Posted on by Chloe_6

How does the firewall on the Linux system work?

#!/bin/bash

iptables -F
iptables -t nat -F
iptables -X
iptables -t nat -X

iptables -P INPUT DROP
iptables -P OUTPUT DROP
iptables -P FORWARD DROP

iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -i eth1 -p tcp -s 192.168.167.0/24 --dport 22 \
-m state --state NEW -j ACCEPT
iptables -A INPUT -i eth1 -p tcp -m state --state RELATED,ESTABLISHED -j ACCEPT


iptables -A OUTPUT -o lo -j ACCEPT
iptables -A OUTPUT -o eth1 -p tcp -m state --state RELATED,ESTABLISHED -j ACCEPT

iptables -t nat -A PREROUTING -i eth0 -p tcp -d 133.172.114.17 --dport 25 \
-j DNAT --to-destination 192.168.167.23:25
iptables -t nat -A POSTROUTING -o eth0 -p tcp -s 192.168.167.0/24 \
-j SNAT --to-source 133.172.114.17

echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -A FORWARD -i eth1 -p tcp -s 192.168.167.0/24 --dport 80 \
-m state --state NEW -j ACCEPT
iptables -A FORWARD -i eth1 -p tcp -s 192.168.167.0/24 --dport 443 \
-m state --state NEW -j ACCEPT
iptables -A FORWARD -i eth0 -p tcp -d 192.168.167.23 --dport 25 \
-m state --state NEW -j ACCEPT
iptables -A FORWARD -p tcp -m state --state RELATED,ESTABLISHED -j ACCEPT

With this firewall script, I'm needing to answer this question:
For the following four groups of iptables commands, explain:
the overall effect of each group of commands, and
the purpose of each command within the group.

  1. lines 12 and 18,
  2. lines 13 and 19.
  3. lines 21, 32 and 34.
  4. lines 23, 28, 30 and 34.