On 24 Oct 2018, at 10:01, Viktor Dukhovni wrote:
My reading of RFC 1035 is that DNS name "compression"
via "pointers" is restricted to name strictly earlier
in the DNS message:
4.1.4. Message compression
In order to reduce the size of messages, the domain system utilizes
a
compression scheme which eliminates the repetition of domain names
in a
message. In this scheme, an entire domain name or a list of labels
at
the end of a domain name is replaced with a pointer to a prior
occurance
---------------
of the same name.
Not strictly to do with loops but we noticed that not all nameservers
use the same compression algorithm. See section 9.1 and appendix B of
https://datatracker.ietf.org/doc/draft-ietf-dnsop-dns-capture-format
John
And yet, here and there I see mention of having to take care to avoid
"loops",
but loops are impossible in a monotone strictly decreasing sequence.
Is there a later RFC that relaxes the constraint and allows pointers
to names
later in the message? I'm having a bit of trouble finding the later
text...
Secondarily, can the pointer point to some odd-ball location earlier
in the
message that is not semantically a label in its original context, but
just
happens to carry data that decodes as the desired label? Or, are
pointers
only valid to prior locations that are corresponding labels in their
original
context?
--
Viktor.
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John Dickinson
http://sinodun.com
Sinodun Internet Technologies Ltd.
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