Avoiding Spams with SPF and greylisting within Exim

A year ago, I posted an article describing a way to slay Spams with both Bogofilter and SpamAssassin embedded in exim. This method was proven effective for my mailboxes:  since then, during a timespan of one year, Bogofilter caught ~ 85 % of actual spams, SpamAssassin (called only if mail not already flagged defavorably by Bogofilter) caught ~ 15 %. Do the math, I had almost none to flag by hand.

Why would I change such setup? For fun, obviously 🙂

Actually, I made no change, I just implemented SPF (Sender Policy Framework) and greylisting.

I noticed that plenty of spams were sent to my server @thisdomain claiming to be sent by whoever@thisdomain. These dirty spams were easily caught by the duo Bogofilter / SpamAssassin, but still, it annoyed me that @thisdomain was misused. SPF allows, using DNS records, to list which servers/computers are allowed to send mails from addresses @thisdomain.   SPF checks are predefined in Exim out of the box, so I’ll skip its configuration. The relevant DNS record (with bind9), allowing only two boxes (primary and secondary mail servers) designated by their IP to send mails @thisdomain, looks like:

thisdomain. IN  TXT  "v=spf1 ip4:78.249.xxx.xxx ip4:86.65.xxx.xxx -all"

Result: Since I implemented SPF on my domains, there was no change in the number of spam caughts. However, during this period, my primary server list of temporary bans dropped from 200/100 IPs to 40/20 IPs. I cannot pinpoint with certainty the cause of this evolution because the temporary bans list depends on plenty of things. But, surely, pretending to send mails @thesedomainsgrilledbySPF surely lost some interest for spambots. Implementing SPF is actually not about helping ourselves directly but indirectly: reducing effectiveness of spambots helps everybody.

I use greylisting on my secondary mail server since a while and I noticed over years that this one almost never had to ban IPs. Not that he never received spam, but that he almost never received mails from very obvious spam sources identified at STMP time.  Seems that most very obvious spam sources never insist enough to pass through greylisting. I guess that most spambots are coded to skip any mail server that does not immediatly accept a proper SMTP transaction, because it has no time to waste, considering how little is the percentage of spams sent actually reaching someone real.

This greylisting use the following files (an assumes memcached and libcache-memcached-perl are properly installed):

So I gave a try using greylist my primary mail server, but with a very short waiting time, because 5 minutes, for example, to receive mail from a not-yet-known  source is not acceptable. So I edited the relevant conf.d/main/ file to GREY_MINUTES = 0.5 and GREY_TTL_DAYS = 25.

Result: no changes regarding the number of caught spams. However, like on the secondary mail server, the number of banned IPs is near to none. Looks like most obvious spam sources don’t wait even only 30 seconds – actually, it’s a very acute choice as they would be anyway banned if they did.

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Slaying Spams with both Bogofilter and SpamAssassin embedded in exim

Ads are spam. Good thing with the internet’s ads is that you can set up countermeasures.

(Disclaimer: yes, there is nothing new here, just an example of setup)

I have plenty of email addresses from different providers, some are definitely history. I could go through the websites of all of these and set up forwarding for the one I no longer use but still want to be able to get mail from, just in case. Well, I would do that if I was using my mail client to fetch mails – because otherwise fetching mails would actually take ages.

But, as I have a local home underclocked 🙂 server, I find way easier and potent to, instead, use ESR’s fetchmail to download them all to a single account that is accessed by my mail client through IMAPS. I have a /etc/fetchmailrc like:

poll pop.free.fr with proto POP3
user 'XXX' there with password 'XXX' is 'localuser' here
poll imap.gmail.com with proto IMAP
user 'XXX@gmail.com' there with password 'XXX' is 'localuser' here with ssl
user 'XXZ@gmail.com' there with password 'XXZ' is 'localuser' here with ssl

Fetchmail download mails than then relies on the installed SMTP, which is Exim, to deliver it to end user account mailbox accessible through IMAPS.

What’s so nifty nifty about? Well, mails will also be filtered for spam. As it happens on the local home server, it will be unnoticeable for the end user that is me. We’ll use several anti-spam tools, not caring about redundancy and time-consumption: DNSBLs, Bogofilter, SpamAssassin, razor2.

So, here we go. Note that Exim (exim4) in Debian use the user Debian-exim. localuser is the recipient end-user, it belongs to the group localuser name after himself.
We will add Debian-user to the group localuser and create a system group dedicated to spamchecking to easily share bayesian databases.:

# addgroup --system spamslayer
# adduser Debian-exim spamslayer
# adduser Debian-exim localuser
# adduser localuser spamslayer

* Bogofilter is a bayesian spam filter . It is said to be faster and lesser time consuming than the SpamAssassin’s own bayesian filter so will run mails through it first. It is installed with the debian package.

Edit /etc/bogofilter.cf as follows:

bogofilter_dir=/var/lib/bogofilter
db_transaction=yes

The bayes directory must be created by hand:

# mkdir /var/lib/bogofilter
# chgrp spamslayer /var/lib/bogofilter
# chmod 2777 /var/lib/bogofilter

* SpamAssassin is a powerful, at the cost of time-consumption, spam-killer. It is installed with the debian package.

In the following site-wide config /etc/spamassassin/local.cf, I use bayesian filters, razor2, several DNSBLs and I adjust some tests according to my needs:

# Save spam messages as a message/rfc822 MIME attachment instead of
# modifying the original message (0: off, 2: use text/plain instead)
#
# Keep as it is because bogofilter would not learn properly otherwise,
# as it cannot distinguish report from the spam.
report_safe 0
# Set which networks or hosts are considered 'trusted' by your mail
# server (i.e. not spammers)
#
trusted_networks 192.168.1.
# Locales
#
# (I only receive mails in English or French)
ok_locales en fr
# Set the threshold at which a message is considered spam (default: 5.0)
#
required_score 3.3
# Use Bayesian classifier (default: 1)
#
# (I created the relevant directory)
use_bayes 1
bayes_file_mode 0777
bayes_path /var/lib/spamassassin-bayes/bayes
score BAYES_20 0.3
score BAYES_40 0.5
score BAYES_50 0.8
score BAYES_60 1
score BAYES_80 2
score BAYES_95 2.5
score BAYES_99 6
# Bayesian classifier auto-learning (default: 1)
#
# (I may change that, not sure about it)
bayes_auto_learn 1
# Set headers which may provide inappropriate cues to the Bayesian
# classifier
#
bayes_ignore_header X-Bogosity
bayes_ignore_header X-Spam-Flag
bayes_ignore_header X-Spam-Status
# use razor
# (/etc/razor is the standard debian path)
use_razor2 1
razor_config /etc/razor/razor-agent.conf
score RAZOR2_CF_RANGE_51_100 3.2
# some rbl checks are already made by exim, at RCPT time, not all.
skip_rbl_checks 0
rbl_timeout 30
score RCVD_IN_SBL 15
score RCVD_IN_XBL 15
score RCVD_IN_SORBS_HTTP 15
score RCVD_IN_SORBS_SOCKS 15
score RCVD_IN_SORBS_MISC 15
score RCVD_IN_SORBS_SMTP 15
score RCVD_IN_SORBS_ZOMBIE 15
# adjust some tests scores: lower DUL test
score FROM_ENDS_IN_NUMS 0.2
score FROM_HAS_MIXED_NUMS 0.2
score FROM_HAS_MIXED_NUMS3 0.2
score RCVD_IN_NJABL_DUL 0.1
score RCVD_IN_SORBS_DUL 0.1
# lower stupid test
score DNS_FROM_SECURITYSAGE 0.0
# adjust some tests scores
score FAKE_HELO_HOTMAIL 3
score FORGED_HOTMAIL_RCVD 3
score HTML_FONT_BIG 2.4
score NO_REAL_NAME 2
score RCVD_IN_BL_SPAMCOP_NET 3
score SUBJ_ILLEGAL_CHARS 4.8
score EXTRA_MPART_TYPE 2.8
score SUBJ_ALL_CAPS 2.6
# increase all scores related to drugs: what do I care, duh
score DRUGS_ANXIETY 5
score DRUGS_ANXIETY_EREC 5
score DRUGS_ANXIETY_OBFU 5
score DRUGS_DIET 5
score DRUGS_DIET_OBFU 5
score DRUGS_ERECTILE 5
score DRUGS_ERECTILE_OBFU 5
score DRUGS_MANYKINDS 10
score DRUGS_MUSCLE 5
score DRUGS_PAIN 5
score DRUGS_PAIN_OBFU 5
score DRUGS_SLEEP 5
score DRUGS_SLEEP_EREC 5
score DRUGS_SMEAR1 5
# same goes for porn
score AMATEUR_PORN 5
score BEST_PORN 5
score DISGUISE_PORN 5
score DISGUISE_PORN_MUNDANE 5
score FREE_PORN 5
score HARDCORE_PORN 5
score LIVE_PORN 5
score PORN_15 5
score PORN_16 5
score PORN_URL_MISC 5
score PORN_URL_SEX 5
score PORN_URL_SLUT 5

The bayes directory must be created:

# mkdir /var/lib/spamassassin-bayes
# chown Debian-exim /var/lib/spamassassin-bayes
# chmod 0777 /var/lib/spamassassin-bayes

Obviously, it implies that razor2 must be properly installed. We install the debian package then set it up. Remember it must run with user Debian-exim, so we do:

# chown -R Debian-exim:spamslayer /etc/razor
# su Debian-exim
$ razor-admin -home=/etc/razor -register
$ razor-admin -home=/etc/razor -create
$ razor-admin -home=/etc/razor -discover

To save ressources, we start SpamAssassin as a daemon (spamd), that will be called using its specific client (spamc). Before using the initd script, edit as follows /etc/defaut/spamassassin:

# Change to one to enable spamd
ENABLED=1
# SpamAssassin uses a preforking model, so be careful! You need to
# make sure --max-children is not set to anything higher than 5,
# unless you know what you're doing.
OPTIONS="--create-prefs --max-children 5 --helper-home-dir -u Debian-exim -g spamslayer"
# Cronjob
# Set to anything but 0 to enable the cron job to automatically update
# spamassassin's rules on a nightly basis
CRON=1

All that being do, you’ll want to (re)start the daemon with the relevant initd script (/etc/init.d/spamassassin restart here).

* Now we’ll tune Exim to call all by himself first Bogofilter and then SpamAssassin, if necessary only. We use splitted configuration in /etc/exim4/conf.d/. That is debian-specific I think but it does make any difference anyway.

First we define useful transports in /etc/exim4/conf.d/transport/35_spamblock (the name 35_spamblock is arbitrary and the number does not matter here):

spamslay_bogofilter:
driver = pipe
command = /usr/sbin/exim4 -oMr spamslayed-bogofilter -bS
use_bsmtp = true
transport_filter = /usr/bin/bogofilter -l -p -e
home_directory = "/tmp"
current_directory = "/tmp"
# must use a privileged user to set $received_protocol
# on the way back in!
user = Debian-exim
group = spamslayer
log_output = true
return_fail_output = true
return_path_add = false
message_prefix =
message_suffix =
#
spamslay_spamd:
driver = pipe
command = /usr/sbin/exim4 -oMr spamslayed-spamd -bS
use_bsmtp = true
transport_filter = /usr/bin/spamc
home_directory = "/tmp"
current_directory = "/tmp"
# must use a privileged user to set $received_protocol
# on the way back in!
user = Debian-exim
group = spamslayer
log_output = true
return_fail_output = true
return_path_add = false
message_prefix =
message_suffix =

Second we define routers, here in /etc/exim4/conf.d/router/450_spamblock – the order matters, here it is just after 400_exim4-config_system_aliases and before 500_exim4-config_hubuser:

# spam checking
# first bogofilter
spamslay_router_bogofilter:
debug_print = "R: bogofilter for $local_part@$domain received with protocol $received_protocol with X-Spam-Flag=$h_X-Spam-Flag and X-Bogosity=$h_X-Bogosity"
# When to scan a message :
# - it isn't already flagged as spam
# - it has not yet been spamslayed at all
# - it isn't local ($received_protocol eq "" or local)
condition = "${if and{ {!eqi{$h_X-Spam-Flag:}{yes}} {!eq{$received_protocol}{spamslayed-bogofilter}} {!eq{$received_protocol}{spamslayed-spamd}} {!eq{$received_protocol}{local}} {!eq{$received_protocol}{}} }}"
driver = accept
transport = spamslay_bogofilter
#
# second spamd
spamslay_router_spamd:
debug_print = "R: spamd for $local_part@$domain received with protocol $received_protocol with X-Spam-Flag=$h_X-Spam-Flag and X-Bogosity=$h_X-Bogosity"
# When to scan a message :
# - it isn't already flagged as spam
# - it has not yet been spamslayed with SA
# - it isn't local ($received_protocol eq "" or local)
condition = "${if and { {!eqi{$h_X-Spam-Flag:}{yes}} {!match{$h_X-Bogosity:}{^Spam}} {!eq {$received_protocol}{spamslayed-spamd}} {!eq{$received_protocol}{local}} {!eq{$received_protocol}{}} }}"
driver = accept
transport = spamslay_spamd
#
# This route will send any mail that got here to the devnull alias, that
# should be configured in /etc/aliases to be a real link to /dev/null.
# This route should get only mails that have spam score higher than 14.
# This will affect users mails!
spamslay_killit:
condition = "${if ge{$h_X-Spam-Level:}{\*\*\*\*\*\*\*\*\*\*\*\*\*\*} {1}{0} }"
driver = redirect
data = spam
file_transport = address_file
pipe_transport = address_pipe

* Next step, now that spams are flagged, it makes sense to put them apart in the Maildir that will be accessed through IMAPS. I do this with procmail. We set umask for procmail (the IMAP server is configured as such too) to make sure Debian-exim can access stored mails (we want mode 0640, group read access, so the umask is 666-640=026). Here’s the relevant bit of /home/localuser/.procmailrc:

UMASK=026
IMAPDIR=$HOME/.Maildir/
SPAM=$IMAPDIR".Poubelle.Spam/"
#
:0
* ^X-Spam-Status: Yes
$SPAM
:0
* ^X-Spam-Flag: YES
$SPAM
#
:0
* ^X-Bogosity: Spam
$SPAM

At the same time, we make sure Debian-exim can access mails already there (so not affected by umask):

# cd /home/localuser
# chmod 750 -Rv .Maildir
# chmod 0640 -v `find .Maildir -type f`

(PS: you may want to enforce a more restrictive policy, depending on how your server is accessed – but, anyway, Debian-exim is by essence able to tamper with mails you receive, so it won’t make a big difference)

* Training bayesian filters.

Now that spam ended up in a specific Maildir, both SpamAssassin and Bogofilter bayesians filters must be trained to be effective.

We add the following in /etc/cron.d/bayes:

# trains bayesian filters
SPAMDIR="/home/localuser/.Maildir/.Poubelle.Spam/cur/ /home/localuser/.Maildir/.Poubelle.Spam/new/"
#
# spamd: can handle by itself bogofiltered headers
25 * * * * Debian-exim /usr/bin/sa-learn --spam $SPAMDIR
#
# bogofilter: not able to clean inappropriate cues from spamd, will do it
# by removing:
# - informational SpamAssassin headers
# - SpamAssassin score and decision (irrelevant)
# (-u was not set as it is discouraged perf-wise in bogofilter's manual)
28 * * * * Debian-exim for file in `find $SPAMDIR -type f`; do cat $file; done | grep -v -E "^X-Spam-(Checker|Flag|Level|Report)" | sed s/"^X-Spam-Status.*score.*required.*tests="//g | /usr/bin/bogofilter --register-spam

Obviously, if you want it to learn from plenty of different users, you’ll have to think of something more elaborated 🙂
Anyway, regarding plenty of users, it would actually probably wise to think twice about the whole concept of sharing bayesian filters that may not at all be accurate for very differents users.

One alternative would have been to avoid meddling with Exim and to run both bogofilter and spamd via procmail. Sure, it would not have been site-wide setup but for a few users, ~/.procmailrc can be replicated easily. But actually I enjoy messing with Exim, that’s kind of a hobby. I skipped here the part where we call DNSBLs in Exim (working out-of-the-box anyway). And on a production server, with the SMTP wide opened to the web, it is possible to follow this approach just to shut off spammers at SMTP-time -which induces a huge resources gain- and even ban them.