[CentOS] Strange behaviour of iptables in centos 7

[James Hogarth]

On 8 March 2016 at 09:22, anax <anax at ayni.com> wrote:

>
>
> On 03/08/2016 09:43 AM, James Hogarth wrote:
>
>> On 8 Mar 2016 07:36, "anax" <anax at ayni.com> wrote:
>>
>>>
>>> Hi
>>> strange behaviour of iptables on a centos 7.0 machine:
>>> The following rule is in the iptables of said machine:
>>>
>>> [root at myserver ~]# iptables -L -v -n --line-numbers |grep 175\.
>>> 9        9   456 DROP       all  --  *      *       175.44.0.0/16
>>>
>> 0.0.0.0/0
>>
>>> [root at myserver ~]#
>>>
>>> The corresponding enty in /etc/sysconfig/iptables looks like:
>>>
>>> [root at myserver ~]# grep 175 /etc/sysconfig/iptables
>>> -A INPUT -s 175.44.0.0/16 -j DROP
>>> [root at myserver ~]#
>>>
>>> The rule must be there since ages, because it has number 9 out of 76
>>>
>> similar rules.
>>
>>>
>>> Today, on the same machine (I rechecked it to make sure not to confound
>>>
>> machines), I see the following extract of the ftplog:
>>
>>>
>>> <snip>
>>> 175.44.4.127    2915
>>> 175.44.26.128   2021
>>> 175.44.26.138   1322
>>> 175.44.6.186    1290
>>> 175.44.24.88    1219
>>> 175.44.4.199    1212
>>> </snip>
>>>
>>> saying that from this IP addresse there have been this many connections
>>>
>> to the ftp server on that machine during the last two days, which means
>> that the iptables haven't dropped the connection to the machine. As far as
>> I know, the ftp server is behind the iptables. I also checked to see in
>> man
>> iptables, wheather the IP address is represented correctly.
>>
>>>
>>> What im I missing?
>>>
>>>
>> Please provide the full iptables listing as a snippet from one section is
>> not useful.
>>
>> Keep in mind iptables does not go by the most specific entry but rather
>> the
>> first matching rule hit.
>>
>> If there are any rules prior to this drop that would permit the traffic
>> then of course the traffic would be permitted.
>>
>> Also 7.0? Please get that system updated asap as you are missing many
>> important (and higher) issues being fixed.
>> _______________________________________________
>> CentOS mailing list
>> CentOS at centos.org
>> https://lists.centos.org/mailman/listinfo/centos
>>
>>
> Hi James
>
> [root at myserver ~]# cat /etc/centos-release
> CentOS Linux release 7.2.1511 (Core)
> [root at myserver ~]#
>
> [root at myserver ~]# uname -a
> Linux myserver.mydomain.com 3.10.0-327.4.4.el7.x86_64 #1 SMP Tue Jan 5
> 16:07:00 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
> [root at myserver ~]#
>
>
>
A joyful thing to see ;)

As for your issue itself - the rules seem sound to drop any packets
arriving at the server from that /16 network.

Are you sure that the iptables rule was added before the transfer logs you
see?

That it didn't happen that someone (or some process) saw abuse of ftp and
then inserted the DROP rule afterwards?

Remember position isn't always useful to gauge age of the rule since you
can insert anywhere ... and only 9 packets have been matched by that rule
in the full output...