On Sun, 07 Mar 2010, 16:19 +0000, Christian Weisgerber
wrote:> ntpd is a convenient source of multicast packets for testing purposes.
> When I add
>
> broadcast 224.0.1.1
>
> to my ntp.conf, ntpd sends a multicast packet with TTL 1 every 64
> seconds. Just as expected. However, when I explicitly specify the
> TTL as in
>
> broadcast 224.0.1.1 ttl 1
>
> it sends packets with TTL 32. Trying a few other numbers confirms
> that it multiplies the specified TTL by 32. That is not expected.
> (I also don't recall this happening the last time I tried it, but
> that may have been years ago.)
>
> Is this simply a bug in ntpd?
No, it's just that the ntp's server configuration statements don't
use
their ttl option to specify network ttl value, but as zero-based index
into ntp's ttl value array. The default array is as you describe
[1,32,64,96,128,160,192,224] but can be overridden by the ttl
configuration statement.
So the following lines in your ntp.conf would result in your multicast
server transmitting packets with ttl=4.
ttl 2 4 6 8
broadcast 224.0.1.1 ttl 1
I tripped over this last year when experimenting with ntp multicast.
I had to resort to the source code to understand what was happening.
It is actually documented.
<http://www.eecis.udel.edu/~mills/ntp/html/miscopt.html#ttl>
--
John Marshall
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