Connecting to a Soekris box from your computer via a console cable is a DTE to DTE connection and so you need to purchase or make up a null modem cable. This page on RS232 serial null modem cable wiring proved invaluable.
With the documentation above, I created a null modem with loop back handshaking cable using two wire-able female DB9-to-RJ45 converters and a standard straight-through network cable following the pin positions in the above link.
To locally connect and loop back pins 1+4+6 and 7+8, I snipped those wires, striped the ends and just twisted together and covered with electrical tape. Pin 5 is connected to its opposite number and you only then need to ensure that pins 2 and 3 are crossed over.
This worked a charm with my USB serial port. After I tried about a dozen various unlabeled console cables I have lying around that is…