Don't you think it's much easier to program that stuff directly in Linux and not having another device in between?
The Arduino is good in accessing electronics and low level devices by protocols like I2C, SPI and the like. You'll find even a profibus library for Arduino but it definitely does not make sense to use an Arduino to emulate a TCP/IP based protocol for a Linux computer. On Linux you don't have the memory constraints you have on an Arduino and you have the multi-tasking support of a full-fledged OS.