From owner-freebsd-net@FreeBSD.ORG  Tue Sep 29 06:50:09 2009
Return-Path: <owner-freebsd-net@FreeBSD.ORG>
Delivered-To: freebsd-net@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id 5E240106568F
	for <freebsd-net@freebsd.org>; Tue, 29 Sep 2009 06:50:09 +0000 (UTC)
	(envelope-from bakul@bitblocks.com)
Received: from mail.bitblocks.com (mail.bitblocks.com [64.142.15.60])
	by mx1.freebsd.org (Postfix) with ESMTP id 3D1D08FC14
	for <freebsd-net@freebsd.org>; Tue, 29 Sep 2009 06:50:09 +0000 (UTC)
Received: from bitblocks.com (localhost.bitblocks.com [127.0.0.1])
	by mail.bitblocks.com (Postfix) with ESMTP id C87045B30;
	Mon, 28 Sep 2009 23:32:19 -0700 (PDT)
To: Adam Vande More <amvandemore@gmail.com>
In-reply-to: Your message of "Mon, 28 Sep 2009 15:21:17 CDT."
	<6201873e0909281321s51e3d485i1074954d43b657d6@mail.gmail.com> 
References: <6201873e0909281321s51e3d485i1074954d43b657d6@mail.gmail.com>
Comments: In-reply-to Adam Vande More <amvandemore@gmail.com>
	message dated "Mon, 28 Sep 2009 15:21:17 -0500."
Date: Mon, 28 Sep 2009 23:32:19 -0700
From: Bakul Shah <bakul@bitblocks.com>
Message-Id: <20090929063219.C87045B30@mail.bitblocks.com>
Cc: freebsd-net@freebsd.org
Subject: Re: tap dhcp 
X-BeenThere: freebsd-net@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Networking and TCP/IP with FreeBSD <freebsd-net.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-net>,
	<mailto:freebsd-net-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-net>
List-Post: <mailto:freebsd-net@freebsd.org>
List-Help: <mailto:freebsd-net-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-net>,
	<mailto:freebsd-net-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Tue, 29 Sep 2009 06:50:09 -0000

On Mon, 28 Sep 2009 15:21:17 CDT Adam Vande More <amvandemore@gmail.com>  wrote:
> Am I doing something wrong here?
> 
> kldload if_tap if_bridge
> sysctl net.link.tap.user_open=1
> ifconfig tap0 create
> ifconfig bridge create
> ifconfig bridge0 addm tap0 addm em0
> 
> it# dhclient tap0
> DHCPDISCOVER on tap0 to 255.255.255.255 port 67 interval 6
> DHCPDISCOVER on tap0 to 255.255.255.255 port 67 interval 6
> DHCPDISCOVER on tap0 to 255.255.255.255 port 67 interval 13
> DHCPDISCOVER on tap0 to 255.255.255.255 port 67 interval 18
> DHCPDISCOVER on tap0 to 255.255.255.255 port 67 interval 18
> No DHCPOFFERS received.
> No working leases in persistent database - sleeping.
> 
> Although dhclient em0 works as expected.

This is behaving as it should.  Here is why.

Normal behavior of the tap device:
* What you send on if tap0 can be read on /dev/tap0. [if == interface]
* What you write on /dev/tap0 can be received on if tap0.

Your bridge setup:

bridge0
   |---- em0 -- physical network
   |---- tap0--/dev/tap0-virtual machine


For this setup the behavior is modified as follows:

* What you write to /dev/tap0 can be received on if tap0 + it
  will be sent out to the phys. net over em0 (but you can't
  receive this packets on if em0).

* What you send on if tap0 can be read on /dev/tap0 + it will
  be sent out to the phys. net over em0 (but you can't
  receive this packets on if em0).

* What you receive from a phys. device can be read on em0 +
  it will be sent out on tap0 (so you can read it from
  /dev/tap0) but you can't receive this packet on if tap0.

So you can see that DHCPDISCOVER pkt sent on tap0 will be
seen by the dhcp server on a physical net but you can't
receive the response on if tap0.

Now typically an emulator like qemu will connect its emulated
VM's interface (say re0) to /dev/tap0.  If the VM does
dhclient re0, it will get its ip address from the dhcp server
on the phys net.

If you want dhclient tap0 to work, run a dhcp server on the
VM!