SSH from A through B to C, using private key on B

Question

I'm looking for a simple way to SSH from my local machine, A, through a proxy, B, to a destination host, C. The private key that goes with the public key on C is on B, and I can't put that key on my local machine. Any tips?

Also, I'd like to be able to do this using ~/.ssh/config.

Thanks!

Answer 1

I've worked this out now. Here is the solution, which is rather straightforward. I should have seen it sooner:

~/.ssh/config:

Host B
  HostName 1.2.3.4
  User myuser
  IdentityFile ~/.ssh/rsa_key
  ControlMaster auto
  ControlPath ~/.ssh/socket/master-%l-%r@%h:%p

Host C.*
  User customer_username
  Port customer_port
  IdentityFile remote/path/to/ssh/key
  ForwardAgent yes
  ProxyCommand ssh accessable.server nc %h %p

Host C.server-1
  HostName 2.3.4.5

'B' is the Proxy server that you are jumping through. It should be configured as you normally would configure access to a server.

'C' is the destination host. It needs to be configured to use 'B' in the connection process. The identity file in 'C' is the path to the ssh-key on 'B'. The ProxyCommand uses Netcat to open the connection to 'C' from 'B'. Netcat, or nc, will need to be installed on 'B'.

Hope this helps others.

Answer 2

Check if following is working.

ssh -t B ssh C

Use following command if you want to use key stored on B.

ssh -t B ssh -i /path/to/identity_on_B C

Here we are specifying command i.e ssh -i /path/to/identity_on_B C to be executed on B instead of a login shell.

Answer 3

My other answer is not completely correct. The fact is there is no way to use ~/.ssh/config to automatically use an identity file on a jump box instance. What I had to do was write a simple script to basically list my ssh keys on the remote instance, and then add the one I selected to my local ssh agent. This is not very clean, but allows me to keep all keys on a remote location rather than locally.

Here's the script if anyone is interested:

#!/usr/bin/ruby

require "rubygems"
require "fileutils"

# Get key list
key_list = (`ssh jumpbox "cd ~/.ssh/ ; ls id_rsa*" | sed 's/id_rsa_/  /g' | sed     's/id_rsa//g'`)
puts ' '
puts 'Available customer keys:'
puts key_list

# Get customer name input
puts ' '
puts 'Enter customer name: '
customer_name = gets.chomp

# Add key to ssh-agent
key_name = "~/.ssh/id_rsa_#{customer_name}"
puts ' '
puts "Adding #{key_name} to local ssh-agent"
`ssh jumpbox "ssh-add ~/.ssh/id_rsa_#{customer_name}"`
exit 0