iptables question

hey i bought a book on linux yesterday

it went into iptables only slightly.

anyway, it gave on line examples, of how to accept, deny and drop packets on ports.

my question is

can u deny access at all ports at the beggining, and then open each port you use???????

for example, something like this :

# iptables -A INPUT -p tcp -s 0/0 --destination-port all -j DENY
# iptables -A INPUT -p tcp -s 0/0 --destination-port 21 -j ACCEPT
# iptables -A INPUT -p tcp -s 0/0 --destination-port 52 -j ACCEPT

thanx

 

 

 

 

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