126. linux ssh examples: rsync across portforwarded ssh and helping remotely via ssh behind firewalls

Even bog-standard ssh is pretty neat, since there's little that can't be done in the terminal. However, firewalls can be annoying and if you set somebody up with linux you will have to be prepared to support them for years to come -- at least with debian wheezy the odd breakage happen, and people tend to be less forgiving with linux problems than with windows problems.

Anyway.

1. rsync across a server 
A can connect to B, B can connect to C. A can't connect directly to C. B and C can't connect directly to A. B and C can connect to each other either direction. An example is when B is your home router and C sits on your local network, while A has a public IP but sits behind a corporate firewall.

You want to rsync from A to C

On A, do
ssh user_at_B@B_ip_address -L 5555:C_ip_address:22
then in another terminal
rsync -avz  --progress --stats -e "ssh -p 5555" /home/user_a/work user_c@localhost:/home/user_c/Documents

2. Helping someone in the terminal across a server
A and C can't connect to each-other. A and C can both connect to B. B can't connect to A or C. A wants to connect to C to start e.g a screen session to help out. An example is when both users A and C can connect to a lab router from their respective home, but their ISPs are preventing direct ssh access between them.

Setting up reverse ssh, on C, do
autossh -R 19998:localhost:22 user_B@B_ip_address

Connecting from A to C, do
ssh user_B@B_ip_address -L 19999:localhost:19998
then in another terminal
ssh user_C@localhost -p 19999

You can then set up a screen session to both help and teach.
http://verahill.blogspot.com.au/2012/02/debian-testing-wheezy-64-attach-to.html
http://verahill.blogspot.com.au/2012/03/fun-with-gnu-screen-setting-up-screenrc.html