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Re: protocol support for intercepting proxies

From: Mark Andrews <Mark_Andrews@isc.org>
Date: Tue, 19 Jun 2007 11:35:56 +1000
Message-Id: <200706190135.l5J1Zugm015971@drugs.dv.isc.org>
To: Adrien de Croy <adrien@qbik.com>
Cc: Henrik Nordstrom <henrik@henriknordstrom.net>, HTTP Working Group <ietf-http-wg@w3.org>

> Henrik Nordstrom wrote:
> > mån 2007-06-18 klockan 18:24 +1200 skrev Adrien de Croy:
> >   
> > I hereby call you to look into the problem of why interception is
> > needed, and to try to come up with ideas to address that problem. In the
> > long run a proper solution to that problem will benefit you and everyone
> > else.
> >
> >   
> I can start with why we implemented an intercepting proxy.  Apart from 
> commercial / competitive issues that is.
> Basically the main thing we identified was that customers wanted it.  
> Feedback was pretty explicit on this.
> Why they wanted it, I guess is because proxy auto-detect or proxy 
> operation wasn't doing it for them. 
> Proxy auto-detect
> ------------------
> There are plenty of our customers that use proxy auto-detect and are 
> quite happy with it.
> However, proxy auto detect can require some configuration.  Originally 
> proxy auto-detect worked by doing a DNS lookup for the name "WPAD".  
> WinGate's DNS server would respond to this with its own IP address.  The 
> client would then make an HTTP request for a file called wpad.dat.  We 
> would auto-generate this and send it to the client.  So, basically 
> customers got proxy auto-detect for free.  I presume other vendors 
> adopted a similar approach.
> then someone at MS decided DHCP would be a better option for discovering 
> proxy config, and option 252 was created whereby the DHCP client issues 
> a request for option 252 as part of a DHCP_INFORM message.  DHCP_INFORM 
> was a late addition to the DHCP spec, also option 252 is still listed by 
> the IANA as "private".  So there are a bunch of DHCP servers out there 
> that don't support DHCP_INFORM nor option 252.
> So, we have some browsers that use DNS (which they can control, but 
> which seems to be being deprecated), and others that rely on their OS 
> platform using a private DHCP option that is not formally specified.  
> Some client platforms don't provide a mechanism where applications (such 
> as a browser) can obtain info from DHCP clients or servers (i.e. 
> influence DHCP requests, and access response data).  Most notable OS 
> that springs to mind is most if not all versions of windows.
> finally, enabling or disabling proxy auto-detection is under user 
> control.  Heaven forbid!
> I'm not aware of any browsers that do a sanity check on the values that 
> get returned from a WPAD DNS lookup, or the URL from DHCP option 252 
> either.  We just presume that the DNS and DHCP server responses are 
> trustworthy.
> So in short, IMHO proxy auto detect is fraught with problems.
> Proxy operation
> ----------------
> There are zillions of applications out there that use HTTP for whatever 
> purpose.  Many of these are cobbled together.  Many do not consider the 
> existence of proxies at all, or can't do proxy auth, or any sort of HTTP 
> auth.  Many do not present any UI, but operate as background service 
> processes.  Thankfully with APIs like MS INETAPI on Windows, developers 
> now have an easier way to write such things, and they gain support for 
> things like chunking, auth etc.  But there are still a bunch of broken 
> apps out there.
> Couple this with the sys admin that still wants to force everything 
> through the proxy anyway, and your only option is connection 
> interception, where the proxy pretends to behave like a server (right 
> down to the header level).
> Many of these apps are very old, and aren't being maintained, but are 
> still in use and relied on.
> This isn't an argument for changes to the protocol actually since 
> proposed changes would break all these apps.
> > Trying to solve this in HTTP is short-sighted and doomed for failure
> >   
> Actually I see the configuration of HTTP proxy as being an HTTP protocol 
> configuration element, and therefore I don't see why it shouldn't be 
> solved in HTTP.  It's not like we're setting a parameter unrelated to 
> HTTP.  Pushing the config out to other services multiplies the number of 
> applications that need to be maintained.  A sys admin doesn't need to 
> keep just their proxy and clients up to date, but also their DHCP 
> servers and other servers.  If these are all built into a hardware box, 
> there aren't many options.
> You might laugh, but one of our biggest support issues used to be people 
> configuring their proxy config to use port 21 for FTP proxy in IE.  They 
> "knew" that port 21 was the port to be used for FTP.  In the end, we had 
> to write an HTTP handler in WinGate's FTP proxy to tell them to change 
> their config when it detected an HTTP request for an FTP URL.
> There are lots of people out there with a little knowledge, enough to 
> make it dangerous to get them to do any sort of manual configuration.
> If we could assume that all HTTP agents could work through a proxy, and 
> do proxy auth, we should be looking at a more foolproof mechanism for 
> proxy auto configuration.  The form that takes has many possibilities, 
> you don't like the one proposed so far, that I can live with.  But there 
> are potentially others that would solve concerns, and achieve the goals 
> of minimum impact on other systems.  The fewer the links in the chain 
> IMO the better.  So firstly I think such a mechanism should be
> 1. under network not individual control (so the network can set the 
> requirement to use the proxy, not the client, and it can be managed 
> centrally).
> 2. not reliant on DHCP.  Why must we force everyone on the planet to use 
> DHCP just to get an auto proxy config.  Why must we force all OS vendors 
> to open up their DHCP clients so that applications can obtain DHCP 
> configuration data.

	To be perfectly honest.  Applications SHOULD have access to
	DHCP configuration results. 

> So maybe DNS SRV records are the way to go here.  Some customers lock 
> down even DNS though, since the proxy does the DNS lookup for normal 
> proxy operations.  Also most OS provided DNS resolver implementations 
> (i.e. winsock) only support A record lookups, so unless you want browser 
> authors to write their own DNS resolvers (and deal with the 
> configuration issues around that), they won't have much luck on many 
> OSes even getting a SRV record.

	Most OS's support looking up any record you can think about.

	For Windows you need DNSQuery*().  Even the embedded versions
	of DNSQuery_W support SRV lookups.

	The problem with SRV is that you need to know the query name.

> the one common factor that all browsers have is HTTP support.
> In the end, the best solutions may be platform-specific, i.e. Active 
> Directory policy based, or extensions to network logons.  But I'm 
> struggling :)
> Adrien
> > imho.
> >
> > Regards
> > Henrik
> >   
> -- 
> Adrien de Croy - WinGate Proxy Server - http://www.wingate.com
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews@isc.org
Received on Tuesday, 19 June 2007 01:36:18 UTC

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