[CentOS] NTP server problem behind firewall

Mon Sep 3 14:32:41 UTC 2012
Giles Coochey <giles at coochey.net>

On 03/09/2012 15:18, Artifex Maximus wrote:
> On Mon, Sep 3, 2012 at 11:15 AM, Leonard den Ottolander
> <leonard at den.ottolander.nl> wrote:
>> On Sun, 2012-09-02 at 07:46 +0000, Artifex Maximus wrote:
>>> Any idea what is wrong?
>> The iptables rules you specify only allow clients from your local
>> network access to your "proxy" ntp server. However, you do not specify
>> any rules for eth1 to allow that ntp server to synchronise with the
>> remote servers it is using. So unless you are using a local time source
>> that might be your problem.
>> Btw, when specifying rules for the external ntp servers you might want
>> to specify IPs as well to restrict access.
> Thanks. You are right ntp proxy is absolutely what I want. Mine
> description was not clean probably. So this is the setup:
> GPSNTP( - eth1 myserver eth0 - clients(
> Because GPSNTP is on a physically separated network I need this proxy
> for my clients. My server is able to synchronize with GPSNTP so rules
> are fine for that (because my output chain is ACCEPT per default). My
> clients whom are cannot synchronize with my server even if I allow NTP
> port which I do not understand.
So at this stage, doing a "tcpdump -i eth0 -s 0 -w capture.cap" and 
getting one of your clients to try to sync time with your server and 
then repeating this with the firewall turned off (when it purportedly 
works) ought to give you enough information to be able to view the 
packet capture and see what is going wrong.


Giles Coochey, CCNA, CCNAS
NetSecSpec Ltd
+44 (0) 7983 877438
giles at coochey.net