Just a few comments.

- The below commands drops ALL future connections to the IMAP ports and not just the one from that specific IP address.
- It all depends on the ordering of the rest of your iptables rules. A lot of iptables setups have an accept related / established in the top of the INPUT chain and then indeed the traffic will continue as long as the connection is established. If you put a correct drop rule in the top of your iptables INPUT chain it will block all traffic including any related/established.

Fail2Ban is able to insert such a drop rule in the top of the INPUT chain and thereby block all further tries.
This is exactly how I have setup my fail2ban and it works.

The first few lines of my iptables input chain look like this:

  29M 2249M f2b-dovecot  tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            multiport dports 110,143,993,995
9969K 2545M f2b-sasl   tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            multiport dports 25,465
9691K 2788M ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0           
 134M  257G ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            state RELATED,ESTABLISHED

Jan Hugo Prins


On 5/23/22 23:16, Hippo Man wrote:
OOPS! I incorrectly copied and pasted the iptables command in my previous message. Here is the correct iptables command:

iptables -I INPUT -p tcp -m multiport --destination-port 143,993 -d aaa.bbb.ccc.ddd -j DROP

This command successfully blocks *future* connections to ports 143 and 993 from that IP address, but as I mentioned, it doesn't kill the currently open connection.

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On Mon, May 23, 2022 at 4:54 PM Hippo Man <hippoman@gmail.com> wrote:
Thank you, but fail2ban doesn't do what I need. Here is why ...

I have used fail2ban and also my own homegrown log monitor program for this purpose. In both cases, I can detect the failed imap logins and then cause the following command to be run ...

iptables -I INPUT -p tcp --destination-port aaa.bbb.ccc.ddd -j DROP

However, this does not drop connections that are existing and already open. It will only drop *future* connections from that IP address to port 143.

This is why I want to kill the existing connection. Even after that "iptables" command is issued, the entity which is connected to the imap port can continue to send more and more imap commands.

If I can drop the TCP connection as soon as an imap login fails and also issue that kind of "iptables" command, then the client would have to reconnect in order to retry other login attempts. Those future connections would then be successfully blocked by that iptables rule.

And even if I issue a "tcpdrop" command instead of just the "iptables" command, it doesn't kill the already-open connection. It just force-blocks future connections.

I'm thinking of patching the dovecot source code to create a personal version which immediately disconnects from the socket after login failure. Of course, I would prefer not to do that, if there is another way to accomplish this.

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On Mon, May 23, 2022 at 4:24 PM Jan Hugo Prins <jhp@jhprins.org> wrote:
Look at fail2ban.
Should be able to do that for you.

Jan Hugo


On 5/23/22 21:11, Lloyd Zusman wrote:
I'm running dovecot 2.2.13 under Debian 8.

I'd like to force an immediate TCP socket disconnect after any imap login attempt that fails.

Right now, if invalid credentials are supplied during an imap login, the client can keep retrying logins with different credentials. However, I want to prevent that from occurring by causing the socket connection to be closed as soon as there is any failed login attempt.

I haven't been able to find any dovecot configuration setting which could control this behavior, but I'm hoping that I just missed something.

Thank you very much for any suggestions.

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 hippoman@gmail.com
 Take a hippopotamus to lunch today.