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Re: [Captive-portals] Discovering captive portal API URL via DNS?



> On Sep 3, 2019, at 4:44 PM, Lorenzo Colitti <[email protected]> wrote:
> 
> All,
> 
> During discussions with captive portal operators about implementing the capport API, one of the stumbling blocks that keeps coming up is that the captive portal operator does not always control the DHCP configuration and thus cannot easily use RFC7710.

Thanks for bringing this up.

I wanted to clarify the issue a bit before diving into the mitigations. Do these captive portal operators have *no* relationship to the DHCP configuration? Presumably, the captive portal enforcement is done somewhere on path, in the router, or between the router and the Internet connection. And, if there is redirection of DNS going on, presumably, this is a DNS server that the captive portal (or the operator of the network) has some control over, and is provisioned via DHCP. For such portals, I would assume that it would be as easy to add a DHCP CAPPORT option as it would be to add the DNS server address of the captive portal's resolver (assuming the DHCP implementation supports the option).

Since the mitigation below is specific to modifying the DNS, I assume that we are talking about captive portal solutions that work, in part, by intercepting DNS.

Thanks,
Tommy
> 
> The WG has previously rejected the option of using a well-known DNS name to discover the URL, because the API itself requires TLS, and without a hostname it is not possible (or at least not easy) to validate the server. However, what if the client did a CNAME query for capport.arpa (or equivalent other local-only, non-DNSSEC-signed name), got back a CNAME for the real server, and then assumed that the API server was https://<targetofcname>/capport-api ?
> 
> Alternatively, Erik and Warren suggest RFC 7553. In this scheme the client would do a URI lookup for "capport.arpa" or equivalent, and would take the result of that URL as the API endpoint.
> 
> Thoughts?
> 
> Regards,
> Lorenzo
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