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The ephemeral port range is per host because TCP connections are identified by the four-tuple: local IP, local port, remote IP, remote port. You can have active connections from localhost:12345 to both some_server:80 and some_other_server:80 or even some_server:443.

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Didn't realize it was a 4 tuple, I was thinking of just the binding to the local port. Still, increasing the ephemeral range will cause problems with applications relying on ports outside the default ephemeral range.

It can, though it's not likely. The kernel only uses free ports even in the ephemeral range, so if an application binds to something in the ephemeral range first, then the kernel just won't pick it for new connections. You've effectively removed one port out of tens of thousands.

You may be in trouble if the kernel happened to choose an ephemeral port for an outbound connection and then an application tried to bind to it for receiving new connections.

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