2020年9月4日 星期五

[Linux 文章收集] How To List and Delete Iptables Firewall Rules

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Introduction
Iptables 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
* Clear Packet and Byte Counters
* Delete rules
* Flush chains (delete all rules in a chain)
* Flush all chains and tables, delete all chains, and accept all traffic


List Rules by Specification
To list out all of the active iptables rules by specification, run the iptables command with the -S option:
$ sudo iptables -S
-P INPUT DROP
-P FORWARD DROP
-P OUTPUT ACCEPT
-N ICMP
-N TCP
-N UDP
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m conntrack --ctstate INVALID -j DROP
-A INPUT -p udp -m conntrack --ctstate NEW -j UDP
-A INPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -m conntrack --ctstate NEW -j TCP
-A INPUT -p icmp -m conntrack --ctstate NEW -j ICMP
-A INPUT -p udp -j REJECT --reject-with icmp-port-unreachable
-A INPUT -p tcp -j REJECT --reject-with tcp-reset
-A INPUT -j REJECT --reject-with icmp-proto-unreachable
-A TCP -p tcp -m tcp --dport 22 -j ACCEPT

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:
$ sudo iptables -S TCP
-N TCP
-A TCP -p tcp -m tcp --dport 22 -j ACCEPT

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:
# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT udp -- anywhere anywhere udp dpt:domain
ACCEPT tcp -- anywhere anywhere tcp dpt:domain
ACCEPT udp -- anywhere anywhere udp dpt:bootps
ACCEPT tcp -- anywhere anywhere tcp dpt:bootps

Chain FORWARD (policy ACCEPT)
target prot opt source destination
DOCKER-USER all -- anywhere anywhere
...

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:
# iptables -L INPUT
  1. Chain INPUT (policy ACCEPT)  
  2. target     prot opt source               destination  
  3. ACCEPT     udp  --  anywhere             anywhere             udp dpt:domain  
  4. ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:domain  
  5. ACCEPT     udp  --  anywhere             anywhere             udp dpt:bootps  
  6. ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:bootps  

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:
* target: If a packet matches the rule, the target specifies what should be done with it. For example, a packet can be accepted, dropped, logged, or sent to another chain to be compared against more rules
prot: The protocol, such as tcp, udp, icmp, or all
* opt: Rarely used, this column indicates IP options
* source: The source IP address or subnet of the traffic, or anywhere
* destination: The destination IP address or subnet of the traffic, or anywhere

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:
# iptables -L INPUT -v
Chain INPUT (policy ACCEPT 329K packets, 763M bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT udp -- virbr0 any anywhere anywhere udp dpt:domain
0 0 ACCEPT tcp -- virbr0 any anywhere anywhere tcp dpt:domain
0 0 ACCEPT udp -- virbr0 any anywhere anywhere udp dpt:bootps
0 0 ACCEPT tcp -- virbr0 any anywhere anywhere tcp dpt:bootps


// Drop packet from ping
# iptables -A INPUT -p icmp --icmp-type echo-request -j REJECT

# ping -c 4 192.168.58.129
PING 192.168.58.129 (192.168.58.129) 56(84) bytes of data.
From 192.168.58.129 icmp_seq=1 Destination Port Unreachable
From 192.168.58.129 icmp_seq=2 Destination Port Unreachable
From 192.168.58.129 icmp_seq=3 Destination Port Unreachable
From 192.168.58.129 icmp_seq=4 Destination Port Unreachable


# iptables -L INPUT -v
  1. Chain INPUT (policy ACCEPT 329K packets, 763M bytes)  
  2. pkts bytes target     prot opt in     out     source               destination  
  3.     0     0 ACCEPT     udp  --  virbr0 any     anywhere             anywhere             udp dpt:domain  
  4.     0     0 ACCEPT     tcp  --  virbr0 any     anywhere             anywhere             tcp dpt:domain  
  5.     0     0 ACCEPT     udp  --  virbr0 any     anywhere             anywhere             udp dpt:bootps  
  6.     0     0 ACCEPT     tcp  --  virbr0 any     anywhere             anywhere             tcp dpt:bootps  
  7.     4   336 REJECT     icmp --  any    any     anywhere             anywhere             icmp echo-request reject-with icmp-port-unreachable  

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:
# iptables -Z

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:
# iptables -Z INPUT

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:
# iptables -Z INPUT 1

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:
# iptables -D INPUT -m conntrack --ctstate INVALID -j DROP

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:
# iptables -L --line-numbers
  1. [secondary_output Example Output: Rules with Line Numbers]  
  2. Chain INPUT (policy DROP)  
  3. num  target     prot opt source               destination  
  4. 1    ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED  
  5. 2    ACCEPT     all  --  anywhere             anywhere  
  6. 3    DROP       all  --  anywhere             anywhere             ctstate INVALID  
  7. 4    UDP        udp  --  anywhere             anywhere             ctstate NEW  
  8. 5    TCP        tcp  --  anywhere             anywhere             tcp flags:FIN,SYN,RST,ACK/SYN ctstate NEW  
  9. 6    ICMP       icmp --  anywhere             anywhere             ctstate NEW  
  10. 7    REJECT     udp  --  anywhere             anywhere             reject-with icmp-port-unreachable  
  11. 8    REJECT     tcp  --  anywhere             anywhere             reject-with tcp-reset  
  12. 9    REJECT     all  --  anywhere             anywhere             reject-with icmp-proto-unreachable  
  13. 10   ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:ssh ctstate NEW,ESTABLISHED  
  14. ...  

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:
# iptables -D INPUT 3

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:
# iptables -F INPUT

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:
# iptables -F

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:
# iptables -P INPUT ACCEPT
# iptables -P FORWARD ACCEPT
# iptables -P OUTPUT ACCEPT

Then flush the nat and mangle tables, flush all chains (-F), and delete all non-default chains (-X):
# iptables -t nat -F
# iptables -t mangle -F

// -F [chain]: Delete all rules in chain or all chains
# iptables -F

// -X [chain]: Delete a user-defined chain
# iptables -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.

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