Subject: Re: unaligned compression

From: "D. J. Bernstein" <[email-masked]>
Date: 30 Nov 1999 22:48:54 -0000

[email-masked] writes:
> Local-compression was killed in last call.

The fact remains that Peter Koch, in his ``security considerations,''
advised client implementors to ``verify'' that conventional pointers
``really point to the start of a label.'' Has Koch deployed clients that
do this?

You say that a server can use unaligned pointers. RFC 1035 seems to
allow them. But Vixie says that a server shouldn't use unaligned
pointers, and Koch says that a server must not use unaligned pointers.

I'm finishing up a new name server. If I compress more effectively than
BIND---which is easier to implement than BIND's approach---will I run
into interoperability problems? If I compress less effectively than BIND
but still fit things into UDP, will I be beseiged by religious fanatics?

---Dan

---

From: [email-masked]
Date: Wed, 01 Dec 1999 19:34:51 +1100

> ``really point to the start of a label.'' Has Koch deployed clients that

	This is not an issue.  Any "deployed" clients are deemed to be
	experimental clients which we don't need to worry about.  Not
	that I am aware of any.

> pointers, and Koch says that a server must not use unaligned pointers.

	Paul said they shouldn't *generate* them.

	How could you verify them as local compression hid them.
	You would just have to trust that the pointer was correct
	if you didn't know the structure of a given type.  Anyway
	this is not a issue.

> into interoperability problems?

	You mean you actually search through the message for existing
	domain names rather than just remembering the ones you put there?

	BIND 4, 8 and 9 just follows the pointers.  Just make sure they
	always point backwards.

> If I compress less effectively than BIND

	You shouldn't be attacked.

	No one's complained about BIND not using all the available
	compression pointers in the question.  Before someone
	submits a bug report this will be fixed.

> ---Dan
	Mark
--
Mark Andrews, Internet Engines Inc. / Internet Software Consortium
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: [email-masked]