Schematic:

    ssh       ssh
A ------> B ------> C
    ^          ^
 using A's   using B's
 ssh key     ssh key

Preconditions:

A is running ssh-agent; A can access B; B can access C; A's ssh public key is present in B:~/.ssh/authorized_keys B's ssh public key is present in C:~/.ssh/authorized_keys

HowTo

Method 1 (Use ssh -t)

A$ ssh -t B ssh C

Method 2 (Use ProxyCommand)

A$ vim ~/.ssh/config

Host C
ProxyCommand ssh -o 'ForwardAgent yes' B 'ssh-add && nc %h %p'

If your ssh private key on B is in a non-standard location(not ~/.ssh/id_rsa), add its path after ssh-add. You should now be able to access C from A:

A$ ssh C
C$

Debug

Use multiple -v option to debugging connection, authentication, and configuration problems.

A$ ssh -vvv C

Example

$ cat ~/.ssh/config
Host sshproxy
HostName 192.168.10.10
User root

Host target
HostName 192.168.0.170
User root
ProxyCommand ssh -o 'ForwardAgent yes' sohuzk 'ssh-add && nc %h %p'

$ ssh target 'tail -f /var/log/example.log'