It previously assumed that pod-to-other-node-nodeIP would be
unmasqueraded, but this is not the case for most network plugins. Use
a HostNetwork exec pod to avoid problems.
This also requires putting the client and endpoint on different nodes,
because with most network plugins, a node-to-same-node-pod connection
will end up using the internal "docker0" (or whatever) IP as the
source address rather than the node's public IP, and we don't know
what that IP is.
Also make it work with IPv6.
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