[ntp:questions] Interface IP address Change.

Frank Kardel kardel at ntp.org
Mon Jun 30 20:47:45 UTC 2008


the core for the dynamic interface code *is* actually event based.
ntpd listens to routing messages on the routing socket. The routing 
socket has been part of BSD and BSD derived stacks for quite some time.
Whenever a routing message that indicates a possible routing change
is received a rescan is started within two seconds. This approach is
even better than just watching interface changes as it detects changing
routes for peer associations on the fly.

Unfortunately the BSD routing socket API is not available in every OS. 
Linux has a similar but different mechanism. Windows has another. For 
these systems ntpd currently uses the periodic scan approach to pick up 

Thus there are two bugs waiting to be closed:
	1) Windows: https://support.ntp.org/bugs/show_bug.cgi?id=991
	2) Linux: https://support.ntp.org/bugs/show_bug.cgi?id=992

For these and other operating systems not supporting the BSD routing
socket interface a small piece of code is needed to support event driven 

I am still waiting for people to provide the respective patches....


kiran shirol wrote:
> Frank,
> Instead of using a polling approach, would it not be better we use a 
> notification model, where in whenever there
> is a change in the IP address a notification be generated from the interface 
> managing module. I am not sure if
> there is such a facility availble currently, I will check the same a get 
> back.
> My point is , the probability of change in the IP address is low and hence 
> periodic polling may not be the best
> approach. Let me know your comments.
> Thanks
> Kiran
> "Frank Kardel" <kardel at ntp.org> wrote in message 
> news:4867F324.7080506 at ntp.org...
>> David Woolley wrote:
>>> Kiran Shirol wrote:
>>>> In one of my setups with just one interface I had configured ntp server.
>>>> Everything works fine till here. Now I change the IP address of the
>>>> interface
>>> Interface addresses aren't supposed to change, not just for ntp.  You 
>>> will probably also confuse named.
>>>> My question is How can I make NTP automatically detect this change and
>>>> rebind
>>>> upon the change in the IP address of the interface ?
>> Since 4.2.4 ntpd detects changed interface addresses. Depending on the
>> OS used this happens sooner or later. The fasted detection is on systems
>> that have the routing socket interface (*BSD and maybe Solaris). For
>> Windows and Linux interface scans are done every five minutes by default. 
>> This default can be changed with the -U <secs between scans>
>> command line switch for faster detects (60s is the current lower limit).
>> If some good souls from the Linux and Windows communities would
>> contribute the code needed the for fast detection on these systems 
>> reaction time would improve there.
>>> I believe some work has been done on working round broken ISPs who 
>>> deliberately, or otherwise, misoperate DHCP.
>> The dynamic interface update code was introduced in 4.2.4 to make
>> ntpd work correctly in dynamic/mobile environments.
>>   I'm not sure what state it
>>> is in, but you should certainly be using the very latest release version, 
>>> if not a development version.
>> 4.2.4 (the current stable version) is just fine.
>>> You are, of course, welcome to submit a patch to the source code.
>> Frank 

More information about the questions mailing list