Subject: [dhcwg] Hardware address in DHCP packets (chaddr)

From: "Gaurav Halwasia (ghalwasi)" <[email-masked]>
Date: Wed, 20 Oct 2010 19:54:47 +0530

Hi Folks,

Is it fine to have "chaddr" as zero in the DHCP 4 message exchange
(DORA) in case client-id is also present. RFC 2131 does not seem to be
clearly mentioning this. I do understand that in case of other dhcp
messages like "LEASEQUERY", we can have chaddr as zero.

Any inputs/clarity on this would be highly appreciated.

Thanks,

Gaurav Halwasia

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From: Ted Lemon <[email-masked]>
Date: Wed, 20 Oct 2010 19:34:44 -0400

On Oct 20, 2010, at 10:24 AM, Gaurav Halwasia (ghalwasi) wrote:
> Is it fine to have “chaddr” as zero in the DHCP 4 message exchange (DORA) in case client-id is also present. RFC 2131 does not seem to be clearly mentioning this. I do understand that in case of other dhcp messages like “LEASEQUERY”, we can have chaddr as zero.

If you don't set chaddr, the DHCP server or relay agent will not have the information it needs to unicast a response.   So if you can set it, you really should.   If you can't, you should set the broadcast bit.   This is a workaround that's been used for layer two implementations such as firewire where there is no layer two address.

---

From: Andre Kostur <[email-masked]>
Date: Wed, 20 Oct 2010 16:35:46 -0700

Why would you want to?  As I recall there's only been two cases where an all-zeros chaddr made sense.  Leasequery and one particular hardware layer which apparently didn't have anything to put into the chaddr field.

Theoretically the DHCP server isn't supposed to be using the chaddr to identify the client if a client ID is supplied.  It may be using it for other auditing and access control features that the DHCP server may support.

On 2010-10-20, at 7:24 AM, Gaurav Halwasia (ghalwasi) wrote:

Hi Folks,

Thanks,
Gaurav Halwasia

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From: "Gaurav Halwasia (ghalwasi)" <[email-masked]>
Date: Thu, 21 Oct 2010 08:33:06 +0530

Hi Ted,

Thanks for your response.

Thanks,

Gaurav

From: Ted Lemon [mailto:[email-masked]] 
Sent: Thursday, October 21, 2010 5:05 AM
To: Gaurav Halwasia (ghalwasi)
Cc: [email-masked]
Subject: Re: [dhcwg] Hardware address in DHCP packets (chaddr)

	Is it fine to have "chaddr" as zero in the DHCP 4 message
exchange (DORA) in case client-id is also present. RFC 2131 does not
seem to be clearly mentioning this. I do understand that in case of
other dhcp messages like "LEASEQUERY", we can have chaddr as zero.

If you don't set chaddr, the DHCP server or relay agent will not have
the information it needs to unicast a response.   So if you can set it,
you really should.   If you can't, you should set the broadcast bit.
This is a workaround that's been used for layer two implementations such
as firewire where there is no layer two address.

---

From: "Gaurav Halwasia (ghalwasi)" <[email-masked]>
Date: Thu, 21 Oct 2010 08:39:24 +0530

Hi Andre,

Thanks for inputs. Actually I was asking this to sort of resolve some
interoperability issues with Alcatel Lucent where in ALU client is just
sending client-id with chaddr as zero. As Ted has also relied on the
other thread, I guess we can accept chaddr as zero in case broadcast
flag is set in the upstream DHCP message and client-id is present. 

Thanks,

Gaurav

From: Andre Kostur [mailto:[email-masked]] 
Sent: Thursday, October 21, 2010 5:06 AM
To: Gaurav Halwasia (ghalwasi)
Cc: [email-masked]
Subject: Re: [dhcwg] Hardware address in DHCP packets (chaddr)

Why would you want to?  As I recall there's only been two cases where an
all-zeros chaddr made sense.  Leasequery and one particular hardware
layer which apparently didn't have anything to put into the chaddr
field.

Theoretically the DHCP server isn't supposed to be using the chaddr to
identify the client if a client ID is supplied.  It may be using it for
other auditing and access control features that the DHCP server may
support.

Hi Folks,

Thanks,

Gaurav Halwasia

[email-masked]

---

From: "David W. Hankins" <[email-masked]>
Date: Tue, 26 Oct 2010 15:46:43 -0700

On Wed, Oct 20, 2010 at 07:34:44PM -0400, Ted Lemon wrote:
> If you don't set chaddr, the DHCP server or relay agent will not have the
> information it needs to unicast a response.   So if you can set it, you
> really should.   If you can't, you should set the broadcast bit.   This is
> a workaround that's been used for layer two implementations such as
> firewire where there is no layer two address.

The chaddr contents are also quite frequently used operationally to
enforce network security (dynamic IP:MAC mappings).  I think if you
can set it, you must.  Otherwise you risk non-interoperability.

-- 
David W. Hankins	"If you don't do it right the first time,
Software Engineer		     you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins