How does Korea perform internet censorship

Snippet from the dialogue with tech-savvy Korean, 12 Feb 2019.

Blocked websites are not disclosed by the government. Currently, known sites are only porn. But people are getting worried since this technology can be used to block access to opposing political parties and websites. Censorship was done in stages. The first stage was blocking ‘problematic’ DNS query in ISP’s DNS, but everyone started using Google public DNS or Cloudflare to mitigate that. After that, the government started modifying HTTP response from blocked websites to redirect to the ‘Warning’ page (Notice that this website is blocked by the government due to various reasons). Since HTTPS was not affected, everyone just started to use HTTPS everywhere + public DNS to browse freely. In response, the government started hijacking all DNS queries to ISP’s DNS server in some ISPs. After many controversies, they stopped DNS hijacking after a day (ISP claims that it was just a routing error, not related to censorship), and started SNI header based HTTPS blocking yesterday.

A lot of information regarding Korean internet censorship could be found here:년%20인터넷%20검열%20논란