This is a basic Linux firewall program.
Look at your firewalls:
We see the output of input, output and forwarding rules.
I don’t need any forwarding, so I’m going to drop all forwarding:
iptables -P FORWARD DROP
Let’s ‘A’dd, or ‘A’ppend a rule with -A. Let’s drop all input from a nearby IP
iptables -A INPUT -s 192.168.0.23 -j DROP
Or we can block all input from a particular port on the full Network.
iptables -A INPUT -s 192.168.0.0/24 -p tcp --destination-port 25 -j DROP
iptables -A INPUT --dport 80 -j ACCEPT
This allows http traffic to an Apache web server over port 80.
However, rules are accepted in order - so a packet cannot be rejected and then accepted.
To delete rule 2 from the INPUT chain:
iptables -D INPUT 3
Alternatively, you can ‘I’nsert a rule at the start, rather than ‘A’ppending it.
iptables -I INPUT -s 192.168.0.13 DROP
Catchall rules state that anything which is not permitted is forbidden. They must be allowed last.
The -j flag accepts ACCEPT/REJECT/DROP. The last two are identical except that “REJECT” acknowledges the rejection.
Flush all existing rules with:
# Allow all loopback (lo0) traffic and drop all traffic to 127/8 # that doesn't use lo0 iptables -A INPUT -i lo -j ACCEPT iptables -A OUTPUT -o lo -j ACCEPT iptables -A INPUT -d 127.0.0.0/8 ! -i lo -j REJECT --reject-with icmp-port-unreachable # Allow established sessions to receive traffic iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT # Allow ICMP pings iptables -A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPT # Allow SSH remote iptables -I INPUT -p tcp --dport 22 -j ACCEPT # Reject all other inbound connections iptables -A INPUT -j REJECT --reject-with icmp-port-unreachable iptables -A FORWARD -j REJECT --reject-with icmp-port-unreachable