Re: RFC 9113 and :authority header field

I might be misunderstanding something, but from my reading of RFC 9113,
sending an empty :authority pseudo-header with a non-empty Host header is
invalid for HTTP requests. That's why google.com rejects these with 400.

<<Clients that generate HTTP/2 requests directly MUST use the ":authority"
pseudo-header field to convey authority information, unless there is no
authority information to convey>>
<<An intermediary that forwards a request over HTTP/2 MUST construct an
":authority" pseudo-header field using the authority information from the
control data of the original request, unless the original request's target
URI does not contain authority information>>

Sending a non-empty Host header means that the URI contains authority
information, so it triggers the h2 requirement to send :authority.

Am I misunderstanding something?
David

On Wed, Jun 29, 2022 at 10:54 AM Roy T. Fielding <fielding@gbiv.com> wrote:

> > On Jun 28, 2022, at 10:52 PM, Willy Tarreau <w@1wt.eu> wrote:
> >
> > Hi Tatsuhiro,
> >
> > On Wed, Jun 29, 2022 at 08:58:47AM +0900, Tatsuhiro Tsujikawa wrote:
> >> RFC 7540 even says that :intermediary MUST omit :authority "when
> translating
> >> from an HTTP/1.1 request that has a request target in
> >> origin or asterisk form (see [RFC7230], Section 5.3)."
> >>
> >> Now RFC 9113 has this text:
> >>
> >>      An intermediary that forwards a request over HTTP/2 MUST construct
> >>      an ":authority" pseudo-header field using the authority
> >>      information from the control data of the original request, unless
> >>      the original request's target URI does not contain authority
> >>      information (in which case it MUST NOT generate ":authority").
> >>      Note that the Host header field is not the sole source of this
> >>      information; see Section 7.2 of [HTTP].
> >>
> >> This means :authority must be included if the host header field exists
> in
> >> an HTTP/1.1 request.
> >
> > My understanding is that Host doesn't necessarily count as "control data"
> > here, and that the goal was to accurately represent an HTTP/1.x request
> > targetting an HTTP/1.0 server after being transported over HTTP/2. For
> > example, let's say that a client passes this to a proxy:
> >
> >     GET http://example.com/ HTTP/1.0
> >     Proxy-connection: keep-alive
> >
> > and nothing more. If instead it gets sent via a gateway that transports
> > it over H2, it could make sense to consider that the scheme is "http",
> > the authority is "example.com", that there's no host, hence the request
> > would be passed as:
> >
> >     :method: GET
> >     :scheme: http
> >     :authority: example.com
> >
> > and that's all. Conversely, let's see the same HTTP/1.0 request sent
> > directly to the origin server:
> >
> >     GET / HTTP/1.0
> >
> > There's no more authority nor host, so a gateway receiving that cannot
> > invent one, unless it uses its own configured name corresponding to its
> > own address, that it expects the client used to construct the request.
> >
> > With HTTP/1.1 there are less ambiguities since Host is mandatory, but
> > the distinction between "proxy requests" and origin requests is still
> > relevant, especially when you don't know whether or not the origin
> > server supports HTTP/1.1 or only 1.0 (and may be confused by the
> > presence of an authority in the request line). For example, if a
> > client sends:
> >
> >  GET / HTTP/1.1
> >  Host: example.com
> >
> > to an HTTP/1.0 server that parses Host, it will work. If it sends
> >
> >  GET http://example.com/ HTTP/1.1
> >  Host: example.com
> >
> > To an HTTP/1.1 server, it will work as well, but it may fail to an
> HTTP/1.0
> > server (or worse, loop over itself if it supports proxing requests and
> > resolves itself as example.com).
>
> Well, this ship has sailed, but I must have missed that original
> discussion.
>
> The premise is incorrect in all respects, since all of those HTTP/1.1
> requests are also valid HTTP/1.0 requests (even with an absolute URI)
> and so is the presence of Host in those requests.
>
> Host is an HTTP/1.x field that was used in HTTP/1.0 requests (in 1995)
> as soon as we reached consensus on the field name. That was long before
> 1.1 was finished and 1.0 obsoleted. Host is a required part of HTTP/1.0 now
> just by virtue of the Internet as deployed, regardless of the
> informational RFC.
>
> [The idea was originally proposed in 1994 by John Franks
>
>
> https://lists.w3.org/Archives/Public/ietf-http-wg-old/1994SepDec/0019.html
>
> but it took a long time to converge on a single syntax
>
>
> https://lists.w3.org/Archives/Public/ietf-http-wg-old/1995JanApr/0067.html
>
> https://lists.w3.org/Archives/Public/ietf-http-wg-old/1995JanApr/0084.html
>
> https://lists.w3.org/Archives/Public/ietf-http-wg-old/1995JanApr/0130.html
>
> https://lists.w3.org/Archives/Public/ietf-http-wg-old/1995SepDec/0291.html
>
> and while we still talk about it as an important addition of HTTP/1.1
> (because
> that's where we chose to document it), the feature is required for 1.0 to
> work with deployed servers.]
>
> So, an HTTP proxy recipient that receives any form of authority/host
> information must forward that information in either Host or :authority,
> no matter what version it is using. Failure to do so introduces a
> security bypass because L7 routers act on that information whether
> or not the client/server pair is aware of their presence.
>
> Hence, an HTTP/1.0 proxy that receives your first example should forward
> that as
>
>     GET / HTTP/1.0
>     Host: example.com
>     Proxy-connection: keep-alive
>
> because the routing doesn't work otherwise due to name-based hosts
> being deployed before HTTP/1.1.
>
> And, no, there is absolutely no reason to concern ourselves with proxies
> that loop over their own hostnames, since that is a self-correcting error
> whenever a full URI is received as the request target.
>
> > If the first request is transported over H2, thus converted from H1 to
> > H2 then back from H2 to H1, adding an authority that was not initially
> > present would introduce exactly this problem. By not adding it and using
> > Host only, the request representation is preserved, and the origin server
> > can receive the same request that the client took care to encode, and not
> > be confused. That's why I'm saying that in this case it's clearly visible
> > that Host isn't part of the "control data" and must not appear in an
> > authority that was not initially encoded.
> >
> > I know it's a bit complicated but we have to deal with history. What
> we're
> > doing in haproxy is that both Host and :authority are used
> interchangeably
> > after having been checked for proper matching, and are modified at the
> > same time if needed, and we have a flag indicating if an authority was
> > present in the incoming request to know if we have to produce one on
> > output or not. That's in the end what seems to preserve the most accurate
> > representation along a chain of multiple versions. This allows us to emit
> > a Host field only if one was present, and an authority only if one was
> > present, regardless of the HTTP version. I don't think that RFC9113
> brings
> > any changes regarding this, it might only be a matter of what constitutes
> > "control data".
>
> Sorry, that is a broken implementation. You need to send Host regardless
> of the original request version.
>
> ....Roy
>
>
>