My SSH and VNC connections freeze a few seconds after they get established when I connect to my home Linux computers from home using their external IP addresses. Everything works fine when I connect to them from an outside computer or from home using their local IP addresses. The Linux computers are connected to a Zyxel DSL router (o2 HomeBox 6441) which also does the port-forwarding.

I was thinking that I might encounter a NAT loopback or hairpinning problem explained here, here and here.

However, what surprises me is that the connections gets established and I can even type a few characters or receive half of a VNC screen. Another surprise is that I can establish an SSH connection if I connect to another external computer and SSH back home form there. But if I enable X11 forwarding, the problem is back and the connection freezes.

I don't see any difference in the ssh -vvv output between the local and failing external connection.

What could be the problem?

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