You need a domain name, xxx.example, then you can uplink data yyyyyy through the tunnel by querying yyyyyy.xxx.example and iodined, authoritative for xxx.example, will receive this query by the captive portal's resolver. To downlink data, you read responses (txt, cname or null records) that iodined will send for your queries.
This probably fills up the cache of the captive portal's resolver (:
I was on a ferry ship with paid sattelite wifi internet once and they only faked DNS responses for gstatic.com, google, etc. But they allowed my domain (DNS queries to it) and stripe.com (because that's how you paid) -- so they only faked responses for a small set of popular domains. IP access to the internet was, apart from stripe, blocked.
And because stripe uses FastlyCDN, they allowed all of fastlycdn network ranges (not useful for iodine), so I could browse reddit (they also use fastly) using stripe's CDN (only html, without i.redd.it image hosting, that's elsewhere).
AFAIR Fastly doesn't have a free plan, so I couldn't tunnel at higher speeds (via HTTP) but only via DNS.
But yeah, sometimes they just outright respond to any query with their IP and you are out of luck.
But that's the beauty of iodine. It will still work because if the captive portal's name servers actually fully resolve requests, it will contact your upstream iodine controlled name servers and forward the response as-is, because that's just how DNS works.
Of course it's also fairly easy to detect/block since your DNS usage will be completely abnormal.