Re: [Doh] Captive portals (was Re: suggested slides for IETF 104 on draft-reid-doh-operator)

"Martin Thomson" <mt@lowentropy.net> Fri, 15 March 2019 11:26 UTC

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Date: Fri, 15 Mar 2019 07:26:10 -0400
From: "Martin Thomson" <mt@lowentropy.net>
To: "Jim Reid" <jim@rfc1035.com>
Cc: doh@ietf.org
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Subject: Re: [Doh] =?utf-8?q?Captive_portals_=28was_Re=3A__suggested_slides_f?= =?utf-8?q?or_IETF_104_on_draft-reid-doh-operator=29?=
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On Fri, Mar 15, 2019, at 18:39, Jim Reid wrote:
> Suppose a DoH client has already bootstrapped itself and knows the 
> address of its chosen DoH server. The device moves to a new net that 
> has a captive portal. 

If the OS catches the captive portal, everything works nicely once the captive portal is dealt with.  If the captive portal manages to evade detection...

The app might blithely continue.  If the resolver is now blocked by the captive portal (remember, we already have an IP address) then name resolution fails the next time it is needed.

The app that Daniel talks about is more sophisticated.  It attempts to detect the new network and then tries to restart its bootstrapping process.  This is highly unreliable both in detecting the move and in detecting the captive portal (after all the OS missed it too).  So the sophisticated app often ends up looking like the dumb one. 

But this isn't that unique.  It's not just name resolution that fails in this case.  We're back that the "you connected to a broken network" mess I described in my original mail.

The only real take-away then is that evading captive portal detection produces a terrible user experience.