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Re: Time to disable "Expect: 100-continue" by default?

From: David Weisgerber <>
Date: Wed, 26 Jul 2017 17:35:41 +0200

I have a strange behaviour with the Expect: 100 implementation as well.
Remote server: Tomcat 8.0.28
Client: libcurl/7.54.0 OpenSSL/1.0.2k zlib/1.2.11 nghttp2/1.22.90 (Win32 /

It seems as if the Tomcat server sends the 100 Continue faster than libcurl
finishes with sending the Headers. Then libcurl hangs and does nothing more...

From wireshark:

Client -> Server:
3lP&5E__at_xCN@NPPOST /diag/rest/v1/file HTTP/1.1
Host: valkyrie:8080
Accept: */*
Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
Content-Length: 24563312
Expect: 100-continue
Content-Type: multipart/form-data;

Server -> Client: (+1ms)
P&53lEA@@CN_at_P&HTTP/1.1 100 Continue

Client -> Server: (+4ms)
Content-Disposition: form-data; name="evaluation"

Content-Disposition: form-data; name="evaluationType"

Content-Disposition: form-data; name="file"; filename="data"

Server -> Client (+30ms)

Server -> Client (+20s)
P&53lE@@6CN_at_P&dHTTP/1.1 400 Bad Request
Server: Apache-Coyote/1.1
Cache-Control: no-cache
Content-Type: text/html;charset=utf-8
Content-Language: en
Content-Length: 1033
Date: Wed, 26 Jul 2017 15:27:40 GMT
Connection: close

Is this a problem of Tomcat or of libcurl or of the underlying operating

Thanks for help,

PS: I get this problem not very often...

Am Mittwoch, 26. Juli 2017, 08:13:46 CEST schrieb Michael Felt:
> On 7/6/2017 9:59 AM, Daniel Stenberg wrote:
> > Hey,
> >
> > libcurl has hueristics to include the "Expect: 100-continue" header in
> > several circumstances when doing POST and PUT requests.
> >
> > This header was designed to allow servers to reject a request before
> > any data has been sent from the client, so that there would be less
> > waste. It is especially useful with various kinds of HTTP auth
> > mechanisms during which the 100 can save us from having to resend the
> > entire (potenitally huge) POST body.
> >
> > In the real world however, there seems to be many more broken servers
> > than working ones when it comes to supporting 100. If curl sees no
> > response within 1000 milliseconds, it continues anyway and this pause
> > is a very common reason for annoyance with curl. So many servers just
> > don't see or care about Expect:.
> >
> > Then there's still the large group of servers that send a 100 response
> > back immediately on all requests with Expect: even if the auth isn't
> > fine, only to deny the request later anyway. Contrary to the intention
> > of the header.
> >
> > The net result of all this is that switching off this Expect: header
> > from libcurl requests is a very common thing. In many cases it removes
> > a 1000ms pause from the request. In most other cases it makes no
> > difference. In a rare few cases, it causes wasted bandwidth and
> > additional roundtrips.
> >
> > The number of requests done "out there" today that unncessarily are
> > waiting 1000 ms on each request is probably substantial. Switching the
> > default could automatically make a huge amount of curl requests go
> > faster in the future.
> >
> > Is it time to remove the automatic Expect: header use and only do it
> > on demand?
> My gut response is:
> a) leave the default behavior as it is. This is a well known behavior.
> Straight "out of the box" configuration reversals "scare me" by
> principal (as this one might be harmless, but hey - this is shooting
> from the hip!).
> b) add a message (perhaps to stderr as well as syslog) that a request
> was made, but no response was made within 1000 msec, ideally in a way
> that statistics could be collected (which servers never respond, e.g.)
> In short, since you had to ask - my gut says this is not a guaranteed
> "100% harmless" change - and I would start with a message, or as a
> configure option (maybe it is that already) and packagers can decide
> and/or learn what works best.
> If putting it is configure is new - then you could put it in with the
> default as is (e.g., true aka enabled), but add some info to README to
> change the configure default to false (aka default request disabled).
> And, this also gives you a moment to review the current documentation on
> how to change the default in the config file (e.g., maybe what the
> configure change does is change the result of a generated (default)
> config file rather than actually changing code.
> Hope this helps!
> > We probably need to add a new option that turns on the current/old
> > behavior if we do this. Alternatively we would just let the user set
> > "Expect: 100-continue" as a custom header and detecting that, we would
> > enable the internal logic for it, but that seems very error-prone and
> > not very convenient.
> >
> > Applications that currently disable Expect: header use should of
> > course not be affected by this sort of change. They would just disable
> > it extra much! =)
> >
> > Thoughts?
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Received on 2017-07-26