SSH – Copying Data Through SSH Tunnel Over Multiple Hops

linuxsshssh-tunneltunnel

We have two main environments in question:

Development and QA

Each environment has two servers:

  • Jump Box
  • Application server

In order to connect to the application server, you must connect to the jump box first, and then SSH to the Application server.

There are a few rules in place courtesy of the firewall:

  • You MUST connect to the application server via the jump box
  • The application server cannot connect to either jump boxes
  • The jump boxes are on the same subnet, and CAN talk to each other.

Our Problem

We have a lot of content (670 GB) on the DEVELOPMENT APPLICATION SERVER, and we need to get this to the QA APPLICATION SERVER.

Copying this data to the jump boxes is not an option because they lack the required amount of space.

I did some research, and learned that we could potentially do a series of tunnels through these servers so that we can stream the data straight from one app server to the other via the tunnels. However, the problem that we cannot connect to the jump box from the application server.

Do we have any options? This is getting to be a desperate situation, and time is of the essence. We do not have time to download the data and re-upload it. Copying across the network on the servers will go quickly, as it is a gigabit connection.

Best Answer

By far, the easiest way is to just copy it via scp. Plus, this syntax actually works unlike some of the other suggestions.

You can't beat this syntax for ease. It allows you to recursively copy, rsync or what ever you'd like without the hassle of considering potentially complex pipes. This syntax is intuitively clear, will be more readily supportable by Sys Admins that follow you and does not make useless use of cat.

scp -3 devappserver:/path/to/copy/from qaappserver:/path/to/copy/to

From the scp man page: -3 Copies between two remote hosts are transferred through the local host. Without this option the data is copied directly between the two remote hosts. Note that this option disables the progress meter.

In the example below

  • Your workstation is named MacBook-Pro.
  • Dev Jump Box is named devjumpserver
  • Dev Application Server is named devapplicationserver
    • Is on LAN DNS zone named .local
  • QA Jump Box is named qajumpserver
  • QA Application Server is named qaapplicationserver
    • Is on LAN DNZ zone named .local
  • We'll perform a test copy of a 670GB /etc/hosts file ;-)
  • Assumption is made that you have SSH public key authentication configured.



Here is an ~/.ssh/config file that sets up the direct access from your workstation to the application servers via the appropriate jump (aka bastion server).

MacBook-Pro:~ barrychapman$ cat ~/.ssh/config
Host *
  ServerAliveInterval 60
Host devapplicationsever
  HostName devapplicationserver.local
  ProxyCommand ssh -i ~/.ssh/id_rsa barrychapman@devjumpserver.example.com -W %h:%p
  User barrychapman
Host qaapplicationserver
  HostName qaapplicationserver.local
  ProxyCommand ssh -i ~/.ssh/id_rsa barrychapman@qajumpserver.example.com -W %h:%p
  User barrychapman

MacBook-Pro:~ barrychapman$



Testing for presence of file on target server, it won't be there.

MacBook-Pro:~ barrychapman$ ssh qaapplicationserver ls /tmp/hosts
ls: cannot access /tmp/hosts: No such file or directory
Killed by signal 1.
MacBook-Pro:~ barrychapman$



Now let's copy a file from Dev Application server to QA Application via your workstation.


MacBook-Pro:~ barrychapman$ scp -3 devapplicationserver:/etc/hosts qaapplicationserver:/tmp/
Killed by signal 1.
Killed by signal 1.
MacBook-Pro:~ barrychapman$



Now let's check for the presence of the copied file on the QA Application Server. It will be there this time.

MacBook-Pro:~ barrychapman$ ssh qaapplicationserver ls /tmp/hosts
/tmp/hosts
Killed by signal 1.
MacBook-Pro:~ barrychapman$

Note

When closing a ProxyCommand connection, you will see the warning message "Killed by signal 1". This is SSH tearing down the ProxyCommand connection and is nothing to be alarmed about. You can get rid of it by adding LogLevel Quiet to your bastion host config stanza.

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