPTY_RETURN_PATH 3.0

If you have configured OpenDMARC on your mail server, you can now add the following lines to add scores to emails that fail DMARC check.

header CUSTOM_DMARC_FAIL Authentication-Results =~ /dmarc=fail/
describe CUSTOM_DMARC_FAIL This email failed DMARC check
score CUSTOM_DMARC_FAIL 3.0

The above code tells SpamAssassin to check if the Authentication-Results header contains the string “dmarc=fail”. If found, increase the score by 3.0.
Body Rules

You can tell SpamAssassin to increase the score of an email if a certain phrase is found in the body. For example, many spammers use the recipient’s email address in the first body line like below.

Hi xiao@linuxbabe.com
Hello xiao@linuxbabe.com
Dear xiao@linuxbabe.com

I don’t want to talk with people who doesn’t bother writing my name in the first line of email. So I created a rule in SpamAssassin to filter this kind of email.

body BE_POLITE /(hi|hello|dear) xiao\@linuxbabe.com/i
describe BE_POLITE This email doesn’t use a proper name for the recipient
score BE_POLITE 5.0

Regular expression in SpamAssassin is case-sensitive by default, you can add the i option at the end to make it case-insensitive.
Add Negative Scores

You can also add negative score to good emails, so there will be less false positives. For example, many of my blog readers ask me Linux questions and I don’t think spammers would include words like Debian, Ubuntu, Linux Mint in the email body, so I created the following rule.

body GOOD_EMAIL /(debian|ubuntu|linux mint|centos|red hat|RHEL|OpenSUSE|Fedora|Arch Linux|Raspberry Pi|Kali Linux)/i
describe GOOD_EMAIL I don’t think spammer would include these words in the email body.
score GOOD_EMAIL -4.0

If the email body contains a Linux distro’s name, then add a negative score (-4.0).

There are some common phrases that is included in legitimate bounce messages, so I can add negatives scores to these email messages.

body BOUNCE_MSG /(Undelivered Mail Returned to Sender|Undeliverable|Auto-Reply|Automatic reply)/i
describe BOUNCE_MSG Undelivered mail notifications or auto-reply messages
score BOUNCE_MSG -1.5

Note that body rules also include the Subject as the first line of the body content.
Meta Rules

In addition to header and body rules, there’s also meta rules. Meta rules are combinations of other rules. You can create a meta rule that fires off when two or more other rules are true. For example, I occasionally receive emails saying that the sender wants to apply for a job and a resume is attached. I have never said on my website that I need to hire people. The attachment is used to spread virus. I created the following meta rule to filter this kind of email.

body __RESUME /(C.V|Resume)/i
meta RESUME_VIRUS (__RESUME && __MIME_BASE64)
describe RESUME_VIRUS The attachment contains virus.
score RESUME_VIRUS 5.5

The first sub rule __RESUME tests if the email body contains the word C.V. or resume. The second sub rule __MIME_BASE64 is already defined in /usr/share/spamassassin/20_body_tests.cf file, as follows, so I don’t need to define it again in local.cf file. This rule tests if the email message includes a base64 attachment.

rawbody __MIME_BASE64 eval:check_for_mime(‘mime_base64_count’)
describe __MIME_BASE64 Includes a base64 attachment

My meta rule RESUME_VIRUS will fire off when both of the sub rules are true, adding a 5.5 score to the email message. Note that sub rule often starts with double underscore, so it has no score in its own right.

Now you learned how to add score if a string is found. What if you want to add score when a string doesn’t exist in the email headers? Well, you can use the ! operator. For example, I have seen spammers using a single word in the From: address. I added the following lines to score this kind of email.

header __AT_IN_FROM From =~ /\@/
meta NO_AT_IN_FROM !__AT_IN_FROM
score NO_AT_IN_FROM 4.0

The first line checks if the @ sign exists in the From: header. The second line defines a meta rule, which fires off when !__AT_IN_FROM is true. !__AT_IN_FROM rule is the opposite of the first header rule, which means when there’s no @ sign in the From: address, the meta rule fires off.

You can also add the following lines to check if a dot exists in the From: address.

header __DOT_IN_FROM From =~ /./
meta NO_DOT_IN_FROM !__DOT_IN_FROM
score NO_DOT_IN_FROM 4.0

Whitelist

You can use the whitelist_from parameter to add a particular email address or domain to your Spamassassin whitelist. For example, add the following two lines at the end of local.cf file.

whitelist_from xiao@linuxbabe.com
whitelist_from *@canonical.com

A whitelisted sender has a -100 default score. They will still be tested by SpamAssassin rules, but it’s super hard for them to reach a 5.0 score.
Blacklist

To blacklist a sender, use the blacklist_from parameter, which has the same format as whitelist_from.

blacklist_from spam@example.com
blacklist_from *@example.org

Checking Syntax and Restart

After saving the local.cf file. You should run the spamassassin command in lint mode to check if there’s any syntax errors.

sudo spamassassin –lint

Then restart SpamAssassin for the changes to take effect. (If you use Amavis with Spamassasin as in iRedMail, you just need to restart Amavis: sudo systemctl restart amavis)

sudo systemctl restart spamassassin

SpamAssassin’s Builtin Whitelist

It’s worth mentioning that SpamAssassin ships with its own whitelist. There are several files under /usr/share/spamassassin/ directory that includes 60_whitelist in the filename. These files contain SpamAssassin’s builtin whitelist. For example, the 60_whitelist_spf.cf file contains a list of addresses which send mail that is often tagged (incorrectly) as spam.
Move Spam into the Junk Folder

Note: iRedMail has this configured out-of-the-box.

I’m going to show you how to move spam to Junk folder with the Dovecot IMAP server and the sieve plugin. This method requires that inbound emails are delivered to the message store via the Dovecot “deliver” LDA (local delivery agent). If you can find the following text in /var/log/mail.log file, then this requirement is satisfied.

postfix/lmtp

or

delivered via dovecot service

Run the following command install dovecot-sieve from Ubuntu software repository.

sudo apt install dovecot-sieve

This package installs two configuration files under /etc/dovecot/conf.d/ directory: 90-sieve.conf and 90-sieve-extprograms.conf. Open the 15-lda.conf file.

sudo nano /etc/dovecot/conf.d/15-lda.conf

Add the sieve plugin to local delivery agent (LDA).

protocol lda {
# Space separated list of plugins to load (default is global mail_plugins).
mail_plugins = $mail_plugins sieve
}

Save and close the file. If you can find the 20-lmtp.conf file under /etc/dovecot/conf.d/ directory, then you should also enable the sieve plugin in that file like below.

protocol lmtp {
mail_plugins = quota sieve
}

Then open the 90-sieve.conf file.

sudo nano /etc/dovecot/conf.d/90-sieve.conf

Go to line 79 and add the following line, which tells Sieve to always execute the SpamToJunk.sieve script before any user specific scripts.

sieve_before = /var/mail/SpamToJunk.sieve

Save and close the file. Then create the sieve script.

sudo nano /var/mail/SpamToJunk.sieve

Add the following lines, which tells Dovecot to move any email messages with the X-Spam-Flag: YES header into Junk folder.

require “fileinto”;

if header :contains “X-Spam-Flag” “YES”
{
fileinto “Junk”;
stop;
}

Save and close the file. We can compile this script, so it will run faster.

sudo sievec /var/mail/SpamToJunk.sieve

Now there is a binary file saved as /var/mail/SpamToJunk.svbin. Finally, restart dovecot for the changes to take effect.

sudo systemctl restart dovecot

Set Message Maximum Size

By default, SpamAssassin does not check messages with attachments larger than 500KB, as indicated by the following line in the /var/log/mail.log file.

spamc[18922]: skipped message, greater than max message size (512000 bytes)

The default max-size is set to 512000 (bytes). A high value could increase server load, but I think the default size is a little bit small. To increase the max-size, edit /etc/default/spamass-milter file and add the following lines at the end.

#Spamc options
OPTIONS=”${OPTIONS} — –max-size=5120000″

The empty — option tells spamass-milter to pass all remaining options to spamc, which understands the –max-size option. I increased the size to 5000KB. Save and close the file. Then restart spamass-milter.

sudo systemctl restart spamass-milter

How to Configure Individual User Preferences

You may want to set custom rules for emails sent to a specific address on the mail server. I like this feature very much. I have a contact email address for this blog, which is only used for keeping contact with readers. I don’t use the contact email address elsewhere, so I can create special spam-filtering rules that apply only to this contact email address.

First, edit the SpamAssassin main configuration file.

sudo nano /etc/spamassassin/local.cf

Add the following line to allow user rules.

allow_user_rules 1

Save and close the file. Next, edit the SpamAssassin environment file.

sudo nano /etc/default/spamassassin

Find the following line.

OPTIONS=”–create-prefs –max-children 5 –helper-home-dir”

We need to change it to

OPTIONS=”–create-prefs –max-children 5 –helper-home-dir –nouser-config –virtual-config-dir=/var/vmail/%d/%l/spamassassin –username=vmail”

Where:

--nouser-config: disable per-user configuration file for local Unix users.
--virtual-config-dir: specify the per-user configuration directory for virtual users. The %d placeholder represents the domain part of email address and %l represents the local part of email address.
--username: run spamd as the vmail user.

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