on the client box my sshd_config is:
+- | Host * | Tunnel yes +-
and on the remote box my sshd_config is:
+- | Protocol 2 | PermitRootLogin without-password | X11Forwarding no | PermitTunnel yes | Subsystem sftp /usr/libexec/sftp-server +-
sudo ssh -w 0:0 firstname.lastname@example.org
On the server:
server# ip link set tun0 up server# ip addr add fec0:1::1/112 dev tun0
On the client:
client# ip link set tun0 up client# ip addr add fec0:1::2/112 dev tun0
This is enough so that you can ping the other side through the tunnel, if there is no firewall rule blocking. The next step is to set routes over the tunnel (don't forget net.ipv6.conf.default.forwarding=1), and then adjust the link MTU to get optimal performance.
server# sysctl net.ipv6.conf.all.forwarding=1
client# ip -6 route add default via fec0:1::1
This will allow your client to ping other networks that the server has access to, given that the targets have routes back to your remote client.