Linux 2.6 Uptime Calculations

[Sean Elble <elbles () sessys com>]

Hi all,

I apologize if this is kind of a brain dead topic for those of you with
knowledge far beyond mine, but I'm curious as to why nmap can sometimes
calculate the uptime for a Linux system and other times cannot. I did a few
searches using Google and marc.theaimsgroup.com, and really didn't come up
with much, other than one theory of my own.

> From what I can tell, it looks like nmap's uptime calculations may be
dependent upon the system timer frequency, at least on Linux. Testing it out
on a few boxes here, it seems to be able to calculate the uptime for Linux
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.

However, on another 2 of my boxes, both of which running FC4, nmap cannot
determine the uptimes. In both cases, cat'ing
/proc/sys/net/ipv4/tcp_timestamps returns 1, so that aspect should be OK, as
far as having the resources for nmap to calculate the uptime (as I
understand it). It also appears both of these FC4 systems are running with
kernels compiled with a 250 Hz timer frequency (one with a stock FC kernel,
the other with a custom compiled vanilla one). Unfortunately, those systems
are running services that do not enable me to take them down to install
kernels with different system timer frequencies. I'm not at all any kind of
a programmer, which is why I thought I'd try asking all of you. I don't
consider it a bug of any kind, just merely a personal curiosity, and if one
of you gets a second to reply, I'd really appreciate it. Thanks, in advance.

-- 
+-------------------------------------------------+
|  Sean Elble                                     |
|  Virginia Tech                                  |
|  Computer Engineering, Class of 2008            |
|  Vice President, VTLUUG                         |
|  E-Mail:   elbles () sessys com                    |
+-------------------------------------------------+



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