[whatwg] WebSocket and proxies

Shannon shannon at arc.net.au
Mon Oct 13 16:22:01 PDT 2008

In the process of testing my WebSocket proposal I discovered the CONNECT 
method has a major restriction. Most proxies disable CONNECT to anything 
but port 443.

The following is from "Squid and the Blowfish":
It is very important that you stop CONNECT type requests to non-SSL 
ports. The CONNECT method allows data transfer in any direction at any 
time, regardless of the transport protocol used. As a consequence, a 
malicious user could telnet(1) to a (very) badly configured proxy, enter 
something like:
... snip example ...
and end up connected to the remote server, as if the connection was 
originated by the proxy.

I verified that Squid and all public proxies I tried disable CONNECT by 
default to non-SSL ports. It's unlikely many internet hosts will have 
443 available for WebSockets if they also run a webserver. It could be 
done with virtual IPs or dedicated hosts but this imposes complex 
requirements and costs over alternatives like CGI.

The availability and capabilities of the OPTIONS and GET protocols also 
varied from proxy to proxy. The IETF draft related to TLS 
(http://tools.ietf.org/html/draft-ietf-tls-http-upgrade-05) has this to say:

3.2 Mandatory Upgrade

   If an unsecured response would be unacceptable, a client MUST send
   an OPTIONS request first to complete the switch to TLS/1.0 (if

          OPTIONS * HTTP/1.1
          Host: example.bank.com
          Upgrade: TLS/1.0
          Connection: Upgrade

So according to this draft spec OPTIONS is the only way to do a 
*mandatory* upgrade of our connection. Once again this failed in testing

=> Proxy-Connection: keep-alive
=> Connection: Upgrade
=> Upgrade: WebSocket/1.0
=> Host: warriorhut.org:8000
<= HTTP/1.0 400 Bad Request
<= Server: squid/3.0.STABLE8

Other proxies gave different errors or simply returned nothing. The 
problem may be related to the Upgrade and Connection headers rather than 
OPTIONS, since I had similar issues using Connection: Upgrade with GET.

I had the most success using GET without a Connection: Upgrade header. 
It seems that the proxy thinks the header is directed at it so it does 
not pass it on to the remote host. In many cases it will abort the 
connection. Using the Upgrade: header without Connection allows the 
Upgrade header through to the actual websocket service.

It seems to me that whatever we try in many cases the connection will be 
silently dropped by the proxy and the reasons will be unclear due to the 
lack of error handling. There seems to be a wide variation in proxy 
behaviour for uncommon operations. I suppose proxy developers could fix 
these issues but whether a significant rollout could be achieved before 
HTML5 is released is questionable.

Given that an asynchronous connection cannot be cached the only reasons 
remaining for going through a proxy are anonymity and firewall 
traversal. Automatically bypassing the users proxy configuration to 
solve the issues above has the potential to break both of these. It 
would be a significant breach of trust for a UA to bypass the users 
proxy and some networks only allow connections via a proxy (for security 
and monitoring).

It seems that we're stuck between a rock and hard place here. In light 
of this I reiterate my earlier suggestion that the time could be better 
spent providing guidelines for communication via an asynchronous CGI 
interface. This would allow reuse of existing port 80 and 443 web 
services which would resolve the cross-domain issues (the CGI can relay 
the actual service via a backend connection) and most of the proxy 
issues above (since proxy GET and CONNECT are more reliable on these ports).


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