You don’t need to write a program to do this in Linux. Just pipe the serial port through netcat:
netcat www.example.com port </dev/ttyS0 >/dev/ttyS0
Just replace the address and port information. Also, you may be using a different serial port (i.e. change the /dev/ttyS0 part). You can use the stty or setserial commands to change the parameters of the serial port (baud rate, parity, stop bits, etc.).