ssh-chain - ssh via a chain of intermediary hosts
This functionality is built into OpenSSH via the -J option as of version 7.3, therefore this tool will no longer be maintained.
Copy the ssh-chain script to somewhere that's in your path. Append the following to ~/.ssh/config or /etc/ssh/ssh_config:
# This should be the last entry
Host *^*
ProxyCommand ssh-chain %h %p
and you're done.
ssh-chain can act as a wrapper to ssh in order to avoid filling your known_hosts file with garbage - just run ssh-chain instead of ssh.
The simple use case is this:
ssh final.example^second.example^first.example
The connection is built right to left, so you'll end up with a set of connections that looks like this:
you -> first.example -> second.example -> final.example
This will also work with scp/sftp and hopefully any other tool that invokes ssh as a backend (e.g. rsync, git, svn, etc.) and all the standard features such as port forwarding should work.
Sometimes you'll have need to specify a username or port for an intermediary host. Since ssh will normally consume these, different (and sort of weird) syntax is used. Ports are specified by appending an underscore (e.g. foo.example_2222) and usernames use a plus instead of an at symbol (e.g. jdoe+foo.example). The far left host still needs to be specified using an at symbol since this doesn't get fed to the ProxyCommand. Example:
jdoe@final.example^johnd+second.example_2222^john+first.example_443
To make host-specific options for hosts other than the first one in the chain work, you need to change lines like this
Host *.foo.example bar.example
User john
Port 2222
to
Host *.foo.example *.foo.example^* bar.example bar.example^*
User john
Port 2222
It's preferable to use OpenSSH 5.4 or newer with ssh-chain. 'netcat mode' (-W) was added then and this is faster then exec'ing netcat on the remote host. ssh-chain auto-detects if -W is available and will remote exec netcat otherwise.