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restrictions

Richard Stallman, founder of the Free Software Foundation published an interesting article on BBC news website. From the article:

To pay so much attention to Bill Gates' retirement is missing the point. What really matters is not Gates, nor Microsoft, but the unethical system of restrictions that Microsoft, like many other software companies, imposes on its customers. But Gates didn't invent proprietary software, and thousands of other companies do the same thing. It's wrong, no matter who does it.

Microsoft, Apple, Adobe, and the rest, offer you software that gives them power over you. A change in executives or companies is not important. What we need to change is this system.

That's what the free software movement is all about. "Free" refers to freedom: we write and publish software that users are free to share and modify.

=> It's not the Gates, it's the bars

Postfix configure anti spam with blacklist

Postfix is free and powerful MTA. You can easily configure Postfix to block spam. You need to add
following directives to /etc/postfix/main.cf file:

=> disable_vrfy_command = yes : Disable the SMTP VRFY command. This stops some techniques used to harvest email addresses.

=> smtpd_delay_reject = yes : It allows Postfix to log recipient address information when rejecting a client name/address or sender address, so that it is possible to find out whose mail is being rejected.

=> smtpd_helo_requi
red = yes
: Require that a remote SMTP client introduces itself at the beginning of an SMTP session with the HELO or EHLO command. Many spam bot ignores HELO/EHLO command and you save yourself from spam. Following lines further restrictions on HELO command:
smtpd_helo_restrictions = permit_mynetworks,
reject_non_fqdn_hostname, Reject email if remote hostname is not in fully-qualified domain form. Usually bots sending email don't have FQDN names.
reject_invalid_hostname, Reject all bots sending email from computers connected via DSL/ADSL computers. They don't have valid internet hostname.
permit

You can put the following access restrictions that the Postfix SMTP server applies in the context of the RCPT TO command.
=> smtpd_recipient_restrictions =
reject_invalid_hostname, - Reject email if it not valid hostname
reject_non_fqdn_hostname, - Reject email if it not valid FQDN
reject_non_fqdn_sender, - Reject the request when the MAIL FROM address is not in fully-qualified domain form. For example email send from xyz or abc is rejected.
reject_non_fqdn_recipient, - Reject the request when the RCPT TO address is not in fully-qualified domain form
reject_unknown_sender_domain, - Reject email, if sender domain does not exists
reject_unknown_recipient_domain, Reject email, if recipient domain does not exists
permit_mynetworks,
reject_rbl_client list.dsbl.org, Configure spam black lists
reject_rbl_client sbl.spamhaus.org,
reject_rbl_client cbl.abuseat.org,
reject_rbl_client dul.dnsbl.sorbs.net,
permit

Open /etc/postfix/main.cf file :
# vi /etc/postfix/main.cf
Set/modify configuration as follows

disable_vrfy_command = yes
smtpd_delay_reject = yes
smtpd_helo_required = yes
smtpd_helo_restrictions = permit_mynetworks,
     reject_non_fqdn_hostname,
     reject_invalid_hostname,
     permit
smtpd_recipient_restrictions =
   permit_sasl_authenticated,
   reject_invalid_hostname,
   reject_non_fqdn_hostname,
   reject_non_fqdn_sender,
   reject_non_fqdn_recipient,
   reject_unknown_sender_domain,
   reject_unknown_recipient_domain,
   permit_mynetworks,
   reject_rbl_client list.dsbl.org,
   reject_rbl_client sbl.spamhaus.org,
   reject_rbl_client cbl.abuseat.org,
   reject_rbl_client dul.dnsbl.sorbs.net,
   permit
smtpd_error_sleep_time = 1s
smtpd_soft_error_limit = 10
smtpd_hard_error_limit = 20

Also force (highlighted using red color) Postfix to limit incoming or receiving email rate to avoid spam.

Save and close the file. Restart postfix:
# /etc/init.d/postfix restart

Watch out maillog file. Now you should see lots of spam email blocked by above configuration directive:
# tail -f /var/log/maillog
Output:

Jan  9 06:07:22 server postfix/smtpd[10308]: NOQUEUE: reject: RCPT from 183-12-81.ip.adsl.hu[81.183.12.81]: 554 Service unavailable; Client host [81.183.12.81] blocked using dul.dnsbl.sorbs.net; Dynamic IP Addresses See: http://www.sorbs.net/lookup.shtml?81.183.12.81; from= to= proto=ESMTP helo=<183-12-230.ip.adsl.hu>
Jan  9 06:07:23 server postfix/smtpd[10308]: lost connection after RCPT from 183-12-81.ip.adsl.hu[81.183.12.81]
Jan  9 06:07:23 server postfix/smtpd[10308]: disconnect from 183-12-81.ip.adsl.hu[81.183.12.81]
Jan  9 06:10:43 server postfix/anvil[10310]: statistics: max connection rate 1/60s for (smtp:81.183.12.81) at Jan  9 06:07:17
Jan  9 06:10:43 server postfix/anvil[10310]: statistics: max connection count 1 for (smtp:81.183.12.81) at Jan  9 06:07:17
Jan  9 06:10:43 server postfix/anvil[10310]: statistics: max cache size 1 at Jan  9 06:07:17
Jan  9 06:16:58 server postfix/smtpd[10358]: warning: 81.92.197.249: address not listed for hostname unassigned.or.unconfigured.reverse.nfsi-telecom.net
Jan  9 06:16:58 server postfix/smtpd[10358]: connect from unknown[81.92.197.249]
Jan  9 06:17:00 server postfix/smtpd[10358]: NOQUEUE: reject: RCPT from unknown[81.92.197.249]: 550 : Recipient address rejected: User unknown in virtual alias table; from=<> to= proto=ESMTP helo=
Jan  9 06:17:00 server postfix/smtpd[10358]: disconnect from unknown[81.92.197.249]

Next time I will write about simple procmail and spamassassin combination to filter out spam :)