Subject: Re: LLMNR authorized range

From: Bernard Aboba <[email-masked]>
Date: Thu, 18 Mar 2004 22:41:55 -0800 (PST)

>LLMNR will make it impossible to resolve names of some devices on the
>same link just because they exist on a different subnet.

No. All but PTR RR queries will be sent to the link-scope multicast
address, so they will be resolved without a problem.

>As far as I can see, that is only a problem with PTR queries, which
>often fail anyway.

Yes. Only PTR queries are sent via TCP.  Given that other
queries will work and applications need to be prepared for
PTR RR queries to fail anyway, you'll only see somewhat degraded
performance.  On the other hand, there is a performance improvement for
other cases, since TTL=1 causes unresolvable PTR RR queries to fail
quickly, rather than having to wait for LLMNR_TIMEOUT.  So this is
really just an optimization issue and I'm not clear
that optimizing for an uncommon case (e.g. TTL=5) makes sense.

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From: Bernard Aboba <[email-masked]>
Date: Sat, 20 Mar 2004 03:47:00 -0800 (PST)

[Joshua Graessley] said:

The current solution of using TCP with a TTL of 1 makes the "authorized
range" pretty simple to understand. Addresses in the authorized range are
those that the host knows about. This highlights a problem with the
current LLMNR draft. If there are two subnets overlaid on a local network
and the hosts are unaware of the opposite subnet, LLMNR will make it
impossible to resolve names of some devices on the same link just because
they exist on a different subnet.

[Ted Lemon] replied:
I guess I don't understand why this is a problem. A host that
you can't talk to without going through a router isn't "local." Why would
we expect this to work?

[Mika Liljeberg] said:
As far as I can see, that is only a problem with PTR queries, which
often fail anyway. And I'd wager overlaid subnets are not a very common
configuration. Do you think this is something we need to worry about?

[Bernard Aboba] In practice, I don't believe this issue will arise.
Section 1 says:

"The assumption is that if a network has a gateway,
then the network is able to provide DNS server configuration."

Therefore in the "overlapping subnet" case LLMNR
assumes that the host has DNS configuration. From early
on, LLMNR has assumed that the gateway supports the
resolution of the names of local hosts using DHCPv4 and
dynamic DNS update. However, this assumption cannot be
made for IPv6 at this point.

So if we are talking about multiple subnets on a link in
IPv4, then a conventional DNS query will be sent first,
and local names can be resolved by the gateway. If we are
talking about IPV6, then Router Advertisement enables
the host to become aware of the multiple subnets,
so that TCP-based queries can be sent on the local
link, rather than forwarded by a router.

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From: "Christian Huitema" <[email-masked]>
Date: Sat, 20 Mar 2004 10:48:18 -0800

> > From: Bernard Aboba <[email-masked]>
> >
> > ... If we are
> Terminology confusion? What subnets are in RA? RA can have multiple
> prefixes to be used on the link (if L=1), but they are not subnets.

Uh? There is no official definition of subnet, but the generally agreed
definition is "the set of addresses that shares a specified prefix".

-- Christian Huitema

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From: Bernard Aboba <[email-masked]>
Date: Thu, 18 Mar 2004 22:41:55 -0800 (PST)

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From: Bernard Aboba <[email-masked]>
Date: Sat, 20 Mar 2004 03:47:00 -0800 (PST)

[Joshua Graessley] said:

[Ted Lemon] replied:

[Mika Liljeberg] said:

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From: "Christian Huitema" <[email-masked]>
Date: Sat, 20 Mar 2004 10:48:18 -0800

-- Christian Huitema