| Re: [chrony-dev] NTP packet with valid header but not data |
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On Wed, Nov 11, 2009 at 09:28:57AM -0800, Bill Unruh wrote:
> I think that the stratum test is needed as that is the only way to prevent
> loops (and is I think the main reason for the existence of strata in the first
> place.) If you accept a machine whose stratum is larger than ours, you can
> have the situation where they get the time from you and you get yours from
> them.
Ok, but I'm not sure it works in chrony as intended. If there is no
other source, it will first make itself unsynced, then the packet
header will be valid and chrony will sync to it later. It looks like
the code is just following RFC 1305. In ntpd there is a check if their
refid is not us to avoid loops, I guess this was added in NTPv4.
> The other proposal sounds OK, but I really have no gut feeling for
> it. The question is can chrony recover from a source marked as
> unreachable, or
> does one bad packet mean that source is forever unreachable. That would not be
> good.
It will recover. Source is marked as unreachable when 9 last responses
had not valid headers or were lost and set reachable when one valid
header is received.
Maybe the only change needed is to set the source reachable only when
both header and data are valid?
--
Miroslav Lichvar
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