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requirements update with design decisions.

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git-svn-id: file:///svn/unbound/trunk@1286 be551aaa-1e26-0410-a405-d3ace91eadb9
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Wouter Wijngaards 2008-10-06 15:07:27 +00:00
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doc/Changelog
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@ -2,6 +2,7 @@
- jostle-timeout option, so you can config for slow links.
- 0x20 fallback code. Tries 3xnumber of nameserver addresses
queries that must all be the same. Sent to random nameservers.
- documented choices for DoS, EDNS, 0x20.
2 October 2008: Wouter
- fixup unlink of pidfile.

37
doc/requirements.txt
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@ -206,3 +206,40 @@ o Case preservation
The casing from the query name is used in preference to the casing
of the authority server. This is the same as BIND. RFC4343 allows either
behaviour.
o Denial of service protection
If many queries are made, and they are made to names for which the
authority servers do not respond, then the requestlist for unbound
fills up fast. This results in denial of service for new queries.
To combat this the first 50% of the requestlist can run to completion.
The last 50% of the requestlist get (200 msec) at least and are replaced
by newer queries when older (LIFO).
When a new query comes in, and a place in the first 50% is available, this
is preferred. Otherwise, it can replace older queries out of the last 50%.
Thus, even long queries get a 50% chance to be resolved. And many 'short'
one or two round-trip resolves can be done in the last 50% of the list.
The timeout can be configured.
o EDNS fallback. Is done according to the EDNS RFC (and update draft-00).
Unbound assumes EDNS 0 support for the first query. Then it can detect
support (if the servers replies) or non-support (on a NOTIMPL or FORMERR).
Some middleboxes drop EDNS 0 queries, mainly when forwarding, not when
routing packets. To detect this, when timeouts keep happening, as the
timeout approached 5-10 seconds, and EDNS status has not been detected yet,
a single probe query is sent. This probe has a sub-second timeout, and
if the server responds (quickly) without EDNS, this is cached for 15 min.
This works very well when detecting an address that you use much - like
a forwarder address - which is where the middleboxes need to be detected.
Otherwise, it results in a 5 second wait time before EDNS timeout is
detected, which is slow but it works at least.
It minimizes the chances of a dropped query making a (DNSSEC) EDNS server
falsely EDNS-nonsupporting, and thus DNSSEC-bogus, works well with
middleboxes, and can detect the occasional authority that drops EDNS.
o 0x20 backoff.
The draft describes to back off to the next server, and go through all
servers several times. Unbound goes on get the full list of nameserver
addresses, and then makes 3 * number of addresses queries.
They are sent to a random server, but no one address more than 4 times.
It succeeds if one has 0x20 intact, or else all are equal.
Otherwise, servfail is returned to the client.