I'm using Linux RHAS 3 u4.
I change tcp paramters to the follow:
net.ipv4.tcp_keepalive_intvl = 5
net.ipv4.tcp_keepalive_probes = 5
net.ipv4.tcp_keepalive_time = 10


Still when open a telnet sasion it not stop by itself after the 35 seconds I supposed it will heppan.

Any ideas?

keepalive deals with how long WAIT states stay alive. They do not (thank God!) kill active sessions. If you want a telnet session to die due to inactivity after it is established then you should probably investigate shell timeouts. (e.g. TMOUT=1800 would be a 30 minute timeout.) These timeouts are sometimes put in place to keep people from staying logged in long after they want finished the task for which they logged in.

The benefit being mainly for security - someone walking by a workstation left logged in as root can easily do a lot of damage - if the session timed out however it reduces the likelihood of this. Of course it also frees up ptys. Note that shell timeouts only do it if someone stays at the shell prompt. If they start an application it won't time that out (unless it is started in background without a nohup) so will not timeout the shell itself.

How do I change this TMOUT ?

Add it to /etc/profile and/or /etc/bashrc for global change (all users) or to .profile or .bashrc for indvidual user in the user's home directory. (bashrc used only for the bash shell, profile is used for sh and ksh.)

TMOUT=<#seconds>;export TMOUT