[ntp:questions] Multicast TTL and FreeBSD trouble

wa6zvp at gmail.com wa6zvp at gmail.com
Wed Aug 17 15:47:23 UTC 2005

Multicast packets from NTPD in FreeBSD do not seem to have the correct
TTL value.  Here is a sample TCPDump from a machine with multcast TTL
set to 2:

08:28:20.88 > v4 bcast strat 2 poll
6 prec -19
                 4500 004c 5756 0000 4011 bd8d aaaa bbbb

TTL is byte 9 of the header, displayed above as '40' or 64 decimal.
Experiments have shown that the TTL always appears in the packet with a
value 32x what is configured, or left shifted by 5 bits.  It seems more
than coincidental that the previous 2 bytes are really a 3 bit value
and a 13 bit value.  Perhaps they were formatted incorrectly, with the
effect of left shifting the TTL.

Current tests include FreeBSD versions 4.8, 5.3 and 5.4, along with NTP
released versions as well as (almost) the most recent DEV version.

The same test were also done on a couple of other platforms:

 RH Linux w/2.4 kernel works correctly, while the xNTP port in Multinet
4.3 on OpenVMS doesn't set the TTL at all (is 0).


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