Here is a very basic set of instruction to setup a simple iptables firewall configuration on linux (redhat)

Enable firewall

log as root thenenter the following command, it launch a text gui

#> setup

first screen: Choose firewall configuration

second screen: choose "Enabled" then "Customize" 

third screen: select you interface in "Trusted Devices", select "Allow Incoming" for "SSH" "Telnet" "FTP" (add eventually other ports, then press "OK" (2 times, then "Quit")

At that point the firewall is enabled. You can start/stop/monitor using service iptables start/stop/status

Change timeout

to changed the tcp established connection timeout

#> echo 120 >    /proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_timeout_established 

Monitor connection in iptables tables

for example if you want to track a connection establish from a host  152.68.65.207

#> cat /proc/net/ip_conntrack |grep 152.68.65.207