[CentOS] chronyd configuration as a local ntp server

Tue Dec 27 14:07:23 UTC 2016
Robert Moskowitz <rgm at htt-consult.com>

'Modern' NTP allows for all sorts of updates to NTP servers, with all 
sorts of attacks.  So to prevent even local hosts from making changes to 
your NTP server, there is the restrict instead of allow command.  Its 
intent is to limit what the server will accept from a host in the 
address range instead of allowing any command from within that range.  I 
use this on my Centos6 servers.

I guess I will have to register to the chronyd list and ask there.


On 12/27/2016 08:49 AM, David Both wrote:
> AFAIK the only thing needed to make your host an NTP server using 
> chrony is to set the allow line to the network address in CIDR format 
> of the network you want to be served, and uncomment it. The restart 
> chronyd. You also need to ensure that port 123 (NTP) is open to your 
> internal network on your filrewall.
> I have a CentOS 6 box that is an NTP server for my network. CentOS 7 
> works the same way.
> On 12/27/2016 08:25 AM, Fred Smith wrote:
>> On Mon, Dec 26, 2016 at 11:04:22PM -0500, Robert Moskowitz wrote:
>>> This is for centos 7 that has chronyd 2.1.1
>>> I am looking into how to use chronyd as my local ntp server.
>>> On my old servers with ntpd I had local access control lines like:
>>> restrict mask nomodify notrap
>>> But in looking for documentation on chronyd I did not find anything
>>> on this at:
>>> https://chrony.tuxfamily.org/doc/2.1/manual.html
>>> In the actual /etc/chronyd.conf there is the sample line:
>>> # Allow NTP client access from local network.
>>> #allow 192.168/16
>>> Does this allow only allow queries?  Does chronyd support the
>>> 'restrict' option?
>> Robert:
>> Years back I used to use Chrony for that  purpose (when I was running
>> Smoothwall on an old PC instead of a commercial router, as I am now)
>> and it did the job remarkably well.
>> One of the designgoals of Chrony was to support networks or computers
>> that are NOT connected full-time, so that time stayed somewhere near
>> correct even if offline for hours or days.
>> But that having been so long ago, now, I don't remember the details.
>> I also don't remember what the "restrict" directive for ntpd does.
>> (to give you an idea of how long ago that was it was when I had a Red 
>> Hat
>> 7.2 or 7.3 workstation as my home PC--pre-RHEL. I could compile 
>> things on
>> that RH box, tar up the necessary results and take that file to the
>> smoothwall box and untar them and with small configuration: voila!)
>> there used to be a chrony mailing list where one could ask such 
>> questions,
>> but I haven't seen traffic on it in years, so it may no longer exist.
>> Fred