SSH from A through B to C, using private key on B [closed]

Schematic:

    ssh       sshA ------> 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 can't access C directly
• A's ssh public key is present in B:~/.ssh/authorized_keys
• B's ssh public key is present in C:~/.ssh/authorized_keys

What I tried

Following this question, I tried the related answer, here is my .ssh/config

Host proxy  HostName 10.10.10.10  User foo  Port 1234  IdentityFile ~/.ssh/id_rsaHost target   HostName 11.11.11.11  User bar  Port 5678  ProxyCommand ssh -o 'ForwardAgent yes' proxy 'ssh-add && nc %h %p'

This works:

$ ssh -t proxy ssh bar@11.11.11.11 -p 5678

This doesn't works:

$ ssh -t proxy ssh targetssh: Could not resolve hostname target: Temporary failure in name resolutionConnection to 10.10.10.10 closed.$ ssh targetCould not open a connection to your authentication agent.kex_exchange_identification: Connection closed by remote hostConnection closed by UNKNOWN port 65535

I also tried with this configuration following this question:

Host proxy  HostName 10.10.10.10  User foo  Port 1234  IdentityFile ~/.ssh/id_rsaHost target   HostName 11.11.11.11  User bar  Port 5678  ProxyCommand ssh -W %h:%p proxy

But when I run ssh target, it keep asking for password

I would like to simply run ssh target but I'm stuck. If someone could help ! Thanks a lot !