[CentOS] iptables question.

Tue Feb 22 00:25:41 UTC 2011
Bill Campbell <centos at celestial.com>

On Mon, Feb 21, 2011, Stephen Harris wrote:
>On Mon, Feb 21, 2011 at 03:32:40PM -0800, Bill Campbell wrote:
>> My problem is that occassionally an IP addresses doesn't appear to be
>> blocked as we continue to see the e-mail messages after the blocks are in
>> place.  Most frequently these occur from courier-imap failed login
>> attempts, less frequently from sshd.
>> To start, iptables is initialized by setting up a named rule set,
>> say on eth0:
>> # these two set up the rule set.
>> iptables -N csblocks
>> iptables -A csblocks -j RETURN
>> # now add it to input, check csblocks on all new connections.
>> iptables -i eth0 -m state --state NEW -j csblocks
>> With all that incoming attempts still seem to get by for a few IP
>> addresses, but certainly not all.
>> Can anybody point out what I'm doing wrong, or why this may happen?
>Connections that are already established may be blocked but traffic
>will continue to flow because you're only blocking on "NEW" traffic.
><connection made>
>login fail
>login fail
>login fail
><BLOCK HAPPENS - perhaps it's the 5th set of connections and it's just
>  tripped the threshold>
>login fail
>login fail
>login fail
><too many failed attempts, disconnected by server daemon>
><new connection blocked>
>You'll see 3 login failures after the block occured because the connection
>was still open.

That makes sense, and was one of the first things I thought of.

On the other hand "lsof -n -i" doesn't show any open connections
to the IP address, and I would think that the forwarding and null
route would prevent that.

INTERNET:   bill at celestial.com  Bill Campbell; Celestial Software LLC
URL: http://www.celestial.com/  PO Box 820; 6641 E. Mercer Way
Voice:          (206) 236-1676  Mercer Island, WA 98040-0820
Fax:            (206) 232-9186  Skype: jwccsllc (206) 855-5792

Historically, inflation is a classic game of legal plunder, more
effective than taxes since the legalized theft is concealed.
  -- T. Hunt Tooley http://mises.org/story/3292