The iptables utility controls the network packet filtering code in the Linux kernel. If you need to set up firewalls and/or IP masquerading, you should install this tool. The /sbin/iptables application is the userspace command line program used to configure the Linux IPv4 packet filtering rules. Since Network Address Translation (NAT) is also configured from the packet filter rules, /sbin.

1. Iptables Tutorial
2. Iptables Command For Os X 12
3. Iptables Command For Os X 10

• Aug 14, 2015 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: sudo iptables -D INPUT 3.
• Nov 28, 2016 If no chain is selected, all chains are listed. As every other iptables command, it applies to the specified table (filter is the default)-F, –flush chain:Flush or Delete the selected chain (if not chain specified then all the chains in the table will deleted).-Z, –zero chain:Zero the packet and byte counters in all chains.

iptables is a user-space utility program that allows a system administrator to configure the IP packet filter rules of the Linux kernelfirewall, implemented as different Netfilter modules. The filters are organized in different tables, which contain chains of rules for how to treat network traffic packets. Different kernel modules and programs are currently used for different protocols; iptables applies to IPv4, ip6tables to IPv6, arptables to ARP, and ebtables to Ethernet frames.

iptables requires elevated privileges to operate and must be executed by user root, otherwise it fails to function. On most Linux systems, iptables is installed as /usr/sbin/iptables and documented in its man pages, which can be opened using man iptables when installed. It may also be found in /sbin/iptables, but since iptables is more like a service rather than an 'essential binary', the preferred location remains /usr/sbin.

The term iptables is also commonly used to inclusively refer to the kernel-level components. x_tables is the name of the kernel module carrying the shared code portion used by all four modules that also provides the API used for extensions; subsequently, Xtables is more or less used to refer to the entire firewall (v4, v6, arp, and eb) architecture.

Iptables Tutorial

iptables superseded ipchains; and the successor of iptables is nftables, which was released on 19 January 2014^[2] and was merged into the Linux kernel mainline in kernel version 3.13.

Overview[edit]

Xtables allows the system administrator to define tables containing chains of rules for the treatment of packets. Each table is associated with a different kind of packet processing. Packets are processed by sequentially traversing the rules in chains. A rule in a chain can cause a goto or jump to another chain, and this can be repeated to whatever level of nesting is desired. (A jump is like a “call”, i.e. the point that was jumped from is remembered.) Every network packet arriving at or leaving from the computer traverses at least one chain.

The origin of the packet determines which chain it traverses initially. There are five predefined chains (mapping to the five available Netfilter hooks), though a table may not have all chains. Predefined chains have a policy, for example DROP, which is applied to the packet if it reaches the end of the chain. The system administrator can create as many other chains as desired. These chains have no policy; if a packet reaches the end of the chain it is returned to the chain which called it. A chain may be empty.

• PREROUTING: Packets will enter this chain before a routing decision is made.
• INPUT: Packet is going to be locally delivered. It does not have anything to do with processes having an opened socket; local delivery is controlled by the 'local-delivery' routing table: ip route show table local.
• FORWARD: All packets that have been routed and were not for local delivery will traverse this chain.
• OUTPUT: Packets sent from the machine itself will be visiting this chain.
• POSTROUTING: Routing decision has been made. Packets enter this chain just before handing them off to the hardware.

A chain does not exist by itself; it belongs to a table. There are three tables: nat, filter, and mangle. Unless preceded by the option -t, an iptables command concerns the filter table by default. For example, the command iptables -L -v -n, which shows some chains and their rules, is equivalent to iptables -t filter -L -v -n. To show chains of table nat, use the command iptables -t nat -L -v -n

Each rule in a chain contains the specification of which packets it matches. It may also contain a target (used for extensions) or verdict (one of the built-in decisions). As a packet traverses a chain, each rule in turn is examined. If a rule does not match the packet, the packet is passed to the next rule. If a rule does match the packet, the rule takes the action indicated by the target/verdict, which may result in the packet being allowed to continue along the chain or may not. Matches make up the large part of rulesets, as they contain the conditions packets are tested for. These can happen for about any layer in the OSI model, as with e.g. the --mac-source and -p tcp --dport parameters, and there are also protocol-independent matches, such as -m time.

The packet continues to traverse the chain until either

1. a rule matches the packet and decides the ultimate fate of the packet, for example by calling one of the ACCEPT or DROP, or a module returning such an ultimate fate; or
2. a rule calls the RETURN verdict, in which case processing returns to the calling chain; or
3. the end of the chain is reached; traversal either continues in the parent chain (as if RETURN was used), or the base chain policy, which is an ultimate fate, is used.

Targets also return a verdict like ACCEPT (NAT modules will do this) or DROP (e.g. the REJECT module), but may also imply CONTINUE (e.g. the LOG module; CONTINUE is an internal name) to continue with the next rule as if no target/verdict was specified at all.

Userspace utilities[edit]

Front-ends[edit]

There are numerous third-party software applications for iptables that try to facilitate setting up rules. Front-ends in textual or graphical fashion allow users to click-generate simple rulesets; scripts usually refer to shell scripts (but other scripting languages are possible too) that call iptables or (the faster) iptables-restore with a set of predefined rules, or rules expanded from a template with the help of a simple configuration file. Linux distributions commonly employ the latter scheme of using templates. Such a template-based approach is practically a limited form of a rule generator, and such generators also exist in standalone fashion, for example, as PHP web pages.

Such front-ends, generators and scripts are often limited by their built-in template systems and where the templates offer substitution spots for user-defined rules. Also, the generated rules are generally not optimized for the particular firewalling effect the user wishes, as doing so will likely increase the maintenance cost for the developer. Users who reasonably understand iptables and want their ruleset optimized are advised to construct their own ruleset.

Other notable tools[edit]

• FireHOL – a shell script wrapping iptables with an easy-to-understand plain-text configuration file
• NuFW – an authenticating firewall extension to Netfilter
• Shorewall – a gateway/firewall configuration tool, making it possible to use easier rules and have them mapped to iptables

See also[edit]

• ipfirewall (ipfw)

References[edit]

1. ^'News of the netfilter/iptables project'. netfilter.org. 2020-06-12. Retrieved 2020-06-14.
2. ^'Linux 3.13, Section 1.2. nftables, the successor of iptables'. kernelnewbies.org. 2014-01-19. Retrieved 2014-01-20.

Literature[edit]

• Gregor N. Purdy (25 August 2004). Linux iptables Pocket Reference: Firewalls, NAT & Accounting. O'Reilly Media, Inc. ISBN978-1-4493-7898-1.

External links[edit]

• 'iptables'. Freecode.
• The netfilter/iptables documentation page (outdated)^{[clarification needed]}
• Detecting and deceiving network scans – countermeasures against nmap

Related

Tutorial

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

Note: When working with firewalls, take care not to lock yourself out of your own server by blocking SSH traffic (port 22, by default). If you lose access due to your firewall settings, you may need to connect to it via the console to fix your access. Once you are connected via the console, you can change your firewall rules to allow SSH access (or allow all traffic). If your saved firewall rules allow SSH access, another method is to reboot your server.

Prerequisites

Before you start using this tutorial, you should have a separate, non-root superuser account—a user with sudo privileges—set up on your server. If you need to set this up, follow the appropriate guide:

Let’s look at how to list rules first. There are two different ways to view your active iptables rules: in a table or as a list of rule specifications. Both methods provide roughly the same information in different formats.

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:

• 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:

Note that the listing now has two additional columns, pkts and bytes.

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.

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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:

Note that the -A option, which is used to indicate the rule position at creation time, should be excluded here.

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.

Iptables Command For Os X 12

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.

Note: Be careful to not lock yourself out of your server, via SSH, by flushing a chain with a default policy of drop or deny. If you do, you may need to connect to it via the console to fix your access.

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.

Iptables Command For Os X 10

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.

Note: This will effectively disable your firewall. You should only follow this section if you want to start over the configuration of your firewall.

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.

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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.