From: Subash Abhinov Kasiviswanathan <redacted>
Date: Wed, 28 Aug 2019 21:26:54 -0600

Reserved ports may have some special use cases which are not suitable
for use by general userspace applications. Currently, ports specified
in ip_local_reserved_ports will not be returned only in case of
automatic port assignment.

In some cases, it maybe required to prevent the host from assigning
the ports even in case of explicit binds. Consider the case of a
transparent proxy where packets are being redirected. In case a socket
matches this connection, packets from this application would be
incorrectly sent to one of the endpoints.

Add a boolean sysctl flag 'reserved_port_bind'. Default value is 1
which preserves the existing behavior. Setting the value to 0 will
prevent userspace applications from binding to these ports even when
they are explicitly requested.

Cc: Sean Tranchetti <redacted>
Signed-off-by: Subash Abhinov Kasiviswanathan <redacted>

I don't know how happy I am about this.  Whatever sets up the transparent
proxy business can block any attempt to communicate over these ports.

Also, protocols like SCTP need the new handling too.