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This question already has an answer here:

I have three linux servers:
1. Database Server 10.10.10.10
2. Application Server 10.10.10.52
3. Backup Server 10.10.10.55

There is a direct connection between Database and Application servers and there is a direct connection between Application and Backup servers, but there is no connection between Database and Backup servers.
I have a file in Database server which I want to copy to Backup server, but I don't have any idea how to do.
If anyone has any idea, please share it with me.
Thanks in advance.

marked as duplicate by Jakuje, techraf, Eric Renouf, GAD3R, Archemar Nov 27 '16 at 15:01

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    ssh tunnel using 2 from 1 to 3. It will slower than a direct connection, ask the networking team for a direct connection. – Rui F Ribeiro Nov 27 '16 at 9:05
  • @RuiFRibeiro The networking team does not allow a direct connection and has asked me to find a solution for it. And you mean that I should copy the file from 1 to 2 and then from 2 to 3? – Abdul Raheem Ghani Nov 27 '16 at 9:07
  • @RuiFRibeiro The mentioned link shows me how to ssh the Database Server. I practiced that, I can ssh Database server, but I am still unable to copy the file directly. – Abdul Raheem Ghani Nov 27 '16 at 9:33
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You have to make a tunnel from host1(1) to host(3) logging in host2(2) with SSH, as only (1) has access to (2), as in:

host1$ ssh -L 9999:host3:22 user@host2

That will create the tunnel to host3, SSH port(22) in localhost, port 9999/TCP

Then in another windows, you scp from host1 with a user present in host 3 as in:

host1$ scp -P 9999 file_to_copy user@localhost:/user/file_to_copy

In that respect, the transfer of the file is done directly from host 1 to host3, and host2 is there just to forward the connection via the SSH tunnel. When you logout from the first ssh, the tunnel is closed.

  • You can't specify the port in the format of localhost:9999 in scp. – Jakuje Nov 27 '16 at 12:25
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Assumption: On Host A scp /tmp/xxx HostB:/tmp/xxx and on HostB scp /tmp/xxx HostC:/tmp/xxx

In cases, traditionally you would use a ssh/scp tunnel to copy from HostA to HostC. Assuming your network has also disabled tunnels (also assuming you also asked about that as a 'direct connection')

You could write a script something like this - for execution from HostB

#!/usr/bin/bash -e
# assumes you have PKI setup so you only have to enter your pass-phrase at most once
file=$1

scp Hosta:$file $file
scp $file HostC:$file
rm $file

After thought: from HostB

scp HostA:$file HostC:$file
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    Not practical really – Rui F Ribeiro Nov 27 '16 at 10:14
  • Your "After thought" command doesn't work, because actually that just SSHes into one system and attempts to connect to the third system from there. – Celada Nov 27 '16 at 12:39
  • scp hosta:this hostc:that actually runs a subprocess on hosta that sends directly to hostc, or tries to, precisely not what this Q wants. You can do ssh [user@]hosta "cat file" | ssh [user@]hostc "cat >file" or to preserve modtime ssh hosta "tar cf - file" | ssh hostc "tar xf -" (add other options like -v to taste) – dave_thompson_085 Nov 27 '16 at 12:45
  • That's the problem with 'afterthoughts' :) - probably worked that way with rcp as well with *nix. My windows scp version copies it to a tmp file on the pc, and then copies that to the destination. As to practical - a network dept that won;t permit a direct connect is not practical, just as not using a tunnel - is a tunnel forbidden? The "security concern" created by not permitting a direct connect is to increase the likelihood of tunnels and/or "process in the middle" solutions. The purpose of my suggestion was to help with an idea on how to proceed - or did you want a solution? – Michael Felt Nov 27 '16 at 12:52
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An alternative to the solution proposed by Rui F Ribeiro is to setup a ProxyCommand in your SSH configuration file (usually ~/.ssh/config):

host <remote>
    ProxyCommand ssh <gateway_user>@<gateway> nc <host> 22
    User <host_user>

Then, you can simply use:

scp /local/path/to/file <remote>:/remote/path/to/file
scp <remote>:/remote/path/to/file /local/path/to/file

Once your SSH config is set up, you can copy files seamlessly, as if you had a direct connection to the remote server.

  • 1
    @RuiFRibeiro: actually we’re dealing with the config file for the SSH client. In Debian (and most certainly other distro), these are /etc/ssh/ssh_config for system-wide configuration, and ~/.ssh/config for user configuration. The /etc/ssh/sshd_config you’re mentioning is the config file for the SSH server. (See man(5) for ssh_config and sshd_config. – Arcturus B Nov 27 '16 at 17:02

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