Here's a cookbook recipe to make a RedHat Linux system genuinely reachable via a serial port. Your mileage may vary; let me know if you have any tips or advice.
1. Make LILO serial-aware.
Add this line somewhere near the top of /etc/lilo.conf:
serial=0,9600n8
This configures ttyS0 (What MS-DOS would call COM1) to be 9600 baud, N-8-1.
2. Make your kernel consider ttyS0 the system console device. Serial support needs to be enabled in your kernel; this seems to be the case in the kernel supplied with RedHat.
Add this to the boot stanza for your kernel in /etc/lilo.conf:
append = "console=ttyS0,9600"3. Make your system provide a login prompt on the serial port. To do this you need to add a getty for the serial port to /etc/inittab.
Add this above the line which begins with "S1:" in your inittab file:
S0:12345:respawn:/sbin/uugetty ttyS0 DT9600 vt1004. Fix the kudzu startup file so it doesn't probe serial, because that hoses things. Alternatively, just take kudzu out of your startup sequence.
Add the option -s to the appropriate line in /etc/rc.d/init.d/kudzu: For example:
/usr/sbin/kudzu -t 30 -s5. Identify the serial port as a "secure tty" -- a tty that root is allowed to log in on.
Add ttyS0 to the top of the file /etc/securettys.
Good luck!