From: Florian Weimer <fw@deneb.enyo.de>
To: Rich Felker <dalias@libc.org>
Cc: musl@lists.openwall.com
Subject: Re: [musl] TCP support in the stub resolver
Date: Sat, 02 May 2020 17:28:48 +0200 [thread overview]
Message-ID: <87wo5umk3z.fsf@mid.deneb.enyo.de> (raw)
In-Reply-To: <20200501220238.GP21576@brightrain.aerifal.cx> (Rich Felker's message of "Fri, 1 May 2020 18:02:38 -0400")
* Rich Felker:
> On Tue, Apr 21, 2020 at 07:26:08PM +0200, Florian Weimer wrote:
>> * Rich Felker:
>>
>> >> I'm excited that Fedora plans to add a local caching resolver by
>> >> default. It will help with a lot of these issues.
>> >
>> > That's great news! Will it be DNSSEC-enforcing by default?
>>
>> No. It is currently not even DNSSEC-aware, in the sense that you
>> can't get any DNSSEC data from it. That's the sad part.
>
> That's really disappointing. Why? Both systemd-resolved and dnsmasq,
> the two reasonable (well, reasonable for distros using systemd already
> in the systemd-resolved case :) options for this, support DNSSEC fully
> as I understand it. Is it just being turned off by default because of
> risk of breaking things, or is some other implementation that lacks
> DNSSEC being used?
It's systemd-resolved. As far as I can tell, it does not provide
DNSSEC data on the DNS client interface.
>> >> > BTW, am I mistaken or can TCP fastopen make it so you can get a DNS
>> >> > reply with no additional round-trips? (query in the payload with
>> >> > fastopen, response sent immediately after SYN-ACK before receiving ACK
>> >> > from client, and nobody has to wait for connection to be closed) Of
>> >> > course there are problems with fastopen that lead to it often being
>> >> > disabled so it's not a full substitute for UDP.
>> >>
>> >> There's no handshake to enable it, so it would have to be an
>> >> /etc/resolv.conf setting. It's also not clear how you would perform
>> >> auto-detection that works across arbitrary middleboxen. I don't think
>> >> it's useful for an in-process stub resolver.
>> >
>> > The kernel automatically does it,
>>
>> Surely not, it causes too many interoperability issues for that. It's
>> also difficult to fit it into the BSD sockets API. As far as I can
>> see, you have to use sendmsg or sendto with MSG_FASTOPEN instead of a
>> connect call to establish the connection.
>>
>> (When the kernel says that it's enabled by default, it means that you
>> can use MSG_FASTOPEN with sysctl tweaks.)
>
> What I mean is that, if you use MSG_FASTOPEN on a kernel new enough to
> understand it, I think it makes a normal TCP connection and sends the
> data if fastopen is not enabled or not supported by the remote host,
> but uses fastopen as long as it's enabled and supported. In this sense
> it's automatic. But of course we'd have to fallback explicitly anyway
> if it's not supported in order to maintain compatibility with older
> kernels.
I found this in the kernel sources. It's a bit worrying.
/*
* The following code block is to deal with middle box issues with TFO:
* Middlebox firewall issues can potentially cause server's data being
* blackholed after a successful 3WHS using TFO.
* The proposed solution is to disable active TFO globally under the
* following circumstances:
* 1. client side TFO socket receives out of order FIN
* 2. client side TFO socket receives out of order RST
* 3. client side TFO socket has timed out three times consecutively during
* or after handshake
* We disable active side TFO globally for 1hr at first. Then if it
* happens again, we disable it for 2h, then 4h, 8h, ...
* And we reset the timeout back to 1hr when we see a successful active
* TFO connection with data exchanges.
*/
It's possible that the retransmit with TCP Fast Open happens as part
of the regular TCP state machine. I can't find an explicit fallback
handler.
>> >> Above 4096 bytes, pretty much all recursive resolvers will send TC
>> >> responses even if the client offers a larger buffer size. This means
>> >> for correctness, you cannot do away with TCP support.
>> >
>> > In that case doing EDNS at all seems a lot less useful. Fragmentation
>> > is always a possibility above min MTU (essentially same limit as
>> > original UDP DNS) and the large responses are almost surely things you
>> > do want to avoid forgery on, which leads me back around to thinking
>> > that if you want them you really really need to be running a local
>> > DNSSEC validating nameserver and then can just use-vc...
>>
>> Why use use-vc at all? Some software *will* break because it assumes
>> that certain libc calls do not keep open some random file descriptor.
>
> Does use-vc do that (keep the fd open) in glibc? It doesn't seem to be
> documented that way, just as forcing use of tcp, and my intent was not
> to keep any fd open (since you need a separate fd per query anyway to
> do them in parallel or in case the server closes the socket after one
> reply).
Sorry, I thought you wanted to keep the connection open to reduce
latency.
next prev parent reply other threads:[~2020-05-02 15:29 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <fce05ab0ed102dec10e4163dd4ce5d8095d2ffd7.camel@web.de>
[not found] ` <20200412211807.GC41308@straasha.imrryr.org>
[not found] ` <d64b1b8801cc5350e9d27dd109dd2446e7d4b860.camel@web.de>
[not found] ` <20200413024746.GD41308@straasha.imrryr.org>
[not found] ` <b38668e94b2781003a14c6dca3d41edf33e347e2.camel@web.de>
[not found] ` <A2FE67B5-A9A9-4A0F-A59D-78FF2AB992B7@dukhovni.org>
[not found] ` <f79a9f0c369607fc38bef06fec521eaf3ab23d8c.camel@web.de>
[not found] ` <6E8A9D4F-18CE-4ADA-A5B4-D14DB30C99E5@dukhovni.org>
[not found] ` <25e70f31f0c4629f7a7d3957649d08be06144067.camel@web.de>
[not found] ` <CECAFB36-DA1B-4EFB-ACD1-294E3B121B2E@dukhovni.org>
2020-04-13 18:35 ` [musl] Re: Outgoing DANE not working Rich Felker
[not found] ` <20200413190412.GF41308@straasha.imrryr.org>
[not found] ` <20200413193505.GY11469@brightrain.aerifal.cx>
[not found] ` <20200413214138.GG41308@straasha.imrryr.org>
[not found] ` <20200414035303.GZ11469@brightrain.aerifal.cx>
[not found] ` <87v9m0hdjk.fsf@mid.deneb.enyo.de>
[not found] ` <20200415180149.GH11469@brightrain.aerifal.cx>
[not found] ` <87imi0haf7.fsf@mid.deneb.enyo.de>
[not found] ` <20200417034059.GF11469@brightrain.aerifal.cx>
[not found] ` <878siucvqd.fsf@mid.deneb.enyo.de>
2020-04-17 16:07 ` Rich Felker
2020-04-18 17:14 ` [musl] TCP support in the stub resolver (was: Re: Outgoing DANE not working) Florian Weimer
2020-04-19 0:03 ` Rich Felker
2020-04-19 8:12 ` [musl] TCP support in the stub resolver Florian Weimer
2020-04-20 1:24 ` Rich Felker
2020-04-20 6:26 ` Florian Weimer
2020-04-20 17:39 ` Rich Felker
2020-04-21 9:48 ` Florian Weimer
2020-04-21 15:02 ` Rich Felker
2020-04-21 17:26 ` Florian Weimer
2020-05-01 22:02 ` Rich Felker
2020-05-02 15:28 ` Florian Weimer [this message]
2020-05-02 15:44 ` Rich Felker
2020-05-02 22:52 ` Bartosz Brachaczek
2020-05-03 8:46 ` Florian Weimer
2020-05-03 16:51 ` Rich Felker
2020-05-03 17:19 ` Florian Weimer
2020-05-03 18:18 ` Florian Weimer
2020-05-03 19:09 ` Rich Felker
2020-05-03 19:34 ` Florian Weimer
2020-05-03 19:45 ` Rich Felker
[not found] ` <20200414061620.GI41308@straasha.imrryr.org>
[not found] ` <20200414160641.GC11469@brightrain.aerifal.cx>
[not found] ` <20200414215951.GJ41308@straasha.imrryr.org>
2020-05-19 1:37 ` [musl] Re: Outgoing DANE not working Rich Felker
[not found] ` <20200519023814.GN68966@straasha.imrryr.org>
2020-05-19 5:44 ` Rich Felker
[not found] ` <20200519090610.GO68966@straasha.imrryr.org>
2020-05-19 14:00 ` Rich Felker
2020-05-19 14:23 ` Wietse Venema
2020-05-19 14:28 ` Rich Felker
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