[Dovecot] feature request: deny IP address via database

Dave McGuire mcguire at neurotica.com
Tue Apr 8 07:02:29 EEST 2008

On Apr 7, 2008, at 4:58 PM, Bill Cole wrote:
>>   Hey folks.  One feature I'd really like to see in dovecot is the  
>> ability to point it at a database (with a configurable query) and  
>> have it allow or deny a connection based on looking up the source  
>> IP address in that database.
>>   I run Postfix, and I've got it configured to use a database  
>> server for its smtpd_client_restrictions checks.  Ideally I'd like  
>> to point dovecot at the same database table.  I have external  
>> tools that maintain that table.
>>   I was thinking of writing it myself, but I'm running 1.0.10; I'd  
>> assume that any such modifications would need to be rewritten for  
>> 1.1.  Then I got to thinking that such functionality would likely  
>> be useful to people other than just me, so..
>>   Thoughts?
> Is there any reason to do this at the application layer rather than  
> the network layer for Dovecot?

   Yup...see my last post on this topic.

> Note that using smtpd_client_restrictions in Postfix *does not*  
> make it deny connections, it just makes it reject mail that is  
> offered on connections.

   Yes, I'm aware of that (I'm pretty comfortable with Postfix) but I  
appreciate your pointing it out.  I'm sorry for my overly loose  
description above.

> It seems to me that for SMTP it is fairly normal to have IP space  
> that you'd want to reject mail from conditionally, such as  
> depending on whether/how the client authenticates or to allow mail  
> to standard role accounts. That sort of conditionality that is only  
> possible at the application layer makes an argument for  
> smtpd_client_restrictions in Postfix in some cases rather than  
> blocking at the network layer (i.e. a packet filter on the host of  
> an external firewall.)

   Yup, I'm right with you there.

> I don't see the same sort of cases for IMAP or POP where you'd want  
> to share a list of not-quite-totally-evil IP addresses with Postfix  
> or anything else, rather than just barring access completely.

   ...but here, I have seen a number of correlations (oddly) between  
dictionary attacks against my IMAP ports, dictionary attacks against  
SMTP authentication, and relay attempts.  I have stuff in place that  
adds, and later removes via a time-based expiration scheme, the  
offending addresses to the database for smtpd_client_restrictions to  
chop them.  The ability to have that table apply to POP3 (for those  
few still using it) and IMAP, as well as the SMTP authentication  
requests, would be Awfully Nice here.

   But as I explained in my other message on this topic, the big deal  
is sharing that list across machines running dovecot in a pseudo-load- 
balanced configuration.  All of my dynamic configuration information  
(everything not in dovecot.conf) is stored in a centralized database,  
with no local state other than currently-active connections.  And if  
one IMAP server is getting hammered on by a dictionary attack from a  
zombie Windows machine, I want *all* of the servers to block (but  
log!) those connections.  I suppose I could probably hack together  
some method of inserting statements in the access lists on the border  
routers (Cisco IOS) but that's a bit dangerous for my taste.

> It is possible to restrict specific users to login via specific  
> networks by using a custom PAM module, the allow_nets extra  
> authentication field, or a custom checkpassword script. You also  
> *might* be able to build a SQL query string using the %r variable  
> and some sort of conditional logic so that authentication from  
> 'bad' IP's fails.

   I could investigate that a bit...that is an interesting idea.


Dave McGuire
Port Charlotte, FL

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