We have about 300 users in the systeme , they mainly use the software called "netterm" to telnet our RH server to access the db in our internal network , I know there are some secure program like ssh , that is more secure for connection , do you think it is good idea to replace the software in our case ? what it the advantage and impact ? could provide any recommendation ? thx in advance.

In my opinion, telnet is(and should) be obsolete by now.

In college one of my professors was a security consultant for this big computer company. The companie's number one piece of advice to its clients was to encrypt everything. They felt this is the first line of defense in computer security. After all, if somebody does break your firewall, sniff your network, and steal some information going around your network what good would it be to them if the packets are encrypted?

So my advice, dump telnet A.S.A.P.

Also, even if nobody from outside the network hacks in, your network is vulnerable from the inside. Do you trust every single one of your 300 employees? They could be sniffing each other's passwords and doing who knows what with that information. (Although... I think that only certain types of network topologies are susceptible to packet sniffing... so depending on what yours is, you might not need ssh... but I think I'm talking out of my ass here in this bracketed off section... in any case, if I were you, I'd feel a lot more secure with ssh).