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<tr> <th>2</th><td>'''Verify Service is Running & Service can Connect to Network'''</td><td>You should learn to read the output of '''netstat -atnp''' and '''netstat -aunp''' to complement the '''systemctl status''' command.</td></tr>
<tr> <th>3</th><td>'''Verify Network Connectivity by Deleting iptables Rules'''</td><td>If you have no idea what's going on and need to confirm that you're still sane - clear all the iptables rules and check your configuration then. Keep in mind that the '''iptables -F''' command will delete all your rules but will not set the deafult policies to ACCEPT. This will tell you for sure whether your problem was (or was not) caused by iptables.<br><br>If you do this - have a ready way to restore the rules you just deleted. Restarting the iptables service is usually a good start and a '''shell script''' to add your custom rules is a reasonable next step.</td></tr>
<tr> <th>4</th><td>'''List your iptables Rules & Perform a "Walk-Thru"'''</td><td>For many decades, when troubleshooting programs that don't run properly, programmers will resort to reading their "source-code" line-by-line and pretend they are the computer to perform the operation. The programmer "walks-through" the code to force them to think like a computer in order to spot and fix subtle problems.<br><br>Therefore, you can follow a packet's path as you understand it should follow. Keep in mind [http://zenit.senecac.on.ca/wiki/index.php/OPS335_Lab_2#How_Firewalls_.28iptables.29_Relate_to_the_Labs_in_this_Course the diagram from the lecture last week]. What chain applies first on which machine? What's the first rule that matches the packet? What happens if no rules match the packet?<br><br>Don't forget that even if you're tracing the path of outgoing traffic - the INPUT chain on your mahchine still applies (for the response that comes back to your request).</td></tr>
<tr> <th>5</th><td>x</td><td>x</td></tr>
#*
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#* Don't forget that even if you're tracing the path of outgoing traffic - the INPUT chain on your mahchine still applies (for the response that comes back to your request).
# At this point you should be able to understand any iptables rules you'll see in this course, including the default ones in CentOS. If you see a rule you don't understand - you can delete it and see what happens. But if you do that - spend some time figuring out what that rule did and why you needed to delete it. It was likely there for a good reason.
