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From: Johannes Berg <johannes-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
To: Guy Harris <guy-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org>
Cc: David Young <dyoung-e+AXbWqSrlAAvxtiuMwx3w@public.gmane.org>,
	radiotap-S783fYmB3Ccdnm+yROfE0A@public.gmane.org,
	Simon Barber <simon-vp0mx6+5gkqFX2APIN6yfw@public.gmane.org>
Subject: Re: [RFC] capture file timestamping
Date: Tue, 25 Aug 2015 19:55:46 +0200
Message-ID: <1440525346.2192.55.camel@sipsolutions.net> (raw)
In-Reply-To: <4E4FE63E-B636-4808-BBFF-6D2CE4EAFB2F-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org>

On Tue, 2015-08-25 at 10:47 -0700, Guy Harris wrote:
> 
> If you have tcpdump 4.6 or later, with libpcap 1.6 or later, on 
> Linux, you should have the --time-stamp-type and --list-time-stamp
> -types options.
> 
> If so, what does
> 
> 	> tcpdump --list-time-stamp-types
> 
> print?
> 
> If it prints "adapter" or "adapter_unsynced", you might want to try 
> those with the --time-stamp-type option, as those time stamp types 
> mean that the time stamp will come from the adapter rather than from 
> Linux.  ("unsynced" means that the time stamps aren't synchronized 
> with the host's clock.)

It does have "adapter_unsynced", but wouldn't the adapter also have to
support it? Ah, yes, if I do
	tcpdump -i wlan0 --list-time-stamp-types

it no longer shows it.

Although perhaps I could possibly convince the driver to report this
timestamp somehow, rather than changing radiotap.

> unless it's based on the TSF (which it probably won't be, as it 
> should, at least in principle, be Epoch time, although I think Linux 
> may have, at some point, changed adapter time stamps in a fashion 
> making them not useful as libpcap time stamps).

That, however, I almost certainly cannot do - I don't see how I'd
synchronize from the adapter time (a free-running 32-bit counter at
microseconds resolution) to the host time.

johannes

  parent reply index

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-08-25 10:07 Johannes Berg
     [not found] ` <1440497229.2192.28.camel-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
2015-08-25 16:28   ` David Young
     [not found]     ` <20150825162841.GR6823-e+AXbWqSrlAAvxtiuMwx3w@public.gmane.org>
2015-08-25 16:46       ` Johannes Berg
     [not found]         ` <1440521168.2192.50.camel-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
2015-08-25 17:47           ` Guy Harris
     [not found]             ` <4E4FE63E-B636-4808-BBFF-6D2CE4EAFB2F-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org>
2015-08-25 17:55               ` Johannes Berg [this message]
2015-08-26 22:48           ` Simon Barber
     [not found]             ` <55DE4244.9030407-vp0mx6+5gkqFX2APIN6yfw@public.gmane.org>
2015-09-17 16:51               ` Johannes Berg

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