$ tar xvzf wtf:foo.tar
    tar (child): Cannot connect to wtf: resolve failed

There is no mention of this in the man page: http://linuxcommand.org/lc3_man_pages/tar1.html ; apart from the rather surprising --rsh-command and --rmt-command options.

This actually is documented in the "info" pages:

http://www.gnu.org/software/tar/manual/tar.html#SEC152 :

    ‘-f [hostname:]file’
    ‘--file=[hostname:]file’

        Use archive file or device file on hostname.

    If the file name contains a ‘:’, it is interpreted as ‘hostname:file
    name’. If the hostname contains an at sign (‘@’), it is treated as
    ‘user@hostname:file name’. In either case, tar will invoke the command
    rsh (or remsh) to start up an /usr/libexec/rmt on the remote machine