RFE: dnsbl-support for dovecot

Reindl Harald h.reindl at thelounge.net
Tue Jun 17 19:20:27 UTC 2014

Am 17.06.2014 21:04, schrieb Jochen Bern:
> On -10.01.-28163 20:59, Reindl Harald wrote:
>> i admit that i am not a C/C++-programmer, but i think
>> doing the DNS request and in case it has a result block
>> any login attemt should be not too complex
> Can't say that I actually ever *did* it, but according to the docs, the
> following should work:
> 1. Use http://wiki2.dovecot.org/Authentication/MultipleDatabases to have
> login requests go through a
> http://wiki2.dovecot.org/AuthDatabase/CheckPassword first. Insert %r
> into the args to pass the rip to the external executable.
> 2. Make that executable return failure if there is a matching DNSBL
> entry. (Note that in the case of a *dictionary* attack, offenders should
> appear in your resolver's local cache shortly, so you can set very low
> timeouts.) Configure the database as "result_failure = return-fail"
> (according to the docs, that should make dovecot generate a log entry)
> and "result_success = continue" (which will pass processing to the
> *actual* userdb/passdb).
> 3. *Now* you can take advantage of having the lookup being done by an
> external executable, instead of (hard)code(d) within dovecot: Use the
> iptables "recent" module to (temporarily) block packets from SRCs on a
> local dynamic blacklist, and let the executable feed any matches it
> encounters to that list through the /proc/net interface as well.
>>        echo +addr >/proc/net/xt_recent/DEFAULT
>>               to add addr to the DEFAULT list

thanks - but all that workarounds is wwhat i want to avoid

simply because the gain is not high enough, the possible points
of failures are increased while having already a trustable DNSBL
and how DNSBL is working - any answer means listed - no response
or a NXDOMAIN means "don't block the client"

finally the way a RBL works also means in case of failures
you only have the DNS request timeout but no false positives

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