CheckIPForwarding reports whether IP forwarding is enabled correctly for subnet routing and exit node functionality on any interface.

NewAltReadWriteCloserConn returns a net.Conn that wraps rwc (for Read, Write, and Close) and c (for all other methods).

NewDrainBufConn returns a net.Conn conditionally wrapping c, prefixing any bytes that are in initialReadBuf, which may be nil.

NewOneConnListener returns a net.Listener that returns c on its first Accept and EOF thereafter.