How can I get DNS resolution to work inside a Mac OS X `chroot` on El Capitan?

The problem here is that requests and responses on the /var/run/mDNSResponder socket use the "ancillary data" feature of the recvmsg and sendmsg system calls, and socat doesn't proxy ancillary data. I was able to get a custom proxy that does relay the ancillary data to work and provide DNS to processes inside a chroot.