[ntp:questions] Re: Multicast TTL and FreeBSD trouble

David L. Mills mills at udel.edu
Thu Aug 18 01:20:25 UTC 2005


Roger,

There is a mapping table which converts integers to actual TTL values. 
See at or near line 2954 of ntp_proto.c. This is intended to follow 
guidelines for administrative scoping. I don't remember which RFC and 
the implementation has never been tested for conformance to my 
knowledge. What to do in IPv6 is even of more murk. I am happy to listen 
to suggestions.

Dave

wa6zvp at gmail.com wrote:
> 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:
> 
> 08:28:20.88 123.123.234.234.123 > 224.0.1.1.123: 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).
> 
>         Roger,
>         roger(-at-)hmc(-dot-)edu
> 




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