If given an IP address, it can be a host name ("localhost"), an IPv4 dotted-quad ("127.0.0.1") or an IPv6 address enclosed in square brackets ("[::1]").
(VCL4.1 and higher) If given an absolute Path ("/path/to/listen.sock") or "@" followed by the name of an abstract socket ("@myvarnishd") accept connections on a Unix domain socket.
The user, group and mode sub-arguments may be used to specify the permissions of the socket file. These sub-arguments do not apply to abstract sockets.