SSH Cheat Sheet

Updated   Read time 5 min(s) 24 sec(s)   (512 views).  Misc  

SSH has several features that are useful when performing a security audit of a system. This article is a simple reminder for myself of the syntax for the most useful features.

This page does not attempt to replace the SSH man page, only to supplement it with examples.

Configuration Files

~/.ssh/config

It's sometimes easier to configure options on your SSH client system in ~/.ssh/config for hosts you use a lot rather than having to type out long command lines.

Using ~/.ssh/config also makes it easier to use other tools that use SSH (e.g. scp and rsync). It's possible to tell other tools that SSH listens on a different port, but it's a pain.

Host 10.0.0.1
Port 4321
User church1e
ForwardX11 yes
DynamicForward 127.0.0.1:1080
RemoteForward 80 127.0.0.1:8000
LocalForward 4321 10.1.1.100:4321

I will explain more about the above lines in the subsequent sections of this article.

~/.ssh/authorized_keys

During a security audit, you might want to add an authorized_keys file to let you log in using an SSH key.

The authorized_keys file lives in a user's home directory on the SSH server. It holds the public keys of the users allowed to log into that user's account.

  • Generate a public/private key pair like this: 
    # ssh-keygen -f mykey
  • If you want to shortest possible key (because your arbitrary-file-write vector is limited), use this example: 
    # ssh-keygen -f mykey -t rsa -b 768
  • Connect to the target system (10.1.1.100) like this — you need to know the username of the user you added an authorized key to: 
    # ssh -i mykey user@10.0.0.1

NOTE: The authorized_keys file might not work if it's writable by other users. If that is the case run "chmod 600 ~/.ssh/authorized_keys" to set the correct file permissions.

X11 Forwarding

If your SSH client is also your desktop (X-Server) then you can launch X-clients applications (e.g. Firefox) inside your SSH session and display them on your desktop. This works well with for almost all UNIX/Linux desktops and from cygwin's X-server on Windows.

Command Line:

% ssh -X 10.0.0.1

A faster but less secure alternative is to run:

% ssh -Y 10.0.0.1

~/.ssh/config:

ForwardX11 yes

A faster but less secure alternative is to have the entry:

ForwardX11Trusted yes

Local Forwarding

Allows services on the remote network accessible to your host via a local listener.

NOTE: You need to be root to bind to TCP port <1024. Higher ports are used in the examples below.

Example 1

The service running on the remote host on TCP port 4321 is accessible by connecting to 14321 on the SSH client system.

Command line:

% ssh -L 127.0.0.1:14321:127.0.0.1:4321 user@10.0.0.1

~/.ssh/config:

LocalForward 127.0.0.1:14321 127.0.0.1:4321

Example 2

Same thing, but other hosts on the same network as the SSH client can also connect to the remote service (can be insecure).

Command line:

% ssh -L 0.0.0.0:14321:127.0.0.1:4321 10.0.0.1

~/.ssh/config:

LocalForward 0.0.0.0:14321 127.0.0.1:4321

Example 3

In this example, 10.1.1.100 is a host that's accessible from the SSH server. We can access the service it's running on TCP port 4321 by connecting to 14321 on the SSH client.

Command line:

% ssh -L 127.0.0.1:14321:10.1.1.100:4321 10.0.0.1

~/.ssh/config:

LocalForward 127.0.0.1:14321 10.1.1.100:4321

Remote Forwarding

Allows services on your local system/network accessible to the remote host via a remote listener. This sounds like an odd thing to want to do, but perhaps you want to expose a services that lets you download your tools.

NOTE: You need to be root to bind to TCP port <1024. Higher ports are used in the examples below.

Example 1

The SSH server will be able to access TCP port 80 on the SSH client by connecting to 127.0.0.1:8000 on the SSH server.

Command line:

% ssh -R 127.0.0.1:8000:127.0.0.1:80 10.0.0.1

~/.ssh/config:

RemoteForward 127.0.0.1:8000 127.0.0.1:80

Example 2

The SSH server will be able to access TCP port 80 on 192.168.0.100 (a host accessible from the SSH client) by connecting to 127.0.0.1:8000 on the SSH server.

Command line:

% ssh -R 127.0.0.1:8000:192.168.0.100:80 10.0.0.1

~/.ssh/config:

RemoteForward 127.0.0.1:8000 192.168.0.100:80

Example 3

The SSH server will be able to access TCP port 80 on 192.168.0.100 (a host accessible from the SSH client) by connecting to TCP port 8000 on the SSH server. Any other hosts able to connect to TCP port 8000 on the SSH server will also be able to access 192.168.0.100:80. This can sometimes be insecure.

Command line:

% ssh -R 0.0.0.0:8000:192.168.0.100:80 10.0.0.1

~/.ssh/config:

RemoteForward 0.0.0.0:8000 192.168.0.100:80

SOCKS Proxy

Set up a SOCKS proxy on 127.0.0.1:1080 that lets you pivot through the remote host (10.0.0.1):

Command line:

% ssh -D 127.0.0.1:1080 10.0.0.1

~/.ssh/config:

Host 10.0.0.1
DynamicForward 127.0.0.1:1080

You can then use tsocks or similar to use non-SOCKS-aware tools on hosts accessible from 10.0.0.1:

% tsocks rdesktop 10.0.0.2

SSH Agents

SSH agents can be used to hold your private SSH keys in memory. The agent will then authenticate you to any hosts that trust your SSH key.

This has the following advantages:

  • You don't have to keep entering your passphrase (if you chose to encrypt your private key)
  • But you still get to store your private SSH key in an encrypted format on disk.

Using an SSH agent is probably more secure than storing your key in cleartext, but agents can be hijacked.

Using an SSH Agent

First start your agent:

% eval `ssh-agent`

Then add your keys to it — you'll need to enter your passphrase for any encrypted keys:

% ssh-add ~/dir/mykey

Hijacking SSH Agents

If you see SSH agents running on a system (ssh-agent), you might be able to use it to authenticate you to other hosts — or other accounts on that host. Check out ~/.ssh/known_hosts for some ideas of where you might be able to connect to.

You can use any agents running under the account you've compromised. If you're root you can use any SSH agent.

SSH agents listen on a unix socket. You need to figure where this is for each agent (e.g. /tmp/ssh-hBaSkkG73Jyq/agent.900). You can then use the agent like this:

% export SSH_AUTH_SOCK=/tmp/ssh-hBaSkkG73Jyq/agent.900
% ssh-add -l # lists the keys loaded into the agent
% ssh user@host # will authenticate you if server trusts key in agent

The following command shows how you could inspect the environment of every ssh-agent process on a Linux system. It should yield a list of unix sockets for SSH agents.

% ps auxeww | grep ssh-agent | grep SSH_AUTH_SOCK | sed 's/.*SSH_AUTH_SOCK=//' | cut -f 1 -d ' '
/tmp/ssh-hBaSkkG73Jyq/agent.900

Agent Forwarding

If you enable SSH agent forwarding then you'll be able to carry on using the SSH agent on your SSH client during your session on the SSH server. This is potentially insecure because so will anyone else who is root on the SSH server you're connected to. Avoid using this feature with any keys you care about.

Additional resources