Date: Tue, 05 Nov 2013 15:18:29 -0700 From: Ian Lepore <ian@FreeBSD.org> To: John-Mark Gurney <jmg@funkthat.com> Cc: FreeBSD Stable List <freebsd-stable@FreeBSD.org>, Dan Nelson <dnelson@allantgroup.com>, "illoai@gmail.com" <illoai@gmail.com>, "freebsd-virtualization@freebsd.org" <freebsd-virtualization@FreeBSD.org> Subject: Re: how is the MAC for tap(4) computed? Message-ID: <1383689909.31172.169.camel@revolution.hippie.lan> In-Reply-To: <20131105182435.GT73243@funkthat.com> References: <CAGBxaXn27u3J6iJBoDkW_R2=_kcHzSBU=d0v5eDUvnoAsucSjw@mail.gmail.com> <20131104170955.GH63947@dan.emsphone.com> <CAHHBGkpwWSbRxPxhvGwwzTB5SU-dNi9PfeCuOBUB82thspHEtQ@mail.gmail.com> <20131105182435.GT73243@funkthat.com>
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On Tue, 2013-11-05 at 10:24 -0800, John-Mark Gurney wrote: > illoai@gmail.com wrote this message on Mon, Nov 04, 2013 at 16:08 -0500: > > On 4 November 2013 12:09, Dan Nelson <dnelson@allantgroup.com> wrote: > > > In the last episode (Nov 04), Aryeh Friedman said: > > >> There seems to be a very high rate of MAC address collisions when tap is > > >> running on different machines.... is there anyway to make the selection > > >> of MAC more random > > > > > > It looks like it's generated based on the number of ticks since boot, plus > > > the unit number of the tap device: > > > > > > http://fxr.watson.org/fxr/source/net/if_tap.c#L434 > > > > > > So if you have devices created on boot on a bunch of machines, chances are > > > high that you'll get conflicts. Maybe instead of using the 'ticks' value, > > > kern.hostid could be used instead? That has much better randomness than > > > 'ticks'. > > > > With physical interfaces you can use something like > > ifconfig ath0 ether 00:2d:44:88:ff:00 > > (assuming the device & the driver support changing MAC > > addresses) > > > > I've never tried it with a virtual interface, but it should work if the > > device supports it. > > But remeber when choosing an address like that, that you set the second > low bit of the first octect to one to designate that it's a locally > administered address... > > See wikipedia for more info: > https://en.wikipedia.org/wiki/MAC_address#Address_details > Several ethernet drivers for embedded systems have logic to cook up a random address (probably it should be commonized somewhere) in which they set the first 3 bytes to 'b','s','d' and the low bytes to a random number. The nifty thing about the value 62:73:64 (beside that it's bsd) is that it sets the locally-administered bit in the address. -- Ian
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