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SCP establishes a new connection when copying files, which sometimes can make it hard if there are firewalls involved.

For example,

Host-A <--> Host-B <--> Host-C <--> Host-D

  • Host-A is connected to Host-B via SSH, and then Host-B is connected to Host-C via SSH, and then to Host-D.
  • Host-A cannot connect to other computers other than Host-B, Host-B can only connect to Host-A and Host-C, Host-C can only connect to Host-B and Host-D, and last Host-D can only connect to Host-C.

The command (on Host-D) scp /home/user/file Host-A:/home/user/newfile will attempt to create a new SSH tunnel between the two, which is not what i want. I want the existing SSH tunnel chain to copy the file back through the previous hosts until it reaches Host-A.

How can i do this?

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You can re-use existing connections. Either use command-line switch -M, or (client) config option ControlMaster. See this answer on StackOverflow for details.

I've used this extensively, combined with ControlPersist to to close the connection only after a delay. This avoids the connection time you would otherwise incur, and drastically improves performance when you're closing and re-connecting multiple times (as I often do during interactive use).

Do note that it does not work very well for connecting to some hosts — GitHub, for example — so you may need to explicitly exclude certain hosts from using this option, if you set it up as your default.

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    Oh, and if the connection hangs, as it does occasionally, you can simply delete the ControlMaster file for the connection, and re-connect. – jpaugh Feb 1 '18 at 21:23

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