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If I nmap a host, then it outputs an "Uptime" too. What does it mean, is it trustable? Does it gives correct times?

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For another data point that shows "uptime", take a look at Michal Zalewski's p0f (lcamtuf.coredump.cx/p0f.shtml). It's a passive scanner that takes relatively good guesses at the OS that generated various TCP connections. It includes "uptime" when it finds a Linux-generated SYN packet. –  Bruce Ediger May 16 '11 at 14:37
    
What would it change to you if a server had run for 3 years or 1 hour? If you need this information for your own hardware, look for monitoring solutions. –  tiktak May 16 '11 at 20:32
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up vote 5 down vote accepted

Uptime is the time that the system has been "up" and running. So, an uptime of 100 hours means the machine has been running non-stop for 100 hours.

Nmap guesses the uptime of the system. According to the Usage and Examples page:

The uptime guess is labeled a “guess” because various factors can make it completely inaccurate.

I take that to mean it's not trustable. It can be helpful, though.

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Some OS's use uptime of the machine as a base of sequence numbers in certain packets. That's technically an information leak, but they do it anyway for backwards compatibility's sake. So these cases, uptime observed by NMap is actually quite accurate.

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