How is software (O/S, firewall) installed on small firewall SOHO appliances that look like routers and that have no CD-ROM, no PS2 connection, only have LAN ports, RS-232 console port and power connections? These appliances are usually configured with flash memory, SDRAM and CPU.
There are a number of ways this can be done.
(1) Over the network. Typically when this is done with a network update, a digital signature is used to make sure the right software (or at least authorized software) is being loaded. Another trick is to force the update to come from some known IP address (like an internal address configured in known to be the sysadmin's workstation) or some known MAC address.
(2) Over the serial port. This has the advantage of requiring someone to be present, so it's more likely it is going to be done by an authorized person.
(3) Something else -- for example, some systems might have a compact flash port that you can put updates on.
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