How to configure a shortcut for an SSH connection through a SSH tunnel

Question

My company production servers (FOO, BAR...) are located behind two gateway servers (A, B). In order to connect to server FOO, I have to open a ssh connection with server A or B with my username JOHNDOE, then from A (or B) I can access any production server opening a SSH connection with a standard username (let's call it WEBBY).

So, each time I have to do something like:

ssh johndoe@a
...
ssh webby@foo
...
# now I can work on the server

As you can imagine, this is a hassle when I need to use scp or if I need to quickly open multiple connections.

I have configured a ssh key and also I'm using .ssh/config for some shortcuts.

I was wondering if I can create some kind of ssh configuration in order to type

ssh foo

and let SSH open/forward all the connections for me. Is it possible?

Edit

womble's answer is exactly what I was looking for but it seems right now I can't use netcat because it's not installed on the gateway server.

weppos:~ weppos$ ssh foo -vv
OpenSSH_5.1p1, OpenSSL 0.9.7l 28 Sep 2006
debug1: Reading configuration data /Users/xyz/.ssh/config
debug1: Applying options for foo
debug1: Reading configuration data /etc/ssh_config
debug2: ssh_connect: needpriv 0
debug1: Executing proxy command: exec ssh a nc -w 3 foo 22
debug1: permanently_drop_suid: 501
debug1: identity file /Users/xyz/.ssh/identity type -1
debug2: key_type_from_name: unknown key type '-----BEGIN'
debug2: key_type_from_name: unknown key type 'Proc-Type:'
debug2: key_type_from_name: unknown key type 'DEK-Info:'
debug2: key_type_from_name: unknown key type '-----END'
debug1: identity file /Users/xyz/.ssh/id_rsa type 1
debug2: key_type_from_name: unknown key type '-----BEGIN'
debug2: key_type_from_name: unknown key type 'Proc-Type:'
debug2: key_type_from_name: unknown key type 'DEK-Info:'
debug2: key_type_from_name: unknown key type '-----END'
debug1: identity file /Users/xyz/.ssh/id_dsa type 2
bash: nc: command not found
ssh_exchange_identification: Connection closed by remote host

Answer 1

As a more concrete version of Kyle's answer, what you want to put in your ~/.ssh/config file is:

host foo
  User webby
  ProxyCommand ssh a nc -w 3 %h %p

host a
  User johndoe

Then, when you run "ssh foo", SSH will attempt to SSH to johndoe@a, run netcat (nc), then perform an SSH to webby@foo through this tunnel. Magic!

Of course, in order to do this, netcat needs to be installed on the gateway server; this package is available for every major distribution and OS.

Answer 2

You can use the ProxyCommand directive in your ~/.ssh/config file, for example to use netcat as the relay:

host server2
    ProxyCommand ssh server1 nc server2 22

The you would just use 'ssh server2'. The man page information for this directive is found in 'man ssh_config'

Answer 3

I prefer a different approach that maintains a pre-authenticated tunnel to the gateway server. In ~/.ssh/config:

Host a
    ControlMaster auto
    ControlPath ~/.ssh/control-master/%r@%h:%p

Then in .bashrc:

s () {
        if ( ssh -O check a 2>&1 > /dev/null 2>&1 )
        then
                ssh -t a ssh $1
        else
                if [[ -S ~/.ssh/control-master/insyte@a:22 ]]
                then
                        echo "Deleting stale socket..."
                        rm ~/.ssh/control-master/insyte@a:22
                fi
                echo "Opening master session..."
                if ssh -Nf a
                then
                         ssh -t a ssh $1
                fi
        fi
 }

So to connect to foo:

s foo

The first time you connect it will authenticate you against "a" and open a persistent, backgrounded ssh tunnel. Subsequent calls to "s" will open almost instantaneously through the pre-authed tunnel.

Works great.

Answer 4

If all your first command is doing is invoking ssh to another box, why not do ssh box1 'ssh box2' ?

Answer 5

weppos:~ weppos$ ssh foo -vv
[..]
bash: nc: command not found

Netcat isn't installed on a. When you run ssh host "command arg", command is executed on host, not on your local machine.

Answer 6

This type of functionality exists in newer versions of OpenSSH and can be used by doing

ssh -W server2 server1

Where server2 is your intended destination and server1 is your proxy host. You can make this easier by using the ProxyCommand option in your ssh config, something like:

host = *.example.com
user = packs
port = 22
ProxyCommand ssh -W %h:%p server1