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Host proxy-host
     Hostname host1
     port 22
     User username
     RequestTTY yes
     ForwardAgent yes
     IdentityFile /key-for-proxy.pem

Host target-host
     Hostname host2
     port 22
     User username
     IdentityFile /key-for-target.pem
     ProxyCommand ssh -A proxy-host /bin/nc %h %p