[ntp:questions] Max number of lines that ntpdc -c monlist could display

mills at udel.edu mills at udel.edu
Tue May 8 17:55:30 UTC 2007


Danny,

The max packet size used by ntpd in response to a request is 468 data 
bytes plus headers. The problem is not the packet size, but the number 
of packets. Even with the default max lines, this can result in a burst 
up to 80 packets. While modern gatways and routers can usully handle 
such a burst, it might set off flood alarms in the firewalls.

Dave

Danny Mayer wrote:

> Martin Burnicki wrote:
> 
>>Danny Mayer wrote:
>>
>>>Ronan Flood wrote:
>>>
>>>>mayer at ntp.isc.org (Danny Mayer) wrote:
>>>>
>>>>
>>>>>Eugen COCA wrote:
>>>>>
>>>>>>What is the maximum number of lines that
>>>>>>
>>>>>>ntpdc -c monlist
>>>>>>
>>>>>>command could display ?
>>>>>
>>>>>#define     MAXMONMEM       600     /* we allocate up to 600 structures */
>>>>
>>>>I have that set to 1920 in ntp-4.2.2, which I calculate is the maximum
>>>>number of IPv4 addresses that can be returned to ntpdc without changing
>>>>the protocol (max 128 UDP packets).  Note that I need to use an earlier
>>>>version of ntpdc, which is not IPv6 aware, to get the full list.  I also
>>>>had to increase ntpdc's receive socket buffer size to cope
>>>>(rbufsize = INITDATASIZE * 9), but that might be an OS issue.
>>>>
>>>
>>>Can you file a bug report on this?
>>
>>Shouldn't this be handled with bug #286:
>>"ntpdc -nc monlist output failure when amount of clients is over 512"
>>https://ntp.isc.org/bugs/show_bug.cgi?id=286
>>
>>Martin
> 
> 
> Yes, almost identical. So Ronan please add your comments to that bug
> report. I think that the biggest problem is timeouts on the client side
> probably due to the size of the packet with possible additional
> limitations by firewalls not expecting such large packets.
> 
> Danny
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