[CentOS] Re: spam control

Scott Silva ssilva at sgvwater.com
Wed Oct 25 20:50:57 UTC 2006

David Mackintosh spake the following on 10/25/2006 10:54 AM:
> Jerry Geis wrote:
>> Gents,
>> I have added the following to /etc/mail/sendmail.mc and rebuilt it
>> trying to control spam. I still get about 25 spam messages a day.
>> Is there something else that can help control spam?
>> Thanks
>> jerry
>> ---------------------------
>> dnl #
>> dnl # dnsbl - DNS based Blackhole List/Black List/Rejection list
>> dnl # See http://www.sendmail.org/m4/features.html#dnsbl
>> dnl #
>> FEATURE(`dnsbl', `bl.spamcop.net',     `"Spam blocked see:
>> http://spamcop.net/bl.shtml?"$&{client_addr}')dnl
>> FEATURE(`dnsbl', `relays.ordb.org',    `"Spam blocked see:
>> http://ordb.org/lookup/?host="$&{client_addr}')dnl
>> FEATURE(`dnsbl', `cbl.abuseat.org',    `"Spam blocked see:
>> http://cbl.abuseat.org/lookup.cgi?ip="$&{client_addr}')dnl
>> FEATURE(`dnsbl', `sbl.spamhaus.org',   `"Spam blocked see:
>> http://spamhaus.org/query/bl?ip="$&{client_addr}')dnl
>> FEATURE(`dnsbl', `list.dsbl.org',      `"Spam blocked see:
>> http://dsbl.org/listing?"$&{client_addr}')dnl
>> dnl #
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>> http://lists.centos.org/mailman/listinfo/centos
> This question is probably inappropriate for this list, but maybe someone
> can answer it.
> Let's pretend I have a network behind a firewall. And let's pretend that
> the users behind that firewall are both beyond my control, and have a
> non-zero population of idiots.  And further, let's pretend that these
> idiots have done something to land my firewall's internet IP on a
> blacklist.
> So now lets pretend I have a different system on the internet, running
> sendmail, that I would like to use to relay mail out through, for myself
> and a few carefully selected non-idiot users.  And lets further pretend
> that this server is a secondary MX for a whole bunch of domains and so
> gets pounded with spam.
> OK, I set up this server so that it grants RELAY permission in
> /etc/mail/access to the IP address that is on the blacklist and
> everything works.
> Now I see the above post and think that adding dnsbl features to this
> sendmail might be a good way of reducing inbound spam.
> So my question is: if my system has granted RELAY permission to a system
> which is in a dnsbl used by the sendmail configuration, does the
> sendmail RELAY, or does it deny the connection attempt?
> Thanks for wading through this completely hypothetical situation.
> :)
If you allow in the access file, it should override the dns blacklist. So if
you allowed an address in the blacklist, it should let it through.


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