ssh agent forward

1. Start ssh-agent

#local
eval `ssh-agent -s`

2. Add identity

ssh-add

3. Configure ssh forwarding

# local
cat "
Host github.com
    ForwardAgent yes
" >> .ssh/config

4. Add public key to server2

ssh-copy-id user@server2

5. SSH login server2 in server1 with passwd

#first, pls login server1, then execute follow command 
ssh -T user@server2 #yeah!, you can login.