Configure an antispam gateway (part I)

In this tutorial i’ll show how to ensure and apply many feature for our mail gateway try to avoid most of the mail spam. I’ll explain all the options that we’ll setup to understand it and modify it to any scenario. For this tutorial I used a machine with Centos 6.0 with the postfix mail software and other virtual machine Debian Squeeze running bind9 for DNS request. We’ll configure SPF (sender profile framework) to check the DNS records txt for check which servers are allowed to send mails with the domains that we’ll received mails in our servers, also we’ll configure some RBL to check in real time server black list and avoid these servers.

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1.- Install postfix and setup to autostart service:


# yum install postfix

# chkconfig --levels 235 postfix on

2.- Edit some options for ensure our mail server and avoid a lot of spam:


# vi /etc/postfix/main.cf


myhostname = mail.mydomain.com #host name


myorigin = mydomain.com #Domain name for mail senders


mydestination = mydomain.com #Local domains


mynetworks = 127.0.0.1/32, 192.168.1.0/24 # Networks that are allowed to send


inet_interfaces = localhost, 192.168.1.200


relay_domains = hash:/etc/postfix/relay_domains #File with accepted domains



parent_domain_matches_subdomains = debug_peer_list smtpd_access_maps #Not accept subdomains


smtpd_recipient_restrictions =

reject_unauth_destination #Deny domains that are not included in relay_domains

check_policy_service unix:private/policy #Check SPF policy

reject_unauth_pipelining

reject_unknown_sender_domain #Deny the request for the mail from address for non exist domain name

reject_unknown_recipient_domain #Deny the request for the rcpt to address for non exist domain name

reject_invalid_hostname #Deny the helo request for invalid FQDN host name

reject_non_fqdn_helo_hostname #Deny the helo request for non exist FQDN host name

reject_non_fqdn_sender #Deny the request for mail from address that don't follow a FQDN

reject_non_fqdn_recipient #Deny the request for rcpt to address that don't follow a FQDN

permit_mynetworks #Allow send mails from our network


smtpd_banner = $myhostname Microsoft ESMTP MAIL service ready


smtpd_helo_required = yes

3.- Edit the file relay_domains to add the domain names that are allowed for send mails to our server:


# vi /etc/postfix/relay_domains

mydomain.com OK

4.- Compile our relay_domains file:


# postmap /etc/postfix/relay_domains

5.- Configure SPF in our domain (192.168.1.57):


@ IN TXT "v=spf1 mx ip4:192.168.1.200/24 ~all"


6.- Configure spf in postfix:


# yum install perl-CPAN wget perl-NetAddr-IP perl-Module-Build perl-Test-Base perl-Net-DNS perl-Net-DNS-Nameserver

# perl -MCPAN -e shell

cpan[1]> install Net::DNS::Resolver::programmable

cpan[2]> install Error

cpan[3]> q

# perl -MCPAN -e "get('Mail::SPF')"

# cd /root/.cpan/sources/authors/id/J/JM/JMEHNLE/mail-spf

# tar -xzvf Mail-SPF-v2.8.0.tar.gz

# cd Mail-SPF-v2.8.0/

# perl Build.PL

# ./Build

# ./Build test

# ./Build install


7.- Download spf software and setup postfix:


# wget https://launchpad.net/postfix-policyd-spf-perl/trunk/2.009/+download/postfix-policyd-spf-perl-2.009.tar.gz

# tar -xzvf postfix-policyd-spf-perl-2.009.tar.gz

# cp postfix-policyd-spf-perl-2.009/postfix-policyd-spf-perl /usr/libexec/postfix/postfix-policyd-spf-perl

# vi /etc/postfix/master.cf

policy unix - n n - - spawn

user=nobody argv=/usr/bin/perl /usr/libexec/postfix/policyd-spf-perl


8.- Configure database RBL (Realtime Blackhole List):



# vi /etc/postfix/main.cf

smtpd_recipient_restrictions=

...

reject_rbl_client zen.spamhaus.org

reject_rbl_client relays.ordb.org

reject_rbl_client blackholes.wirehub.net

reject_rbl_client list.dsbl.org

permit_mynetworks


8.- Add Iptables rule:



# vi /etc/sysconfig/iptables

-A INPUT -m state --state NEW -m tcp -p tcp --dport 25 -j ACCEPT



Image

9.- Restart services postfix and iptables:


# service iptables restart

# service postfix restart

Test our configuration postfix:

- forces to use ehlo to establish a connection with mail server:

Image

- Domains don’t listed in relay_domains file are not allowed to send mails with our server:

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- check SPF records and reject non valid mails from other servers:

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logs for spf:

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