Re: Hello from Mozilla

On Wed, 5 Aug 2009, Amos Jeffries wrote:
> On Wed, 5 Aug 2009 00:57:18 +0000 (UTC), Ian Hickson <ian_at_hixie.ch> wrote:
> > On Thu, 30 Jul 2009, Robert Collins wrote:
> >> On Wed, 2009-07-29 at 23:48 +0000, Ian Hickson wrote:
> >> >
> >> > Surely once the HTTP server has decided that it can Upgrade, it
> >> > doesn't actually need to worry about sending back something
> >> > HTTP-valid at all, since we can just say the entire connection was
> >> > always using the WebSocket protocol, and was never HTTP.
> >>
> >> 10.1.2 101 Switching Protocols
> >>
> >> The server understands and is willing to comply with the client's
> >> request, via the Upgrade message header field (section 14.42), for a
> >> change in the application protocol being used on this connection. The
> >> server will switch protocols to those defined by the response's
> >> Upgrade header field immediately after the empty line which
> >> terminates the 101 response.
> >>
> >> So the server has to send a response before switching.
> >
> > The above text has no normative conformance criteria ("the server
> > will" is an informative statement that is unsupported as far as I can
> > tell).
>
> Point to note: if webserver fails to switch the WebSockets handshake to
> verify said switch actually occurred will fail. Detecting such bad servers
> and acting properly.
> If web server successfully switched then WebSockets client needs to handle
> the HTTP response correctly and begin to do a WebSockets native handshake
> and verify the link is safe _after_ the 101 message is received.

I don't understand what you are trying to say here.

> > In any case, the WebSocket protocol handshake does do a complete HTTP
> > Upgrade including the empty line, it just requires that the handshake
> > be done in a particular way. It's "profiling" the HTTP spec.
>
> No, our whole point repeated over and over in different ways is that the
> specd description for the upgrade request is _incomplete_. And does not
> account for an enormous number of byte-level variations. All of which
> can be safely discarded without affecting security.
>
> Being sensitive to whether the server replies "101 Blah" versus "101
> blah" absolutely cripples WebSockets. We want to help you fix this
> problem.

I still do not understand why anything gets crippled. Maybe you could show
an example of how you expect this problem to occur?

> >> This is because if it doesn't, how can it say 'I won't upgrade'.
> >
> > It can just not upgrade. Returning anything but the correct handshake
> > will be treated as a failed connection by the WebSocket client.
>
> Correct.

So why would it need to say "I won't upgrade"?

> > On Thu, 30 Jul 2009, Robert Collins wrote:
> >> >
> >> > Suppose we had no handshake at all, and that there was no data
> >> > framing, so that as soon as we connected to a port, we could send
> >> > arbitrary data down.
> >> >
> >> > A Web page, say evil.example.net, could open a Web Socket
> >> > connection to http://www.corp.example.com/, send it a GET request
> >> > for /secret-plans, and then forward the contents of the file to a
> >> > remote host. If they could then trick someone on example.com's
> >> > intranet to look at this file, and assuming www.corp.example.com
> >> > did nothing more than rely on connectivitity for authentication
> >> > (pretty common in small intranets), then evil.example.net could
> >> > steal the company's secret plans.
> >>
> >> Can't the web page just send an ajax request to corp.example.com
> >> anyway?
> >
> > It can't read the response, no.
>
> Can you explain this please?
> AFAIK, Sending a request then closing the connection immediately is the
> only way said response might be unreadable. I don't understand what you
> mean.

A script on a Web page can cause a GET request to be sent to an arbitrary
URL, but it has no way to obtain the contents of the result of that
request unless that server opts in (using CORS) to allowing the script to
see the contents of the response.

> >> Dumping the ability to look like HTTP altogether is your best bet
> >> IMO: even on port 443. For port 443, someone running a websocket
> >> server should just not run an HTTPS server there. Or define port 815
> >> as websocket/s.
> >
> > Port 815 is the preferred port, but in certain situations, only ports
> > 80 and 443 are allowed, and port 80 is mitm'ed, so that only leaves
> > 443.
> >
> > You don't always have the option of using a different host, that's why
> > we need to share the port sometimes. (There have also been statements
> > to the effect that just sending a new protocol over ports 80 or 443
> > without using the Upgrade mechanism would be a violation of the
> > semantics of those ports.)
>
> Understood. However strange protocols (torrent is one example) on port 80
> can be detected and dealt with easily.
> Something that appears to be perfect HTTP cannot be detected as non-HTTP.
> When HTTP operations are done to the traffic it will break badly and in
> nasty invisible ways.

Could you explain how a WebSocket connection can break in an invisible
way? An example would be very helpful here.

> >> If its not, it should bail. That seems reasonable :). We can
> >> definitely (and have been :)) advising on how to do that with HTTP,
> >> but it means *having a decent HTTP stack*. The response to a HTTP
> >> request is always an HTTP response. Upgrade: changes the way the wire
> >> behaves after that.
> >
> > It seems that just having a WebSocket "stack" is adequate also.
>
> When all layers of the stack confirm to the appropriate protocols, yes
> agreed.
> Point of this discussion is that the WebSockets layer _when sitting on top
> of HTTP_ needs to be using HTTP layer correctly.

WebSockets, even when initiating the connection by talking to an HTTP
server and then switching to WebSockets, isn't layered on HTTP. It's just
doing the bare minimum required to allow the HTTP server to understand the
request and get out of the way. Once the handshake is complete, there is
no HTTP anywhere on the stack at all.

> >> > Yes, but the client is a WebSocket client, not an HTTP client, so
> >> > why would it send anything but the WebSocket handshake?
> >>
> >> Because its asking an HTTP server to upgrade.
> >
> > How can it know that ahead of time?
>
> When using port 80 is must assume to be doing so.
> a) because it _is_
> b) because any WebSockets server on port 80 is expecting that HTTP upgrade
> request of yours to come in.
>
> Therefore, doing proper HTTP Upgrade and discarding the junk headers is
> no loss, remains secure, and a net gain when MITM are present doing
> perfectly normal HTTP things.

I disagree that it remains secure, but leaving that aside, could you
explain how it is a net gain when MITM are present? Under what conditions
would a MITM allow a WebSocket Upgrade to actually succeed end-to-end,
even if we do everything according to HTTP's rules instead of the
simplified WebSocket rules?

> >> That makes it an HTTP client and a WebSocket client... as in fact you
> >> note below ..
> >
> > It's both. It's not an HTTP client in that it doesn't do anything but
> > what the WebSocket spec says it should do. It's an HTTP client in that
> > it speaks something that can be interpreted as HTTP.
>
> Your spec says otherwise. Simply by defining the byte-level content of
> the headers the server and client are now speaking non-HTTP. Were they
> spec'd as speaking HTTP this entire argument would not be ocurring.

They're not speaking HTTP, they're speaking something that can be
interpreted as HTTP. I think the distinction is quite important.

> >> > > Ian, are you absolutely certain that everywhere you use "the
> >> > > internet", there is no "man in the middle" between you and the
> >> > > server you're speaking to?
> >> >
> >> > In the case of TLS connections, yes.
> >>
> >> In corporate networking, TLS MITM is a 'feature': company signed
> >> certificates are used to sign the TLS connection to the corporate
> >> firewall, and the firewall validate the SSL connection to the outside
> >> world. I haven't personally used this, so can't really say much more
> >> about it. Squid's ssl-bump feature can be used for this, but I
> >> believe browser config is needed (again on a corporate basis) to tell
> >> it that this is expected.
> >
> > I can't think of any way of getting a WebSocket connection through
> > such a situation, so there's not much point doing anything to handle
> > this case. If you can think of a way of handling it that _would_ allow
> > WebSocket through, let me know, as then it would make sense to support
> > it.
>
> Don't confuse TLS MITM with regular MITM. Regular MITM are still very
> common and can be tunneled over without trouble just by accepting
> regular HTTP changes during the upgrade request and reply headers.

Could you show me a proof of concept of this? I've not been able to get
any sort of two-way connection to a remote host when there's a MITM proxy
in the middle of the connection, regardless of what packets I send. Any
help you could provide here would be much appreciated.

-- 
Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'