One time when I was staying at a friend's friend's apartment, their DSL connection was blocked with some form of "you haven't paid" page. Looking at tcpdump, I

One time when I was staying at a friend's friend's apartment, their DSL connection was blocked with some form of "you haven't paid" page. Looking at tcpdump, I saw that my laptop was sending NTP queries to random pool.ntp.org and actually getting responses. Quick spot of 3G roaming and I confirmed that I was also able to send and receive NTP packets from my own server. Quickly setup an OpenVPN server on port 123, and ta-daa, working internet access. They had whitelisted all UDP port 123 traffic past their payment wall. It worked fine for several days. Much nicer than a pseudo HTTP proxy :P