The sendfile manpage says: " Presently (Linux 2.6.9): in_fd, must correspond to a file which supports mmap(2)-like operations (i.e., it cannot be a socket); a

The sendfile manpage says: " Presently (Linux 2.6.9): in_fd, must correspond to a file which supports mmap(2)-like operations (i.e., it cannot be a socket); and out_fd must refer to a socket. " so apparently you cannot use sendfile for your scenario.