DNS tunneling transport (like iodine, dnscat)
DNS-based pluggable transport.
Encode data in recursive DNS queries and responses. Your local recursive resolver sends your packets to the right place. A DNS bridge would be an authoritative name server for a particular domain; users would configure a domain rather than an IP address in their Bridge lines. Tools already exist to do DNS tunneling, for example iodine and dnscat. Probably requires a reliability layer and periodic polling by the client.
Provides a way for users behind restrictive firewalls to connect to Tor at the expense of speed.
Mailing list discussions: [tor-dev] obfsproxy dns transport https://lists.torproject.org/pipermail/tor-dev/2014-February/006250.html (Feb 2014)
using OzymanDNS to access Tor via DNS https://lists.torproject.org/pipermail/tor-talk/2006-January/007124.html (Jan 2006)