OPS335 Lab 2b
ADDITIONAL IPTABLES TROUBLESHOOTING
This lab will provide more hands-on experience with troubleshooting iptables issues.
IPTABLES and Routing Troubleshooting Resources
Some articles that you can use as a reference if you are still experiencing problems with iptables:
INVESTIGATION 1: IPTABLES TROUBLESHOOTING CHECKLIST
By now, you have probably discovered that a simple mistake in your iptables rules can have very serious and unexpected consequences for not only your services, but the network connectivity in general. There is a general process (checklist) that you can following to help troubleshoot iptables in order to fix the problem.
Refer to the following IPTABLES Troubleshooting Checklist:
|1||Test Network Connectivity||You can use the steps in lab 1 as a guide, but keep in mind the firewall may be blocking pings and DNS requests.|
|2||Verify Service is Running & listening on the correct interfaces||You should learn to read the output of netstat -atnp and netstat -aunp to complement the systemctl status command.|
|3||Verify Network Connectivity by Deleting iptables Rules||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.|
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.
|4||List your iptables Rules & Perform a "Walk-Thru"||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.|
Therefore, you can follow a packet's path as you understand it should follow. Keep in mind 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?
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 experience in this course, including the default ones in CentOS. If you see a iptables rule that you don't understand, you can delete it and see what happens. But if you simply delete this rule, take the time to figure out what that rule did and why you needed to delete it. It was likely there for a purpose (other than to drive you crazy).
Record troubleshooting checklist for INVESTIGATION 1 in your OPS335 lab log-book
INVESTIGATION 2: HANDS-ON IPTABLES TROUBLESHOOTING
You will now get additional practice on troubleshooting iptables by downloading a running a shell script that will create iptables rules that will cause problems. You will then need to use tools and procedures (IPTABLES Troubleshooting Checklist) to determine the cause of the problem and fix that problem.
Perform the following steps on your HOST:
- Download and run the following script: http://scs.senecacollege.ca/~andrew.smith/ops335/lab_practice_iptables.sh
This will display a menu of exercises. You can choose any of the items in order, but you should attempt all of them. The script will first reset the firewall settings to CentOS defaults and then make some modifications from those defaults.
- Troubleshoot and fix the problem as you would on a real server. The point of the exercises is for you to find the problem using regular troubleshooting tools, not to reverse-engineer the shell script.
- Finish the exercises, and record any information you feel you'll need to remember to solve problems like this in the future (e.g. in an assignment and/or in a practical test).
Record steps, commands, and your observations in INVESTIGATION 2 in your OPS335 lab log-book
COMPLETING THE LAB
Arrange evidence (command output) for each of these items on your screen, then ask your instructor to review them and sign off on the lab's completion:
- List 3 separate techniques that you used to help troubleshoot to detect and fix iptables from running the shell script in the previous section.
- Without booking at the table above, list the 4 tips for troubleshooting iptables (in order).
- After completing this lab, how does the above-mentioned shell script work to cause problems with iptables?