Pigeon wrote: [cut]
in the meantime I still want it to copy data from one serial port to the other so I can continue to dial out as normal from the main box. Of course, Linux can't run my DOS program. But there's a package called snooper which seems to do the same thing. So I installed it on the modem box and set it up to connect the external and modem serialports. Try pon from the main box - nothing.
[cut] Since you seem dead-set against buying a couple of NICs...I would take a completely different tack and try using SLIP, so you'll have regular TCP/IP networking going on over the serial line. Then use the "modem PC" as your gateway, and run PPP in demand mode on the modem PC.
One of the many beauties of this approach is that when you *do* buy a couple of NICs, and even when you replace your dialup with broadband, you'll still can use the same PC as a gateway.