Telegram Bypasses Lufthansa Inflight WiFi Paywall - how?

It accesses 149.154.167.51 via HTTP. That's a Telegram server.

Why does Lufthansa let that through? No idea. It's just HTTP, not even HTTPS. Maybe Lufthansa's being nice to API requests and only redirecting what seems like web browsers to its captive gateway. (That would be very nice — I hate it when stupid API clients are confused by HTML coming from captive gateways.)

Try curl --resolve www.google.com:80:74.125.206.99 http://www.google.com/, curl -H 'User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.98 Safari/537.36' and curl http://www.google.com/ next time. One of them bypasses the DNS, one uses a human-like user-agent and one is plain old curl. See which ones work.

I think Skype was able to connect on more restricted networks because it directly connected to some predefined IP addresses, it didn't need to have DNS working.

I remember this because my home network had the DNS servers down but still Skype miraculously managed to get connected to its servers. Maybe this is the same situation but for Telegram?

Perhaps using ICMP? Ping is usually not blocked I'd imagine