Networking, Snippets, Unix

Jumping SSH Hosts

Sometimes some servers are unreachable to us due to network topology barriers, which are put in place for security reasons.

A common way to allow developers and sysadmins access is to provide public “bastion” or jump hosts, where one can obtain a shell and then use it to connect to one’s destination.

Looking at the picture above, things may seem pretty simple – but how is authentication handled when more than two parties are involved in a connection?

Let’s explore the anti-patterns first, and then move on to the proper way of doing things.

Storing intermediate key pair

In this solution, two key pairs are created: the first for accessing the jump host from the client, and the second for accessing the server from the jump host, like in the section above.

We will get a shell SSH into the jump host using the first key pair, and from there SSH into the server using the second.

However, this involves installing a private key onto a middle (potentially public) system, which is not always desirable. An intruder who compromises the jump host might be able to read the private key there, and use it to access the remote server or any number of machines in the private network.

Forwarding SSH agent

A slightly better, but still potentially dangerous solution involves the helper command we saw in the previous section: ssh-agent.

With the special flag to the ssh command -A we are able to “forward” the local ssh-agent containing our keys to the jump host. This allows us to re-use our agent from there (and all the keys we added to it) as if we were on our own machine.
This way we can re-use our private keys but without storing any of them on the jump host.

To set up a simple authentication based on ssh-agent forwarding we might want to install our public key both on the jump host’s and on the remote server’s authorized_keys file like in the picture.
Then we can add our private key to our local agent like this:

$ ssh-add ~/.ssh/key_name_id_rsa

And forward it when connecting to the jump host:

$ ssh -A myuser@jump-host

From within the jump host, we should be able to keep using any key previously added to our agent, so we are able to SSH normally into the final remote server:

$ ssh myuser@myserver
How could it be insecure?

The ssh-agent is a process which stores our keys unencrypted in memory and communicates through a Unix socket (see the section on the agent to understand why). As was mentioned, SSH knows where to find the agent socket by looking at the variable $SSH_AUTH_SOCK.

When SSH connects with the agent forwarding option -A enabled, it tells the SSH daemon on the jump host to set its own $SSH_AUTH_SOCK variable to a newly created socket. That socket is made to actually point back over the network to the one in our local machine (the communication is routed through a secondary channel).

That way when the SSH client on the jump host wants to connect to the agent, it will be unknowingly communicating with the agent on our local machine instead of its own.

Usually the socket file on the jump host is stored in /tmp and only the user who owns the SSH session can use it.

$ sudo find /tmp -path '*ssh*' -type s
/tmp/ssh-JVEaf5qUm5O1/agent.57796

However, the root user has access to everything.
Therefore, any other user gaining access to the jump host as root could simply set own their $SSH_AUTH_SOCK to point to our socket, and use our ssh-agent as their own.

$ SSH_AUTH_SOCK=/tmp/ssh-JVEaf5qUm5O1/agent.57796 myuser@myserver

That’s why it’s very important to only forward our SSH agent only to servers that we fully trust.

ProxyCommand and ProxyJump

Unlike agent forwarding and intermediate key value pair this method is not going to store or otherwise expose our keys on the bastion.

The SSH client connects the jump host first, then executes a ProxyCommand which forwards the standard input and output to the remote server:

$ ssh -i ~/.ssh/server_id_rsa -oProxyCommand="ssh -i ~/.ssh/jump_id_rsa jump-host nc myserver" myserver

This command looks very long and convoluted. It will get simpler. It is worth to doing things the “old fashioned” way first in order understand all of the underlying components.

It is essentially structured into two SSH commands, connected by the -oProxyCommand directive:

  • The “inner” SSH command passed to the -oProxyCommand option (within the double quotes) connects to the jump host.
    Once connected, it starts a netcat (nc) process on the jump host which carries all its stdin to the server, and all the stdout back. This allows us to run…
  • The “outer” SSH (outside the double quotes), which connects to the server.
    However, instead of connecting the usual way, the -oProxyCommand flag tells it that when it tries to establish a connection to server it should do so using the stdin/stdout of the “inner” command as a transport. That will be the stdin and stdout of the netcat process we started on the jump host, which will be routed directly to the server.

The combination of the two allows you to obtain a shell directly into the server, using the jump host as a proxy.
You will need to provide authentication in the form of private key or password to both commands as in the example.

In later versions of SSH, you can avoid using netcat (which is handy as it might not be installed on your jump host) and instead use the equivalent flag -W in the inner command:

$ ssh -i ~/.ssh/server_id_rsa -oProxyCommand="ssh -i ~/.ssh/jump_id_rsa jump-host -W myserver" myserver

And you can use “special string substitutions” inside the inner comand to avoid repeating the final host and port:

$ ssh -i ~/.ssh/server_id_rsa -oProxyCommand="ssh -i ~/.ssh/jump_id_rsa jump-host -W %h:%p" myserver

ProxyCommand can also be specified in the ~/.ssh/config file instead of the command line for simplicity:

Host server
   ProxyCommand "ssh -i ~/.ssh/jump_id_rsa jump-host -W %h:%p"
   HostName myserver
   User myusername
   Port 1337
   IdentityFile ~/ssh/my_key_for_the_server
A shortcut: -J, or ProxyJump

Luckily, we don’t have to do all that anymore. Later versions of SSH (OpenSSH 7.3) now simplify this common use case with ProxyJump, or the command line flag -J:

$ ssh -J username@jump-host username@server

Connect to the target host by first making a ssh connection to the jump host described by destination and then establishing a TCP forwarding to the ultimate destination from there.

ssh man page

A little drawback: since there is no separation or “inner” or “outer” commands anymore, there would be ambiguity if the user were allowed to specify options (like the private key) for the jump host as well as for the server in the same command.
Therefore we have to put any required configuration for the jump host into our ~/.ssh/config instead:

Host jump
   HostName jump-host
   User myusername
   Port 1337
   IdentityFile ~/ssh/my_key_for_the_jump_host_id_rsa

Once this is done, the simplified version of the command will immediately give us a shell into our destination, securely routing all the traffic through the jump host.

You can even route the traffic through more than one server in case you have multiple levels of separation in the network between a bastion and the final remote host. 
All you need to do is specify comma-separated jump hosts to -J:

$ ssh -J username@jump-host1,username@jump-host2 username@server

It is possible to specify the jumps you want to make in ~/.ssh/config with the equivalent option ProxyJump:

Host server
   ProxyJump jump
   HostName myserver
   User myusername
   Port 1337
   IdentityFile ~/ssh/my_key_for_the_server

Host jump
   HostName jump-host
   User myusername
   Port 1337
   IdentityFile ~/ssh/my_key_for_the_jump_host_id_rsa

Much more clear than its ProxyCommand equivalent.

Is ProxyCommand dead?

While the newer options are far more simple, they are only a shorthand for the most common use case. ProxyCommand still has a place in non-standard scenarios.

You might want to use a more elaborate command than just the simple tcp forwarding/netcat equivalent provided by -J.

For example, from the documentation:

This directive is useful in conjunction with nc(1) and its proxy support. For example, the following directive would connect via an HTTP proxy at 192.0.2.0:

ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p

As another example, Amazon offers the SSM service to connect into its EC2 machines nowadays, discouraging opening them up for SSH connections from the world. With a ProxyCommand trick, it is possible to still set up an SSH connection to them:

$ ssh -oProxyCommand="sh -c \"aws ssm start-session --target %h --document-name AWS-StartSSHSession --parameters 'portNumber=%p'"\" myuser@myec2instance

The ProxyCommand option is used to invoke aws ssm start-session to establish a connection to your target EC2, rather than connecting directly over port 22. The AWS CLI is making API calls over HTTPS, so you only need port 443 open locally; and those calls are secured by IAM authentication and policies.

More info on the AWS documentation

Next: Tunnelling and Port Forwarding →

← Previous: Config

Table of Contents:

  1. Introduction
  2. Authentication
  3. Known Hosts
  4. SSH Agent
  5. Config
  6. Jumping Hosts
  7. Tunnelling and Port Forwarding
  8. X11 Forwarding
  9. Multiplexing and Master Mode

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