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This is sort of a level-up from this question. Suppose that I am on computer A and want to send files to a computer C. We each have separate access to a host B. A and C can not communicate directly, and B cannot initiate a connection to either.

Is there a way I could pipe a large file to SSH and create some sort of stream which I could then tell the user of computer C to connect to? The file is larger than the available space on B. My current solution is to split the large file, upload each segment in turn, and when user at C has finished downloading one chunk notifies me, I delete the chunk and upload the next one.

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up vote 1 down vote accepted

If I understand you correctly, you want to send data from A to C, but neither computer can connect to the other, but they can both connect to B. B can't connect to anyone.

One solution is using port forwarding. C can forward from a port on B to their SSH port (port 22):

ssh -R 1088:localhost:22

1088 is arbitrary, choose a port that isn't already in use. By default this port is only available on that local machine, i.e., only processes running on B can connect to it.

now a user on A can connect directly to C:

ssh B "ssh -p 1088 localhost"

Or for example, this command will send a file from A to C:

cat infile | ssh B "ssh -p 1088 localhost \"cat > outfile\""

Alternatively, A could have forward their port, allowing the user on C to initiate the copy.

Another solution is to use named pipes. On B, execute:

mkfifo named_pipe

Then on A you can send data into the pipe with:

cat infile | ssh B "cat > named_pipe"

This command will hang until someone opens the other end of the pipe and reads the data out of it. For example, the user on C can do:

ssh B "cat named_pipe" > outfile

This will send the file from A through B to C.

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Yes, you can create a named pipe for them to pull. Make sure to create the pipe first, have them connect from computer C, and then start streaming to it.

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