rinetd redirects TCP connections from one IP address and port to
another address and port. rinetd is a single-process server which
handles any number of connections to the address or port pairs
specified in the file /etc/rinetd.conf. Because rinetd runs as a single
process using nonblocking I/O, it is able to redirect a large number of
connections without a severe impact on the machine. This makes it
practical to run TCP services on machines inside an IP masquerading
firewall.

Note: rinetd can not redirect FTP because FTP requires more than one
socket.