[CentOS] ICMP outoging traffic at centos 6.7

Wed Jan 6 16:34:10 UTC 2016
Tris Hoar <trishoar at bgfl.org>

On 06/01/2016 15:56, Shital Sakhare wrote:
> Yes, now I am dropping packets in OUTPUT chain for type 3. Initially, I
> implemented the chain to drop type 0 and 8. But it wont worked and the
> packets were hitting at firewall for multiple ICMP requests. I didn't
> Understand the problem. After posting here I go through all the types of
> ICMP types where I understand to drop packets for "Host unreachability" .
> Thanks for your help Mr. Gordon 🙏.
> On Wed, Jan 6, 2016 at 8:47 PM, Gordon Messmer <gordon.messmer at gmail.com>
> wrote:
>> On 01/06/2016 05:47 AM, Shital Sakhare wrote:
>>> Thanks, Dropped the ICMP type 3 port. Now question to find the cause.
>> Well, based on your tcpdump output, it looks like your rules were
>> rejecting unrelated packets, or tcp/443 packets.  It's hard to be sure
>> since the ICMP was the first packet, so you didn't show the packet it was
>> actually replying to.
>> The ICMP traffic is a result of rejecting rather than dropping that
>> traffic.  That is, I think you're looking at the problem wrong.  The ICMP
>> traffic is simply the result of a choice you made.  Are you dropping type 3
>> in the output chain?

I assume you also have rules in the INPUT chain, and one of them reads 
something like this:

-A INPUT -j REJECT --reject-with icmp-host-prohibited

When traffic matches this rule your server will respond with an ICMP 
request. it sounds like you now have added a rule blocking your server 
from sending these responses. You should investigate what is matching 
the INPUT rule as it could be malicious activity that should be blocked 
further upstream from you (e.g. at the perimeter firewall)
and if you wish to stop your server sending these responses you should 
change the rule to DROP instead of REJECT.


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