Nmap Development mailing list archives
Uptime estimates and TCP timestamp offsets
From: David Fifield <david () bamsoftware com>
Date: Mon, 18 Aug 2008 18:02:58 -0600
Hi, Fyodor asked me to look into why Nmap's estimate of uptime is way off on some operating systems. For instance he told me that scanme.nmap.org has been up for 147 days but Nmap reports its uptime as 47 days. Apart from being inaccurate the uptime otherwise increases normally, i.e., it increases by one day each day. This has been observed on Linux and Mac OS X. Nmap's estimate of uptime is based on a series of TCP timestamp measurements. I found this patch to the Linux kernel that allows setting an offset that is added to all timestamps: http://lkml.org/lkml/2008/2/15/244 That would explain the observed behavior. However by doing a cursory check of the current versions of certain files affected by the patch, it doesn't appear to have been applied. Does anybody know? Maybe it has been applied by individual distributions? I scanned a Mac OS X 10.5.4 machine moments after booting it up. I repeated the experiment four times, rebooting each time. I got Ignoring claimed uptime of 1219 days Ignoring claimed uptime of 1181 days Uptime: 644.073 days Ignoring claimed uptime of 871 days So it certainly seems to be picking a random offset at boot. What do we do? Nmap already throws out very long uptimes, but a plausible uptime (like scanme's 47 days) can still be wrong. I don't think there's a way to detect an operating system adding a random offset to its timestamps, unless you scan across boots. Even though it can be fooled, the uptime calculation isn't useless--it still works for most OSs out there. Maybe just label it "Uptime guess"? David Fifield _______________________________________________ Sent through the nmap-dev mailing list http://cgi.insecure.org/mailman/listinfo/nmap-dev Archived at http://SecLists.Org
Current thread:
- Uptime estimates and TCP timestamp offsets David Fifield (Aug 18)
- Re: Uptime estimates and TCP timestamp offsets Arturo 'Buanzo' Busleiman (Aug 18)
- Re: Uptime estimates and TCP timestamp offsets David Fifield (Aug 18)
- Re: Uptime estimates and TCP timestamp offsets Michael Pattrick (Aug 18)
- Re: Uptime estimates and TCP timestamp offsets David Fifield (Aug 18)
- Re: Uptime estimates and TCP timestamp offsets Michael Pattrick (Aug 18)
- Re: Uptime estimates and TCP timestamp offsets Brandon Enright (Aug 18)
- Re: Uptime estimates and TCP timestamp offsets David Fifield (Aug 18)
- RE: Uptime estimates and TCP timestamp offsets Dario Ciccarone (dciccaro) (Aug 18)
- Re: Uptime estimates and TCP timestamp offsets David Fifield (Aug 18)
- Re: Uptime estimates and TCP timestamp offsets David Fifield (Aug 18)
- Re: Uptime estimates and TCP timestamp offsets David Fifield (Aug 18)
- Re: Uptime estimates and TCP timestamp offsets David Fifield (Aug 19)
(Thread continues...)
- Re: Uptime estimates and TCP timestamp offsets Arturo 'Buanzo' Busleiman (Aug 18)