Need to block all ports and open only select ports on Ubuntu 7.1

Mr.J · 11-18-2008, 01:55 AM

Hi,

I need to block all ports by defaults on linux system and need to allow only few ports to be opened using iptable rules.

ex. to open only ssh port ( 22) , I tried following commands.

iptables -A INPUT -p tcp --dport 22 -j ACCEPT ;
iptables -A INPUT -p udp --dport 22 -j ACCEPT ;
iptables -A OUTPUT -p tcp --dport 22 -j ACCEPT ;
iptables -A OUTPUT -p udp --dport 22 -j ACCEPT ;

iptables -A OUTPUT -p tcp -j DROP ;
iptables -A OUTPUT -p udp -j DROP ;
iptables -A INPUT -p tcp -j DROP ;
iptables -A INPUT -p tcp -j DROP;

iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:ssh
ACCEPT udp -- anywhere anywhere udp dpt:ssh
DROP tcp -- anywhere anywhere
DROP tcp -- anywhere anywhere

Chain FORWARD (policy ACCEPT)
target prot opt source destination

Chain OUTPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:ssh
ACCEPT udp -- anywhere anywhere udp dpt:ssh
ACCEPT udp -- anywhere anywhere udp dpt:ssh
DROP tcp -- anywhere anywhere
DROP udp -- anywhere anywhere

After this when I tried to ssh a machine, it just timed out.

datopdog · 11-18-2008, 03:45 AM

Try.

Code:

iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP
iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT