Tunnel port through ssh gateway useful for tensorboard/notebook behind a firewall

ssh_tunnel_config

ssh -L <local-port>:<2nd-pc-address>:<2nd-pc-port> <1st-pc-address>
=
Host target-forwarding
    Hostname <1st-pc-address>
    User <1st-pc-username>
    LocalForward <local-port> <2nd-pc-address>:<2nd-pc-port>
    IdentityFile <1st-pc-identity-file>

# if unix is the gateway (1st-pc) and has its own configuration

Host unix-gateway
    User <1st-pc-username>
    Port <1st-pc-port>
    HostName <1st-pc-address>
    IdentityFile <1st-pc-identity-file>

Host target-forwarding
    User <2nd-pc-username>
    HostName <2nd-pc-address>
    ProxyCommand ssh -aT unix-gateway nc %h %p
    IdentityFile <2nd-pc-identity-file>

# if windows is the gateway (1st-pc) then have cygwin64/nc.exe installed and do:

Host windows-gateway
    User <1st-pc-username>
    Port <1st-pc-port>
    Hostname <1st-pc-address>
    IdentityFile <1st-pc-identity-file>

Host target-forwarding
    User <2nd-pc-username>
    HostName <2nd-pc-address>
    ProxyCommand ssh -aT windows-gateway c:\\cygwin64\\bin\\nc.exe %h %p
    IdentityFile <2nd-pc-identity-file>