Problem With Mail Server: Stop Flooding With Fail2ban ?

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Hi,

My main mail server is running CentOS 7 with Postfix and Dovecot.

Last week I was surprised to see that Postfix had some troubles on this machine, according to Icinga. I took a peek at the logs:

# journalctl -p err Mar 28 04:37:02 sd-151768 postfix/smtpd[2786]: fatal: no SASL authentication mechanisms Mar 28 04:37:02 sd-151768 postfix/smtpd[2788]: fatal: no SASL authentication mechanisms Mar 28 04:37:02 sd-151768 postfix/smtpd[2790]: fatal: no SASL authentication mechanisms Mar 28 04:37:02 sd-151768 postfix/smtpd[2792]: fatal: no SASL authentication mechanisms Mar 28 04:37:02 sd-151768 postfix/smtpd[2794]: fatal: no SASL authentication mechanisms
…

And in /var/log/maillog I found a tsunami of these:

Mar 28 03:18:33 sd-151768 postfix/smtpd[29589]: warning:
unknown[45.227.253.115]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
Mar 28 03:18:33 sd-151768 postfix/smtpd[29589]: lost connection after AUTH from unknown[45.227.253.115]
Mar 28 03:18:33 sd-151768 postfix/smtpd[29589]: disconnect from unknown[45.227.253.115]

My first reaction was to manually ban the IP addresses / networks which caused the flood, using my firewall:

# firewall-cmd –permanent –add-rich-rule=”rule family=’ipv4′ source address=’45.227.253.0/24′ reject”
# firewall-cmd –reload

I’m already using fail2ban in conjunction with firewalld to prevent brute force SSH attacks.

Q: can I use it in a similar configuration to stop Postfix from getting flooded and brought down to its knees?

Thanks & cheers from the sunny South of France,

Niki

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3 thoughts on - Problem With Mail Server: Stop Flooding With Fail2ban ?

  • I’m pretty sure I encountered this and needed to yum install cyrus-sasl-plain to resolve it.

  • Sorry, re-read your question and realise my suggestion would only help you get SASL authentication working.

  • Hello NIki,

    Juste enable postfix-sasl in jail.conf:

    [postfix-sasl]

    filter = postfix[mode=auth]
    port = SMTP,465,submission,imap,imaps,pop3,pop3s logpath = %(postfix_log)s backend = %(postfix_backend)s enabled = true maxretry = 3
    findtime = 172800
    bantime = 3600

    And enable recidive too:

    [recidive]

    logpath = /var/log/fail2ban.log banaction = %(banaction_allports)s bantime = 1mo findtime = 1w enabled = true

    Add ignoreip = 127.0.0.1 and your jumpoints :)

    Regards, DH

    po 29. 3. 2021 v 21:31 odesílatel Nicolas Kovacs
    napsal:

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