[CentOS] Problem with mail server: stop flooding with fail2ban ?

Jamie Burchell
Wed, 31 Mar 2021 08:19:09 -0000

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

> On 29 Mar 2021, at 20:31, Nicolas Kovacs <info at microlinux.fr>
wrote:
>
> ?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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