Changes

'''[http://en.wikipedia.org/wiki/Iptables Iptables] is the built-in firewall for LINUX''' consisting of a '''list of rules''' (or '''"tables of policies"'''). If data matches a specified <u>policy</u>, it must “jump” to an existing '''condition'''. Simple conditions include '''ACCEPT''', '''REJECT''', '''DROP''' and '''LOG''' but there are also more complex conditions that can be applied and there is even the option to create your own conditions.

<span style="font-family:courier; font-weight:bold;font-size:1.2em;"><u>Here is an Example:</u></span> 

| |'''Let's break down the command to see how it works:-j'''

The <span style="font-family:courier; font-weight:bold">-I</span> switch tells iptables | |means when condition is met, then jump to INSERT this line into the OUTPUT policy. This means it will be the first line in the policy. If we used a <span style="font-family:courier; font-weight:bold">-A</span> switch it would have appended the line and it would be the last line of the policy. If you particular target – Basic targets are writing complex iptables rules where multiple matches can occur'''ACCEPT''', it is important that the lines go in the right order. If you follow the -I with a number'''DROP''', the new rule will be inserted at that location in the chain (for example'''REJECT''', <code>-I 3 OUTPUT</code> will insert and '''LOG'''. The available targets depend on which table contains the rule into the 3rd position in the OUTPUT chain, moving the existing rules down as necessary (the old rule #3 will become the new rule #4, for example).

The '''|-s0/0''' switch specifies the source IP address. 0/0 means a source address of “anywhere.” this has been put into the lab because your ip address will change because it is dynamically assigned. You can change this value if you want to the IP address that has been specifically assigned to your PC.valign="top"

The | |'''-d0/0DROP''' switch specifies the destination address. It makes sense that this address is set to “anywhere” because if we want to block all requests to the WWW, we will never know the specific IP address of web server that is trying to be accessed.

The switch | |means drop the packet – make it disappear - and do not continue processing rules. '''--dport 80REJECT''' tells iptables is similar, but causes an error packet to look at be sent back to the source host. '''ACCEPT''' causes the destination port packet to be processed. '''LOG''' causes an entry to be made in the system logs showing that the packet and see if it was processed. Note that the LOG target is equal to 80. Alternately, the only one that does not stop rule-checking in the chain - so you can filter based on source addresses using log a packet with one rule, and then use a later rule in the <code>--sport</code> switch.chain to DROP, REJECT, or ACCEPT it