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Tunnelling solutions commonly used outside the Tcl world include SSH port forwarding, Zebedee, Stunnel [L1 ], ...
See SSL Tunnel for a script that tunnels through web proxy servers using tls.
SockSpy can also be used for tunnelling. For example, I've used it to access my university's news feed machine from a remote site (the news feed machine would only accept local connections). KPV
See also
Another recipe, for use in establishing a tunnel into a firewalled LAN that, in principle, only allows outbound SSH:
ssh -f -l special_user -N -g -R 6005:localhost:22 external_server
Then, from the external_server prompt:
ssh -p 6005 my_account@localhost
This logs into my_account@firewalled_host.
This should only be used under supervision and with explicit permission from the guardians of firewalled_host, of course.