Source From Here
IntroductionIptables is a firewall that plays an essential role in network security for most Linux systems. While many iptables tutorials will teach you how to create firewall rules to secure your server, this one will focus on a different aspect of firewall management: listing and deleting rules.
In this tutorial, we will cover how to do the following iptables tasks:
List Rules by Specification
To list out all of the active iptables rules by specification, run the iptables command with the -S option:
As you can see, the output looks just like the commands that were used to create them, without the preceding iptables command. This will also look similar to the iptables rules configuration files, if you’ve ever used iptables-persistent or iptables save.
List Specific Chain
If you want to limit the output to a specific chain (INPUT, OUTPUT, TCP, etc.), you can specify the chain name directly after the -S option. For example, to show all of the rule specifications in the TCP chain, you would run this command:
Let’s take a look at the alternative way to view the active iptables rules, as a table of rules.
List Rules as Tables
Listing the iptables rules in the table view can be useful for comparing different rules against each other. To output all of the active iptables rules in a table, run the iptables command with the -L option:
This will output all of current rules sorted by chain.
If you want to limit the output to a specific chain (INPUT, OUTPUT, TCP, etc.), you can specify the chain name directly after the -L option. Let’s take a look at an example INPUT chain:
The first line of output indicates the chain name (INPUT, in this case), followed by its default policy (DROP). The next line consists of the headers of each column in the table, and is followed by the chain’s rules. Let’s go over what each header indicates:
The last column, which is not labeled, indicates the options of a rule. That is, any part of the rule that isn’t indicated by the previous columns. This could be anything from source and destination ports, to the connection state of the packet.
Show Packet Counts and Aggregate Size
When listing iptables rules, it is also possible to show the number of packets, and the aggregate size of the packets in bytes, that matched each particular rule. This is often useful when trying to get a rough idea of which rules are matching against packets. To do so, simply use the -L and -v option together.
For example, let’s look at the INPUT chain again, with the -v option:
Now that you know how to list the active firewall rules in a variety of ways, let’s look at how you can reset the packet and byte counters.
Reset Packet Counts and Aggregate Size
If you want to clear, or zero, the packet and byte counters for your rules, use the -Z option. They also reset if a reboot occurs. This is useful if you want to see if your server is receiving new traffic that matches your existing rules.
To clear the counters for all chains and rules, use the -Z option by itself:
To clear the counters for all rules in a specific chain, use the -Z option and specify the chain. For example, to clear the INPUT chain counters run this command:
If you want to clear the counters for a specific rule, specify the chain name and the rule number. For example, to zero the counters for the 1st rule in the INPUT chain, run this:
Now that you know how to reset the iptables packet and byte counters, let’s look at the two methods that can be used to delete them.
Delete Rule by Specification
One of the ways to delete iptables rules is by rule specification. To do so, you can run the iptables command with the -D option followed by the rule specification. If you want to delete rules using this method, you can use the output of the rules list, iptables -S, for some help.
For example, if you want to delete the rule that drops invalid incoming packets (-A INPUT -m conntrack --ctstate INVALID -j DROP), you could run this command:
Delete Rule by Chain and Number
The other way to delete iptables rules is by its chain and line number. To determine a rule’s line number, list the rules in the table format and add the --line-numbers option:
This adds the line number to each rule row, indicated by the num header.
Once you know which rule you want to delete, note the chain and line number of the rule. Then run the iptables -D command followed by the chain and rule number. For example, if we want to delete the input rule that drops invalid packets, we can see that it’s rule 3 of the INPUT chain. So we should run this command:
Now that you know how to delete individual firewall rules, let’s go over how you can flush chains of rules.
Flush Chains
Iptables offers a way to delete all rules in a chain, or flush a chain. This section will cover the variety of ways to do this.
Flush a Single Chain
To flush a specific chain, which will delete all of the rules in the chain, you may use the -F, or the equivalent --flush, option and the name of the chain to flush. For example, to delete all of the rules in the INPUT chain, run this command:
Flush All Chains
To flush all chains, which will delete all of the firewall rules, you may use the -F, or the equivalent --flush, option by itself:
Flush All Rules, Delete All Chains, and Accept All
This section will show you how to flush all of your firewall rules, tables, and chains, and allow all network traffic. First, set the default policies for each of the built-in chains to ACCEPT. The main reason to do this is to ensure that you won’t be locked out from your server via SSH:
Then flush the nat and mangle tables, flush all chains (-F), and delete all non-default chains (-X):
Your firewall will now allow all network traffic. If you list your rules now, you will will see there are none, and only the three default chains (INPUT, FORWARD, and OUTPUT) remain.
Conclusion
After going through this tutorial, you should be familiar with how to list and delete your iptables firewall rules.
Remember that any iptables changes via the iptables command are ephemeral, and need to be saved to persist through server reboots. This is covered in the Saving Rules section of the Common Firewall Rules and Commands tutorial.
沒有留言:
張貼留言