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Date: Sun, 1 Jun 2014 02:31:03 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Requirements for new dns backend, factoring considerations
I'm partly just writing this as my own notes to document the design
process, and partly looking for feedback on some of these decisions
for the new dns backend. With that said, here's a lost of components
that will be using it:
1. __lookup_name (getaddrinfo/gethostbyname* backend), which must be
able to perform parallel lookups of A and AAAA records, and
multiple search domain suffixes. For A/AAAA, both answers are
needed. For search domains, all results except the best for each
can be thrown away.
2. getnameinfo, which only needs a single PTR query.
3. res_search and res_query, whose requirements are similar to those
of __lookup_name.
4. res_send, which makes a single query using a caller-provided
query packet.
At first it seems like __lookup_name's dns backend could be
implemented on top of res_search, which could be implemented as
multiple calls to res_query in the search order, which could in turn
be implemented as a call to res_send.
The problem however is the different parallelism requirements.
It's suboptimal to implement res_search in terms of res_query because
you have to wait for one search suffix to fail before trying the next
one. At the res_search/res_query level it's not so bad though. The
parallel search can be the fundamental operation, and res_search and
res_query can just call a common back-end with an argument for whether
to search or not.
The problem however is implementing this on top of something that
looks like res_send. Even if not for search paths, res_search and
res_query need parallel A and AAAA queries, whereas res_send has no
means to request both. We could imagine implementing res_send on top
of a hypothetical "res_multisend" with a count of 1. While this would
work, it's not a very friendly interface for implementing res_search
or res_query since they would have to provide a large number of
pre-generated query packets (6 search domains * 2 RR types * up to 280
bytes per query packet = 3360 bytes of stack usage) despite it being
trivial to generate the Nth packet in just 280 bytes of storage. The
storage requirements for storing all the results are even worse
(6*2*512 = 6144) compared to what's actually needed (2*512 = 1024).
The alternative I see is some sort of "res_multisend" that uses a
callback to let the caller generate the Nth packet and a callback to
notify the caller of replies. Then res_send could be implemented with
callbacks that just feed in and save out the single query/response.
And res_search would generate all the query packets but only save the
"current best match" for each address family.
As another alternative, we could drop the goal of doing search
suffixes in parallel. This would have no bearing on lookups of fully
qualified names using the default settings (ndots:1) since the
presence of a dot suppresses search. Where it would negatively impact
performance, though, is for users who want to have several search
domains (think: home network, university network, department-specific
university network, etc.) for quick shortcuts to machines on multiple
different networks.
Another option still is leaving search domains unimplemented (musl has
not supported them up til now, and there hasn't been much request for
them). But if there is, or will be, demand for them, I don't want the
resolver overhaul design to preclude doing them (or preclude making
them perform decently).
Anyway, if we do decide that parallel search suffixes are not
important, parallel A/AAAA queries are still needed and outside the
scope of what res_send can do. It would be plausible to make an
interface like res_send, in terms of which res_send could be
implemented, but which has an extra boolean argument for "send both A
and AAAA versions of this query". Or we could just accept the cost of
having two copies of the query packet (an extra 280 bytes is
negligible, I think) and make a trivial res_multisend that does N(=2)
queries in parallel using packets provided by the caller.
Rich
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