SSH

Reference

On this Wiki:

• SSH Tools

Tips

On ssh command:

ssh -F hostname                             # Find hostname in ~/.ssh/known_hosts (useful if HashKnowHosts enabled)
ssh -l -f ~/.ssh/known_hosts                # Print fingerprint of known host keys
ssh -Lport:host:hostport hostname sleep 60  # Forward port, and exit after 60s if no connection is made

If a script is to be called as a ssh command, use 1>&2 instead of >/dev/stderr to redirect a message to stderr (if not, you'll get an error undefined /dev/stderr):

#! /bin/bash
# This script is called as an ssh command with
#   ssh hostname thisscript.sh
echo "This will go to stdin"
echo "This will go to stderr" 1>&2

Install

After installing ssh (client & server), you have to create an ssh-key:

ssh-keygen

Configuration

SSH can be configured through file ~/.ssh/config. See man ssh_config for more information. The format is as follows:

# Specific configuration options for host host1
Host host1
  Option1     parameter
  Option2     parameter

# General configuration options for all hosts. 
# Options in this section applies if same option was *not already specified* in a relevant host section above.
Host *
  Option1     parameter
  Option2     parameter

The value to use for each option is given by the first section that matches the host specification and that provides a value for that option. So section Host * should always be at the end of the file, since any subsequent section will be ignored.

ProxyCommand

Specify the proxy command used by ssh to deal with proxies. Example of ProxyCommand to use in ssh config file:

# none
  ProxyCommand   none                                            # No proxy
# using nc
  ProxyCommand   /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p  # Example in ssh_config manpage but DOES NOT WORK
# using connect (or connect-proxy)
  ProxyCommand   /usr/bin/connect %h %p                          # No proxy as well
  ProxyCommand   /usr/bin/connect -H proxyserver:8080 %h %p      # Using HTTP proxy proxyserver:8080
  ProxyCommand   /usr/bin/connect -h %h %p                       # Using HTTP proxy defined in env. var HTTP_PROXY
  ProxyCommand   /usr/bin/connect -S socks5server:1080 %h %p     # Using SOCKS5 proxy server socks5server:1080
  ProxyCommand   /usr/bin/connect -s %h %p                       # Using SOCKS5 proxy defined in env. var SOCKS5_SERVER
  ProxyCommand   /usr/bin/connect -S socks4server:1080 -4 %h %p  # Using SOCKS4 proxy server socks4server:1080
# Using socat
  ProxyCommand   socat -ly - PROXY:proxy:%h:%p,proxyport=8080,proxyauth=user:pass
                                                                 # HTTP Proxy
  ProxyCommand   socat -ly - SOCKS4A:socksserver:%h:%p,socksport=1080,socksuser=user
                                                                 # SOCKS4A proxy (%h is resolved by proxy)
# Using ssh-tunnel
  ProxyCommand   /usr/local/bin/ssh-tunnel.pl -f - - %h %p

If a hostname matches several sections, first match found is used. Use ProxyCommand none to override a default proxy configuration:

Host 192.*
  ProxyCommand none                                              # Otherwise setting in Host * would be taken
Host *
  ProxyCommand /usr/local/bin/ssh-tunnel.pl -f - - %h %p         # Default proxy settings

Command-Line

Remote Command Execution

• SSH allows to execute any command on remote SSH host. The syntax is

ssh -t SSH_HOST COMMAND

• To execute a remote command on remote host and stay connected afterwards, use ssh -t, along with bash rcfile, like:

ssh -t SSH_HOST "bash --rcfile PATH_TO_RC_FILE"

Don't miss the quotes around the command. Bash will execute the commands in the rc file, and will open a session. Connection remains open because stdin/stdout is not closed. Option -t allows for connecting with current terminal. Without this option, there will be no terminal connection, so bash would run in batch mode (no prompt), and terminal features like tab completion or color would be missing.

• Another solution is to force bash interactive mode:

ssh SSH_HOST "bash --rcfile PATH_TO_RC_FILE -i"

Since there is no terminal, bash goes by default in non-interactive mode. Interactive mode is forced with option -i, and so prompt will be printed, etc. But this is only a partial solution because there is still no terminal, ie. no color, no TAB auto-completion.

Port Forwarding

Here two drawings that illustrate very clearly the mechanisms of port forwarding in SSH.

First the case of direct port forwarding, where a port is opened for listening on the SSH Server, and forwarded to a host and port accessible to the client.

Then the case of reverse port forwarding, where a port is opened for listening on the SSH Client, and forwarded to a host and port accessible to the server.