Re: Linux 2.6 Uptime Calculations

["赵雷" <zhaolei () gmail com>]

> systems running either a 100 Hz or 1000 Hz system timer frequency (it could
> calculate the uptime for an old Red Hat 7.3 box I still have running when it
> was running a 2.4 kernel with a 100 Hz timer frequency, but has problems
> with a 2.6.15 kernel with a timer frequency of 250 Hz). Nmap also has no
> problems calculating the uptime of a fairly stock CentOS 4.3 system with the
> 2.6.9 kernel, with the timer running at 1000 Hz.

You are right. Nmap does uptime guessing by the returned TCP timestamp
values. Current Nmap only takes into account the frequencies of 2, 100
and 1000Hz; it would not calculate the uptime of a host which uses
250Hz or other frequencies.
This would be solved in the new Nmap OS fingerprinting system. I have
tested a 2.6.15 kernel compiled with 250Hz :)

Regards,
Zhao Lei


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