Exim 4: ACL and others to reject spams
table of contents :
- Introduction
- Remote host IP checking
- Remote host IP checking by DNS black-list
HELOchecking- Sender checking
- Recipient: emails addresses to catch spams
- Recipient: no hack
- Recipient: no relay
- Recipient: manual redirect by the sender
- greylist
- anti-virus: windows executables in attachment
- anti-virus: clamav
- anti-spam external detector: spamassassin
- Checking source of email associated with your domain in whois
- All in one
Introduction
This article is an explained example of exim 4 ACL configuration. For a larger point of view see «Spam Filtering for Mail Exchangers».
With this configuration we got approximately (for one person) 1 spam per day and we reject 20 per day (see the exim rejectlog file).
The context is a host server with a public IP (82.224.147.80, www.maretmmanu.org), wich is also used as local mail server:
###################################################################### # MAIN CONFIGURATION SETTINGS # ###################################################################### # If exim is used localy in batch mode (exim4 -bs) then "$host" is empty, the ": :" adds the empty string. hostlist own_hosts = 127.0.0.1 : : 192.168.113.114 : 192.168.113.113 : 82.224.147.80 domainlist public_domains = maretmmanu.org
Remote host IP checking
We allow connections from our own hosts and a white-list (Some hosts from big internet providers) with no more check. We refuse connections with some hosts (marketing company etc).
acl_check_host:
accept
hosts = +own_hosts : /etc/exim4/filters/host_white.list
deny
log_message = match host_reject.list
hosts = /etc/exim4/filters/host_reject.list
accept
Remote host IP checking by DNS black-list
Hosts listed by the dns list <a style="color: purple;" href="http://www.spamhaus.org/sbl/" rel="nofollow noopener noreferrer" target="_blank">sbl-xbl.spamhaus.org</a>"" are spammers or relays for spams. Often if you refuse the connection for one of these hosts then a new try is done by another relay some seconds later (see my old reject log). A better solution is to do the rejection when the RCPT is received. Then the spammer does not try again (see the new reject log).
acl_check_rcpt:
. . .
drop
log_message = match sbl-xbl.spamhaus.org
dnslists = sbl-xbl.spamhaus.org
HELO checking
Often spammers send for the HELO argument the name or the IP of your host. Here my own domain is maretmmanu.org"" and my own IP is 82.224.147.80.
acl_check_helo:
accept
hosts = +own_hosts
# If the HELO pretend to be this host
deny condition = ${if or {
{eq {${lc:$sender_helo_name}}{maretmmanu.org}}
{eq {${lc:$sender_helo_name}}{82.224.147.80}}
} {true}{false} }
# by default we accept
accept
Sender checking
We refuse some senders, from some marketing companies.
acl_check_sender:
deny senders = /etc/exim4/filters/sender_reject.list
accept
Recipient: emails addresses to catch spams
You can publish a sacrified email address in a web page to trap spammers (some spammers crawl other web pages to get emails). When this email address matches then an error is returned and all the message reception is droped. There are changes that the spammer software will not retry with this recipient removed.
When you write to a suspicious company wich could send you spam or when you write in a newsgroup, you can use a special email, with date (like echant-td-n040531@maretmmanu.org) or with an included identifier (like echant-tr-lemonde@maretmmanu.org). Then if you receive spam for this email you can put it in the drop list (in this example: /etc/exim4/filters/recipients_drop.list).
acl_check_rcpt:
. . .
drop
log_message = match recipients_drop.list.
recipients = /etc/exim4/filters/recipients_drop.list
I use this script in cron.daily/ to update my emails with a date incorporated. The letter before the date is used to trace the origine (web, news, email).
#!/bin/bash
# Update my email wich include the today date
set -e
T=$(tempfile)
D=$(date '+%y%m%d')
function mod_file {
EMAIL="$1"
LETTRE="$2"
CONF="$3"
if [ -f "$CONF" -a -r "$CONF" ]; then
lockfile-create "$CONF"
sed "s/${EMAIL}-td-${LETTRE}[0-9]{6}@maretmmanu.org/${EMAIL}-td-${LETTRE}${D}@maretmmanu.org/g" <"$CONF" >"$T"
cp "$T" "$CONF"
lockfile-remove "$CONF"
fi
}
# The first line will replace echant-td-n040625@maretmmanu.org
# with echant-td-n040626@maretmmanu.org
mod_file echant n /home/manu/.kde/share/config/knoderc
mod_file echant e /home/manu/.sylpheed/accountrc
mod_file echant e /home/manu/.initvar
# For apache we should reload but it is done by
# logrotate from time to time.
mod_file echant w /etc/apache-extern/httpd.conf
rm $T
Recipient: no hack
(From /usr/share/doc/exim4-doc-html/html/C043.txt.gz):
Deny if the local part contains @ or % or / or | or !. These are rarely found in genuine local parts, but are often tried by people looking to circumvent relaying restrictions.
Also deny if the local part starts with a dot. Empty components aren't strictly legal in RFC 2822, but Exim allows them because this is common. However, actually starting with a dot may cause trouble if the local part is used as a file name (e.g. for a mailing list).
acl_check_rcpt:
. . .
# refuse if the recipient string is a hack,
# see exim file example C043.txt.gz
deny
local_parts = ^.*[@%!/|] : ^\.
Recipient: no relay
I refuse to relay spams:
acl_check_rcpt:
. . .
# For the rest, the domain of the recipient address
# must be my public domain. (no relay)
require
log_message = no relay.
domains = +public_domains
Recipient: manual redirect by the sender
The idea is to send an automatic reply, using mail"" command in a filter, to inform that an email is blocked and that the user must use an other email address. This can be used to change a user email which receive to much spam or to protect a public email address.
In a filter:
### reply for echant@maretmmanu.org
if $original_local_part is "echant" then
seen mail from drop@maretmmanu.org subject "Re: $h_subject" file .echant_reponse.txt
finish
endif
greylist
Greylisting use the fact that most of the time spammers softwares do not take account tempory errors to retry later. It's very effective. You can use a daemon like [urlspan]greylistd[/urlspan].
When exim send a «tempory error»:
- nether if the host is in a white list of «good» hosts
- nether if there is no sender: it is a bounce message
- nether if the HELO argument is well configured and the host name seams owned by the sender.
- if the greylist daemon want to
The two lines with «set acl_m9» are used to send the request to the daemon and get the result. The /24"" is here because some big MTA can be spreaded on multiple hosts.
######################################################################
# MAIN CONFIGURATION SETTINGS #
######################################################################
# Mandatory to use "verify = helo"
helo_try_verify_hosts = !+own_hosts
. . .
######################################################################
# ACL CONFIGURATION #
######################################################################
# ACL "subroutine" used by acl_check_rcpt below. Used to detect
# hosts wich have not their own registered domain-name (probably spammer).
# Return ok if the HELO argument correspond to the connected HOST and
# if the argument does not contain an IP in decimal or hexa.
# I have created this ACL subroutine because we can't do a list of "or"
# in ACL (it's a list of "and"), so I use a negation of "and":
# no (no A and no B) = A or B.
acl_clean_helo:
accept
verify = helo
condition = ${if match{$sender_helo_name}{N(d{1,3}[.-]d{1,3}[.-]d{1,3}[.-]d{1,3})|([0-9a-f]{8})|([0-9A-F]{8})N}{false}{true}}
acl_check_rcpt:
. . .
# Greylisting, if the HELO argument seems bad or
# a dialin name (with IP included in the name). Some hosts from big
# providers are in a white list to avoid testing. When there is no
# sender then it is a bounce message, so no greylist.
defer
message = Please try later.
!hosts = /etc/exim4/filters/host_white.list
!senders = :
!acl = acl_clean_helo
log_message = greylisted.
set acl_m9 = ${mask:$sender_host_address/24} $sender_address $local_part@$domain
set acl_m9 = ${readsocket{/var/run/greylistd/socket}{$acl_m9}{5s}{}{}}
condition = ${if eq {$acl_m9}{grey}{true}{false}}
anti-virus: windows executables in attachment
It's a very basic anti-virus: every emails with a windows executable as attachment is rejected.
acl_check_data:
. . .
deny message = This message contains an attachment of a type which we do not accept (.$found_extension)
demime = bat:btm:cmd:com:cpl:dll:exe:lnk:msi:pif:prf:reg:scr:vbs:url
anti-virus:[urlspan]clamav[/urlspan]
I keep this chapter but I have removed clamav from my computer since there are some vulnerabilities announced times to times and because, with other spam filters, it seems not very usefull.
We used an anti-virus not to avoid virus (we have just linux hosts) but to remove unwanted emails.
######################################################################
# MAIN CONFIGURATION SETTINGS #
######################################################################
av_scanner = clamd:/var/run/clamd.ctl
. . .
######################################################################
# ACL CONFIGURATION #
######################################################################
acl_check_data:
. . .
deny message = This message contains a virus or other harmful content ($malware_name)
demime = *
malware = *
anti-spam external detector: [urlspan]spamassassin[/urlspan]
We add a X-SA-Score:"" in the header of all emails, a X-SA-Report:"" for all email with spam score >0, we consider it a spam if score >5 (adding X-SA-Status: Yes"" and we don't accept the email if score >7.
Because of the accept"" we must put this acl block at the end of the acl_check_data.
######################################################################
# MAIN CONFIGURATION SETTINGS #
######################################################################
spamd_address = 127.0.0.1 783
. . .
######################################################################
# ACL CONFIGURATION #
######################################################################
acl_check_data:
. . .
## spamassassin, spams are never big and spamassassin can die on big emails, so we
## limit its use under 500ko.
accept condition = ${if >={$message_size}{500k}{yes}{no}}
warn message = X-SA-Score: $spam_score
spam = nobody:true
warn message = X-SA-Report: $spam_report
spam = nobody:true
condition = ${if >{$spam_score_int}{0}{true}{false}}
warn message = X-SA-Status: Yes
spam = nobody:true
condition = ${if >{$spam_score_int}{50}{true}{false}}
deny message = This message scored $spam_score spam points.
spam = nobody:true
condition = ${if >{$spam_score_int}{70}{true}{false}}
In your ~/.forward"" you can redirect spams (5< score ≤7) in a special inbox:
# Exim filter <<== do not edit or remove this line!
if $h_X-SA-Status: matches "^Yes" then
save $home/.Mailboxes/incoming/spam
finish
endif
Checking source of email associated with your domain in whois
If you have an email published in a whois database (spammers scan these databases) but want emails just from your registrar, you can add this in your ~/.forward"" filter:
# Exim filter <<== do not edit or remove this line!
if $original_local_part is "echant-tr-myregistrar"
then
if $sender_address_domain is "myregistrar.net" then
deliver marcel
else
save $home/.Mailboxes/incoming/spam
finish
endif
endif
All in one
######################################################################
# MAIN CONFIGURATION SETTINGS #
######################################################################
hostlist own_hosts = 127.0.0.1 : 192.168.109.24 : 192.168.109.23 : 82.224.147.80
domainlist public_domains = maretmmanu.org
. . .
# Mandatory to use "verify = helo"
helo_try_verify_hosts = !+own_hosts
av_scanner = clamd:/var/run/clamd.ctl
spamd_address = 127.0.0.1 783
acl_smtp_rcpt = acl_check_rcpt
acl_smtp_mail = acl_check_sender
acl_smtp_connect = acl_check_host
acl_smtp_data = acl_check_data
acl_smtp_helo = acl_check_helo
. . .
######################################################################
# ACL CONFIGURATION #
######################################################################
acl_check_host:
accept
hosts = +own_hosts : /etc/exim4/filters/host_white.list
deny
log_message = match host_reject.list
hosts = /etc/exim4/filters/host_reject.list
accept
acl_check_helo:
accept hosts = +own_hosts
# If the HELO pretend to be this host
deny condition = ${if or {
{eq {${lc:$sender_helo_name}}{maretmmanu.org}}
{eq {${lc:$sender_helo_name}}{82.224.147.80}}
} {true}{false} }
# by default we accept
accept
acl_check_sender:
deny senders = /etc/exim4/filters/sender_reject.list
accept
# ACL "subroutine" used by acl_check_rcpt below.
# Return ok if the HELO argument correspond to the connected HOST and
# if the HELO argument does not contain an IP in decimal or hexa.
# I have created this ACL subroutine because we can't do a list of "or"
# in ACL (it's a list of "and"), so I use a negation of "and":
# no (no A and no B) = A or B.
acl_clean_helo:
accept
verify = helo
condition = ${if match{$sender_helo_name}{N(d{1,3}[.-]d{1,3}[.-]d{1,3}[.-]d{1,3})|([0-9a-f]{8})|([0-9A-F]{8})N}{false}{true}}
acl_check_rcpt:
# refuse if the recipient string is a hack,
# see exim file example C043.txt.gz
deny
local_parts = ^.*[@%!/|] : ^\.
# Relaying with no more check for my own hosts.
accept
hosts = +own_hosts
# For the rest, the domain of the recipient address
# must be my public domain. (no relay)
require
log_message = no relay.
domains = +public_domains
# Reffuse all the message if the recipient is only used by spammers.
drop
log_message = match recipients_drop.list.
recipients = /etc/exim4/filters/recipients_drop.list
drop
log_message = match sbl-xbl.spamhaus.org
dnslists = [urlspan]sbl-xbl.spamhaus.org[/urlspan]
# Greylisting, if the HELO argument seems bad or
# a dialin name (with IP included in the name). Some hosts from big
# providers are in a white list to avoid testing. When there is no
# sender then it is a bounce message, so no greylist.
defer
message = Please try later.
!hosts = /etc/exim4/filters/host_white.list
!senders = :
!acl = acl_clean_helo
log_message = greylisted.
set acl_m9 = ${mask:$sender_host_address/24} $sender_address $local_part@$domain
set acl_m9 = ${readsocket{/var/run/greylistd/socket}{$acl_m9}{5s}{}{}}
condition = ${if eq {$acl_m9}{grey}{true}{false}}
# Default rule: accept except if recipient address is unrouteable.
accept
message = unrouteable address
verify = recipient
acl_check_data:
accept hosts = +own_hosts
# if there is a windows executable as attachment then we reject
deny message = This message contains an attachment of a type which we do not accept (.$found_extension)
demime = bat:btm:cmd:com:cpl:dll:exe:lnk:msi:pif:prf:reg:scr:vbs:url
# clamav
deny message = This message contains a virus or other harmful content ($malware_name)
demime = *
malware = *
## spamassassin, spams are never big and spamassassin can die on big emails, so we
## limit its use under 500ko.
accept condition = ${if >={$message_size}{500k}{yes}{no}}
warn message = X-SA-Score: $spam_score
spam = nobody:true
warn message = X-SA-Report: $spam_report
spam = nobody:true
condition = ${if >{$spam_score_int}{0}{true}{false}}
warn message = X-SA-Status: Yes
spam = nobody:true
condition = ${if >{$spam_score_int}{50}{true}{false}}
deny message = This message scored $spam_score spam points.
spam = nobody:true
condition = ${if >{$spam_score_int}{70}{true}{false}}
# accept by default
accept
