De la mucho mejor página de manual de FreeBSD, resolv.conf:
The resolver configuration file contains information that is read by the
resolver routines the first time they are invoked by a process.
On a normally configured system this file should not be necessary. The
only name server to be queried will be on the local machine, the domain
name is determined from the host name, and the domain search path is
constructed from the domain name.
Запятая в sed отделяет так называемые адреса в диапазоне адресов.
/./,$
означает «От строки, содержащей символ, до последней строки».