Subject: RE: Turning off link-locals?

From: "Christian Huitema" <[email-masked]>
Date: Thu, 23 Jan 2003 23:40:39 -0800

The discussion on "turning off link local" is a little weird. As Keith noted, a DHCP option would be at best the moral equivalent of a "don't trespass" sign; this will not actually prevent a host of configuring whatever address it pleases. However, that does not mean that the administrator is powerless. If the administrator configures all the other hosts on the network to not reply to LL addresses, then the no-trespass sign gets some teeth: an isolated host will indeed be able to configure itself with an LL address, but that will be pretty much useless; it will not be able to interact with the local hosts.
 
So, in practice, the "turn off LL" option would really be an instruction to all hosts to stop using any LL address and stop responding to these addresses. If conforming hosts do that, then the visiting host will in practice be forced to configure an address using DHCP, not LL. In fact, this is pretty much the current behavior of the windows stack: once it has obtained an address, it does not consider that the 169.254/16 prefix is on line, and thus in practice stops interacting with LL addresses.
 
Obviously, there are two options, and these are really two options for conformant hosts: continue interacting with LL addresses after being configured with a routable address, or don't. In sake of the robustness principle, I clearly see the case for continuing interacting. However, Keith also made the point for giving some control to the local admin. The reasonable option would thus be to define an explict DHCP option to "ignore LL", and have that option affect the behavior of the configured hosts.
 
-- Christian Huitema

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From: "Philip Nye" <[email-masked]>
Date: Fri, 24 Jan 2003 10:20:50 -0000

There seems to be a problem of perspective. rfc2563 is an option which
requires some configuration on the part of the network administrator.

By far the most common case is probably that DHCP either gives you an
address so you don't need a LL one, or it doesn't reply (because its not
present or does not recognise your identifier), in which case you are quite
welcome to self assign a v4LL address.

If the network admin has gone to the trouble of configuring a rfc2563 option
then this is probably signalling a positive intention, but even then the
option may well be to go ahead and autoconfigure.

In the case of a small device which doen't have a user interface, the
solution is not to hard code that device to ignore network policy, the
solution is ultimately to change network policy. On the other hand, if the
device has a user interface which lets you the user override network policy
then that is your responsibility.

DoS:
Any DHCP option is subject to attack and this one option doesn't make v4LL
especially vulnerable - the way some on this list are talking its a wonder
anyone uses DHCP at all! If I maliciously wanted to stop clients getting a
LL address, why would I do it by setting up a DHCP server? I would simply
defend all their claims.

If somebody sets up a bogus DHCP server, that is a problem for the network
as a whole (or for the DHC wg) and not one which requires an special v4LL
solution. If the extra DHCP server is an acident rather than malicious, then
perhaps rfc2563 should be ammended to mention this and say that the default
state in the absence of explicit management action must be "autoconfigure".

However, the huge take up of rfc2563 since 1999 (has anyone implemented it
ever?) indicates either that the network community is not interested in this
way of doing things - or that it needs something like the IPv4-LL spec to
raise it from obscurity.

Philip

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From: "John C. Welch" <[email-masked]>
Date: Fri, 24 Jan 2003 07:52:19 -0500

On 01/24/2003 02:40, "Christian Huitema" <[email-masked]>
wrote:

> The reasonable option would thus be to define an explict DHCP option to
> "ignore LL", and have that option affect the behavior of the configured hosts.

So now we redefine DHCP?

john

-- 
John C. Welch
Consultant III
Administrative Computing
(617) 253 - 1368 work
(508) 579 - 7380 cell
bynkii2          AIM

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From: John Schnizlein <[email-masked]>
Date: Fri, 24 Jan 2003 08:40:39 -0500

At 07:52 AM 1/24/2003, John C. Welch wrote:
>> The reasonable option would thus be to define an explict DHCP option 
>> to "ignore LL", and have that option affect the behavior of the 
>> configured hosts.
>
>So now we redefine DHCP?

Of course not. It might be that Christian's suggestion implies 
revision of RFC 2563, for which Ralph has already written a draft.
In any case, defining an option, or redefining that one, it is
just an option, not DHCP that needs fixing. Options are the
deliberately extensible part of DHCP. 

John

Maybe you missed Philip's email: 
At 10:44 AM 1/23/2003, Philip Nye wrote:

>Digging around in DHCP I found RFC2563

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From: Ted Lemon <[email-masked]>
Date: Fri, 24 Jan 2003 09:43:51 -0800

> Obviously, there are two options, and these are really two options for 
> conformant hosts: continue interacting with LL addresses after being 
> configured with a routable address, or don't. In sake of the 
> robustness principle, I clearly see the case for continuing 
> interacting. However, Keith also made the point for giving some 
> control to the local admin. The reasonable option would thus be to 
> define an explict DHCP option to "ignore LL", and have that option 
> affect the behavior of the configured hosts.

I think this is a good compromise, and I don't object to it, but again 
I question whether or not anybody's going to bother to implement it in 
a client.