I'm pretty sure that c-ares is already doing this next server as a
parallel query, just the default timeout isn't where you expect. If you
set it lower, it will start a second request at that point the timeout
is hit, but if the first request responds, it will still use that
response if the next server on the list hasn't yet responded .... its
been a while since I looked at the code, but that seems to be what I
recall. What c-ares does NOT have is an overall query timeout ... that
has been requested previously, but it doesn't currently exist (though I
agree it should). The logic for retries once it hits the end of the
list of nameservers is a bit weird so predicting when a query will
return a failed result is basically impossible from what I recall. So
this seems to be converging on what I originally suggested then, except
now it sounds like also adding the ability to set an overall query timeout.
On 1/19/22 7:04 PM, Dmitry Karpov via c-ares wrote:
> Again, there's a reason happy eyeballs doesn't just hammer all
endpoints returned from getaddrinfo() simultaneously, I'd think the
same reasoning would go for DNS servers ... be kind ... start a second
query after a short delay if we haven't received a response yet (e.g.
200ms).
> It doesn't make sense to hammer more than 1 DNS server if they're
all responsive, you just doubled the network load for DNS for no reason.
Very true! But in my parallel approach, I didn’t mean to start all
parallel queries simultaneously.
I didn’t nail the details, but obviously such approach should be
similar to the Happy Eyeballs even for single stacks.
So, parallel queries in the parallel approach should be started with
some small delays like 200ms in Happy Eyeballs, but the whole name
resolution should be controlled by one constant and deterministic
timeout – i.e. 5s, which shouldn’t depend on the number of the name
servers in the list, as it is currently the case with c-ares.
In my use cases, using c-ares with libcurl, I see different name
resolution timeouts: 5s, 15s,… depending on a number of bad name
servers in the list, which cause some my time critical services to fail.
And we can’t just use 200ms as a DNS timeout per name server and
iterate name servers sequentially, because there are high-latency
satellite links with big RTTs, which require 2s and sometimes more for
name resolutions.
That’s why the parallel approach (with delays between parallel
queries) seems to me as a better solution for bad name servers than
the sequential one.
But as I said, any improvements in this area will be very welcomed
c-ares extensions, especially if they help libcurl with c-ares, used
by a lot of people, to better handle issues with bad name servers.
Thanks,
Dmitry Karpov
*From:* Brad House <b...@brad-house.com>
*Sent:* Wednesday, January 19, 2022 2:37 PM
*To:* c-ares discussions <c-ares@lists.haxx.se>
*Cc:* Dmitry Karpov <dkar...@roku.com>
*Subject:* Re: Feature request for parallel queries for name servers
from different protocol families (IPv4 vs IPv6)
I guess it always depends on the design of whatever is using c-ares.
In my own use cases, I have a single ares_channel running on an event
loop and enqueue my lookups to there ... so it keeps state. Nothing
with thread local storage or anything, just dispatching to that event
loop for any DNS queries that need to be performed. The single
ares_channel can handle multiple simultaneous DNS queries.
Also, since there is a proposed feedback loop, if a DNS server is no
longer reachable, it will re-sort the list for any future requests, so
it would only impact a single request (ok, well, whatever number of
requests came in before the timeout or error occurred).
Again, there's a reason happy eyeballs doesn't just hammer all
endpoints returned from getaddrinfo() simultaneously, I'd think the
same reasoning would go for DNS servers ... be kind ... start a second
query after a short delay if we haven't received a response yet (e.g.
200ms). It doesn't make sense to hammer more than 1 DNS server if
they're all responsive, you just doubled the network load for DNS for
no reason.
On 1/19/22 5:25 PM, Dmitry Karpov via c-ares wrote:
> I wasn't suggesting this be outside of c-ares, I was talking
about implementing this inside of c-ares as a simpler alternative
to your proposal.
OK, I got it know. :)
Pre-sorting name servers based on reachability from previous
queries or/and protocol family may help in some cases, but the
sequential approach, even with sorting, still will have some
issues that the parallel approach allows to solve more efficiently.
For example, the first query when nothing is sorted, may cause
critical connection timeouts aborting some applications, and
storing name server “reachability metrics” which name servers will
be sorted on will require either thread local storage (thus
requiring each thread to go through the same “name server
discovery” procedure as the other app threads using c-ares) or
some global access to the metrics data with proper read/write
accesses, needed by multi-threaded apps.
Also, if run-time conditions change from the previous query then
the sorted list may be not sorted correctly for the current
conditions, and thus not the best server or even bad server may be
tried first, thus increasing name resolution time.
The parallel approach, on the other hand, will provide the fastest
name resolution regardless the previous queries, so it doesn’t
need to store any name server metrics and do pre-processing of the
name server list from OS.
But I agree that implementing parallel approach may be not very
easy and any improvements in this area will be a very welcomed
extension, anyway.
So, if you think that updated sequential approach with smart
sorting is much easier to implement than the parallel one, then
hopefully we can get it in next c-ares updates.
Thanks,
Dmitry Karpov
*From:* Brad House <b...@brad-house.com> <mailto:b...@brad-house.com>
*Sent:* Wednesday, January 19, 2022 12:10 PM
*To:* c-ares discussions <c-ares@lists.haxx.se>
<mailto:c-ares@lists.haxx.se>
*Cc:* Dmitry Karpov <dkar...@roku.com> <mailto:dkar...@roku.com>
*Subject:* Re: Feature request for parallel queries for name
servers from different protocol families (IPv4 vs IPv6)
Commenting below ...
On 1/19/22 2:51 PM, Dmitry Karpov via c-ares wrote:
> Infact, happyeyeballs itself doesn't always do parallel
connection attempts, its an implementation-defined delay
before also attempting the next address in the list.
In case of Happy Eyeballs, a delay between IPv4 and IPv6
connections is constant and typically relatively short –
200-300ms.
But non-functional IPv6 name servers in the server list may
create dynamic delays in connection establishment which can be
very large.
By default, c-ares uses 5s timeout per name server, so it may
take 5s and more (if several IPv6 name servers are in the
list) to get to the connection Happy Eyeballs thus taking
much more than expected 200-300ms.
It would be assumed as part of this patch set, this timer would be
reduced.
> It would be much easier to stay closer to happy eyeballs and
just sort the dns server list using prior result success/fail
(even upfront sorting using some algorithm to interleave
ipv6/ipv4 in a pattern would help,
> maybe with using logic such as from RFC6724 sec 2.1 like we
do in ares_getaddrinfo for returned addresses, but instead of
the nameservers themselves).
Yes, of course, it is possible that c-ares client can
implement some kind of name server sorting/filtering logic
outside of c-ares and just pass a list of “good” name servers
to c-ares, but in this case it has to be more involved into
the name resolution business than it would be desired.
I wasn't suggesting this be outside of c-ares, I was talking about
implementing this inside of c-ares as a simpler alternative to
your proposal.
-Brad
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