Computer networks -- 2007-2008 -- info.uvt.ro/Laboratory 12

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Important! These pages are somehow outdated and it is recommended to consult the newer version at Computer networks -- 2008-2009 -- info.uvt.ro (by Ciprian Crăciun).

Quick links:


Firewalls[edit]

  • concepts:
    • connection tracking;
    • connection state;
    • tables;
    • chains;
    • default policy;
    • interfaces;
    • actions;
    • matching modules:
      • state;
      • mac;
      • tcp, udp;

Firewall front-ends[edit]

Iptables examples[edit]

Strict firewall[edit]

#!/bin/bash

# Loading modules

modprobe ip_tables
modprobe iptable_filter
modprobe iptable_nat

# Reseting filter table

iptables --table filter --flush
iptables --table filter --delete-chain
iptables --table filter --zero
iptables --table filter --policy INPUT DROP
iptables --table filter --policy OUTPUT DROP
iptables --table filter --policy FORWARD DROP

# Reseting nat table

iptables --table nat --flush
iptables --table nat --delete-chain
iptables --table nat --zero
iptables --table nat --policy PREROUTING ACCEPT
iptables --table nat --policy POSTROUTING ACCEPT
iptables --table nat --policy OUTPUT ACCEPT

# Configuring filter table

## Allowing established

iptables --table filter --append INPUT --match state --state ESTABLISHED --jump ACCEPT
iptables --table filter --append OUTPUT --match state --state ESTABLISHED --jump ACCEPT

## Allowing related

iptables --table filter --append INPUT --match state --state RELATED --jump ACCEPT
iptables --table filter --append OUTPUT --match state --state RELATED --jump ACCEPT

## Allowing loop

iptables --table filter --append INPUT --in-interface lo --match state --state NEW --jump ACCEPT
iptables --table filter --append OUTPUT --out-interface lo --match state --state NEW --jump ACCEPT

## Allowing outgoing

iptables --table filter --append OUTPUT --out-interface eth0 --match state --state NEW --jump ACCEPT

## Allowing incoming

iptables --table filter --append INPUT --in-interface eth0 --protocol tcp --destination-port 22 --match state --state NEW --jump ACCEPT

Simple router + NAT firewall[edit]

iptables --table filter --flush
iptables --table filter --delete-chain
iptables --table filter --zero
iptables --table filter --policy INPUT DROP
iptables --table filter --policy OUTPUT DROP
iptables --table filter --policy FORWARD DROP

iptables --table nat --flush
iptables --table nat --delete-chain
iptables --table nat --zero
iptables --table nat --policy PREROUTING ACCEPT
iptables --table nat --policy POSTROUTING ACCEPT
iptables --table nat --policy OUTPUT ACCEPT

iptables --table filter --append INPUT --match state --state ESTABLISHED --jump ACCEPT
iptables --table filter --append INPUT --match state --state RELATED --jump ACCEPT
iptables --table filter --append OUTPUT --match state --state ESTABLISHED --jump ACCEPT
iptables --table filter --append OUTPUT --match state --state RELATED --jump ACCEPT
iptables --table filter --append FORWARD --match state --state ESTABLISHED --jump ACCEPT
iptables --table filter --append FORWARD --match state --state RELATED --jump ACCEPT

iptables --table filter --append INPUT --match state --state NEW --in-interface lo --jump ACCEPT
iptables --table filter --append OUTPUT --match state --state NEW --out-interface lo --jump ACCEPT

iptables --table filter --append INPUT --match state --state NEW --in-interface eth0 --protocol tcp --destination-port 22 --jump ACCEPT
iptables --table filter --append INPUT --match state --state NEW --in-interface eth0 --match mac --mac-source xx:xx:xx:xx:xx:xx --jump ACCEPT

iptables --table filter --append OUTPUT --match state --state NEW --out-interface eth0 --jump ACCEPT

iptables --table filter --append FORWARD --match state --state NEW --in-interface eth0 --match mac --mac-source xx:xx:xx:xx:xx:xx --jump ACCEPT

iptables --table nat --append POSTROUTING --out-interface eth0 --source 192.168.1.0/24 --jump SNAT --to-source 100.100.100.100