Date: Tue, 5 Nov 2013 10:24:35 -0800 From: John-Mark Gurney <jmg@funkthat.com> To: "illoai@gmail.com" <illoai@gmail.com> Cc: Aryeh Friedman <aryeh.friedman@gmail.com>, FreeBSD Stable List <freebsd-stable@freebsd.org>, Dan Nelson <dnelson@allantgroup.com>, "freebsd-virtualization@freebsd.org" <freebsd-virtualization@freebsd.org> Subject: Re: how is the MAC for tap(4) computed? Message-ID: <20131105182435.GT73243@funkthat.com> In-Reply-To: <CAHHBGkpwWSbRxPxhvGwwzTB5SU-dNi9PfeCuOBUB82thspHEtQ@mail.gmail.com> References: <CAGBxaXn27u3J6iJBoDkW_R2=_kcHzSBU=d0v5eDUvnoAsucSjw@mail.gmail.com> <20131104170955.GH63947@dan.emsphone.com> <CAHHBGkpwWSbRxPxhvGwwzTB5SU-dNi9PfeCuOBUB82thspHEtQ@mail.gmail.com>
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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 -- John-Mark Gurney Voice: +1 415 225 5579 "All that I will do, has been done, All that I have, has not."
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