From Jag, try typing in the IP of the red hat machine. The finder will only connect if a file service is running on the NIX machine.
Each machine will be specific as to what services it's offering. Are they offering NFS or AFP, SAMBA, FTP, HTTP, MAIL etc? Depending on what client app you are using, you will only be able to connect to the relevant service which is offered. File services are browsed using the finder. FTP through an ftp client, mail through a mail client etc. If the remote machine hasn't been set-up to offer these services you can't connect.
For the red hat machine to get file services from OSX you will need to set up NFS on your OSX machine. I don't think this is simple to do at the moment, but it's possible. But it is simple to set-up ftp services on the mac. Just check ftp in sharing preferences and your done. Then use whatever ftp client your red hat machine has to connect.
