iptables
iptables is a command line utility for configuring Linux kernel firewall implemented within the Netfilter project. The term iptables is also commonly used to refer to this kernel-level firewall. It can be configured directly with iptables, or by using one of the many console and graphical front-ends. iptables is used for IPv4 and ip6tables is used for IPv6. Both iptables and ip6tables have the same syntax, but some options are specific to either IPv4 or IPv6.
Installation
The stock Arch Linux kernel is compiled with iptables support. You will only need to install the userland utilities, which are provided by the package iptables. The iptables package is an indirect dependency of the meta package, so it should be installed on your system by default.
Console
- FireHOL — Language to express firewalling rules, not just a script that produces some kind of a firewall. It makes building even sophisticated firewalls easy - the way you want it.
- Firetable — Tool to maintain an IPtables firewall. Each interface can be configured separately via its own configuration file, which holds an easy and human readable syntax.
Graphical
- Firewall Builder — GUI firewall configuration and management tool that supports iptables (netfilter), ipfilter, pf, ipfw, Cisco PIX (FWSM, ASA) and Cisco routers extended access lists. The program runs on Linux, FreeBSD, OpenBSD, Windows and macOS and can manage both local and remote firewalls.
- Portmaster — Portmaster is a free and open-source application firewall that does the heavy lifting for you. Restore privacy and take back control over all your computer's network activity.
Basic concepts
iptables is used to inspect, modify, forward, redirect, and/or drop IP packets. The code for filtering IP packets is already built into the kernel and is organized into a collection of tables, each with a specific purpose. The tables are made up of a set of predefined chains, and the chains contain rules which are traversed in order. Each rule consists of a predicate of potential matches and a corresponding action (called a target) which is executed if the predicate is true; i.e. the conditions are matched. If the IP packet reaches the end of a built-in chain, including an empty chain, then the chain's policy target determines the final destination of the IP packet. iptables is the user utility which allows you to work with these chains/rules. Most new users find the complexities of linux IP routing quite daunting, but, in practice, the most common use cases (NAT and/or basic Internet firewall) are considerably less complex.
The key to understanding how iptables works is this chart. The lowercase word on top is the table and the upper case word below is the chain. Every IP packet that comes in on any network interface passes through this flow chart from top to bottom. A common misconception is that packets entering from, say, an internal interface are handled differently than packets from an Internet-facing interface. All interfaces are handled the same way; it is up to you to define rules that treat them differently. Of course, some packets are intended for local processes, hence they come in from the top of the chart and stop at <Local Process>, while other packets are generated by local processes; hence they start at <Local Process> and proceed downward through the flowchart. A detailed explanation of how this flow chart works can be found here.
In the vast majority of use cases, you will not need to use the raw, mangle, or security tables at all. Consequently, the following chart depicts a simplified network packet flow through iptables:
Tables
iptables contains five tables:
- is used only for configuring packets so that they are exempt from connection tracking.
- is the default table, and is where all the actions typically associated with a firewall take place.
- is used for network address translation (e.g. port forwarding).
- is used for specialized packet alterations.
- is used for Mandatory Access Control networking rules (e.g. SELinux -- see this article for more details).
In most common use cases, you will only use two of these: filter and nat. The other tables are aimed at complex configurations involving multiple routers and routing decisions and are in any case beyond the scope of these introductory remarks.
Chains
Tables consist of chains, which are lists of rules which are followed in order. The default table, , contains three built-in chains: INPUT, and which are activated at different points of the packet filtering process, as illustrated in the flow chart. The nat table includes PREROUTING, , and chains.
See for a description of built-in chains in other tables.
By default, none of the chains contain any rules. It is up to you to append rules to the chains that you want to use. Chains do have a default policy, which is generally set to , but can be reset to , if you want to be sure that nothing slips through your ruleset. The default policy always applies at the end of a chain only. Hence, the packet has to pass through all existing rules in the chain before the default policy is applied.
User-defined chains can be added to make rulesets more efficient or more easily modifiable. See Simple stateful firewall for an example of how user-defined chains are used.
Rules
Packet filtering is based on rules, which are specified by multiple matches (conditions the packet must satisfy so that the rule can be applied), and one target (action taken when the packet matches all conditions). The typical things a rule might match on are what interface the packet came in on (e.g eth0 or eth1), what type of packet it is (ICMP, TCP, or UDP), or the destination port of the packet.
Targets are specified using the or option. Targets can be either user-defined chains (i.e. if these conditions are matched, jump to the following user-defined chain and continue processing there), one of the special built-in targets, or a target extension. Built-in targets are , , and , target extensions are, for example, and LOG. If the target is a built-in target, the fate of the packet is decided immediately and processing of the packet in current table is stopped. If the target is a user-defined chain and the fate of the packet is not decided by this second chain, it will be filtered against the remaining rules of the original chain. Target extensions can be either terminating (as built-in targets) or non-terminating (as user-defined chains), see iptables-extensions(8) for details.
Traversing Chains
A network packet received on any interface traverses the traffic control chains of tables in the order shown in the flow chart. The first routing decision involves deciding if the final destination of the packet is the local machine (in which case the packet traverses through the INPUT chains) or elsewhere (in which case the packet traverses through the chains). Subsequent routing decisions involve deciding what interface to assign to an outgoing packet. At each chain in the path, every rule in that chain is evaluated in order and whenever a rule matches, the corresponding target/jump action is executed. The 3 most commonly used targets are , , and jump to a user-defined chain. While built-in chains can have default policies, user-defined chains can not. If every rule in a chain that you jumped fails to provide a complete match, the packet is dropped back into the calling chain; see the following illustration. If at any time a complete match is achieved for a rule with a target, the packet is dropped and no further processing is done. If a packet reaches a jump to the ACCEPT target, it will not traverse any further rules of the chain and subsequent chains of the table. Its processing will jump to the first chain of the next table in order. See also Traversing tables and chains and Accept Target of the frozentux tutorial.
Modules
There are many modules which can be used to extend iptables such as connlimit, conntrack, limit and recent. These modules add extra functionality to allow complex filtering rules.
Configuration and usage
iptables is a systemd service and is started accordingly. The Arch iptables package installs an empty set of rules in which will be loaded when you start the unit for the first time. As with other services, if you want iptables to be loaded automatically on boot, you must enable it.
iptables rules for IPv6 are, by default, stored in , which is read by . You can start it the same way as above.
After adding rules via command-line as shown in the following sections, the configuration file is not changed automatically — you have to save it manually:
# iptables-save -f /etc/iptables/iptables.rules
If you edit the configuration file manually, you have to reload iptables.
Or you can load it directly through iptables:
# iptables-restore /etc/iptables/iptables.rules
Showing the current rules
The basic command to list current rules is (), which is similar in output format to the iptables-save utility. The main difference of the two is that the latter outputs the rules of all tables per default, while all iptables commands default to the table only.
When working with iptables on the command line, the () command accepts more modifiers and shows more information. For example, you can check the current ruleset and the number of hits per rule by using the command:
If the output looks like the above, then there are no rules (i.e. nothing is blocked) in the default table. Other tables can be specified with the -t option.
To show the line numbers when listing rules, append --line-numbers to that input. The line numbers are a useful shorthand when #Editing rules on the command line.
Resetting rules
You can flush and reset iptables to default using these commands:
# iptables -F # iptables -X # iptables -t nat -F # iptables -t nat -X # iptables -t mangle -F # iptables -t mangle -X # iptables -t raw -F # iptables -t raw -X # iptables -t security -F # iptables -t security -X # iptables -P INPUT ACCEPT # iptables -P FORWARD ACCEPT # iptables -P OUTPUT ACCEPT
The command with no arguments flushes all the chains in its current table. Similarly, deletes all empty non-default chains in a table.
Individual chains may be flushed or deleted by following and with a argument.
Editing rules
Rules can be edited by appending a rule to a chain, inserting it at a specific position on the chain, replacing an existing rule, or deleting it. The first three commands are exemplified in the following.
First of all, our computer is not a router (unless, of course, it is a router). We want to change the default policy on the chain from to .
# iptables -P FORWARD DROP
The Dropbox LAN sync feature broadcasts packets every 30 seconds to all computers it can see. If we happen to be on a LAN with Dropbox clients and do not use this feature, then we might wish to reject those packets.
# iptables -A INPUT -p tcp --dport 17500 -j REJECT --reject-with icmp-port-unreachable
# iptables -nvL --line-numbers
Chain INPUT (policy ACCEPT 0 packets, 0 bytes) num pkts bytes target prot opt in out source destination 1 0 0 REJECT tcp -- * * 0.0.0.0/0 0.0.0.0/0 tcp dpt:17500 reject-with icmp-port-unreachable Chain FORWARD (policy DROP 0 packets, 0 bytes) num pkts bytes target prot opt in out source destination Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes) num pkts bytes target prot opt in out source destination
Now, say we change our mind about Dropbox and decide to install it on our computer. We also want to LAN sync, but only with one particular IP on our network. So we should use to replace our old rule. Where is our other IP:
# iptables -R INPUT 1 -p tcp --dport 17500 ! -s 10.0.0.85 -j REJECT --reject-with icmp-port-unreachable
We have now replaced our original rule with one that allows to access port 17500 on our computer. But now we realize that this is not scalable. If our friendly Dropbox user is attempting to access port 17500 on our device, we should allow them immediately, not test them against any firewall rules that might come afterwards!
So we write a new rule to allow our trusted user immediately. Using to insert the new rule before our old one:
# iptables -I INPUT -p tcp --dport 17500 -s 10.0.0.85 -j ACCEPT -m comment --comment "Friendly Dropbox"
And replace our second rule with one that rejects everything on port 17500:
# iptables -R INPUT 2 -p tcp --dport 17500 -j REJECT --reject-with icmp-port-unreachable
Our final rule list now looks like this:
Allowing multicast traffic
Protocols that use multicast identification (e.g. SANE searching for network scanners) will send traffic to the network's broadcast IP and responses will come back from a specific client's IP. Since these IP addresses are different, iptables will not recognize the response as or , and it will block the response. See for how to accept multicast traffic without creating an overly-permissive firewall.
First, create an ipset hash container. The timeout is the window of time to accept client responses.
# ipset create upnp hash:ip,port timeout 3
Second, create a rule to add outgoing multicast traffic to the ipset hash.
# iptables -A OUTPUT -d 239.255.255.250/32 -p udp -m udp -j SET --add-set upnp src,src --exist
Third, create a rule to allow incoming traffic that matches against the ipset hash.
# iptables -A INPUT -p udp -m set --match-set upnp dst,dst -j ACCEPT
Finally, remember to save the new rules (see #Configuration and usage and ipset#Making ipset persistent), and ensure and are enabled so the rules load upon system start.
Guides
Logging
The LOG target can be used to log packets that hit a rule. Unlike other targets like or , the packet will continue moving through the chain after hitting a LOG target. This means that in order to enable logging for all dropped packets, you would have to add a duplicate LOG rule before each rule. Since this reduces efficiency and makes things less simple, a chain can be created instead.
Create the chain with:
# iptables -N logdrop
And add the following rules to the newly created chain:
# iptables -A logdrop -m limit --limit 5/m --limit-burst 10 -j LOG # iptables -A logdrop -j DROP
Explanation for limit and options is given below.
Now whenever we want to drop a packet and log this event, we just jump to the chain, for example:
# iptables -A INPUT -m conntrack --ctstate INVALID -j logdrop
Limiting log rate
The above chain uses the limit module to prevent the iptables log from growing too large or causing needless hard drive writes. Without limiting an erroneously configured service trying to connect, or an attacker, could fill the drive (or at least the partition) by causing writes to the iptables log.
The limit module is called with . You can then use to set an average rate and to set an initial burst rate. In the example above:
iptables -A logdrop -m limit --limit 5/m --limit-burst 10 -j LOG
appends a rule which will log all packets that pass through it. The first 10 consecutive packets will be logged, and from then on only 5 packets per minute will be logged. The "limit burst" count is reset every time the "limit rate" is not broken, i.e. logging activity returns to normal automatically.
Viewing logged packets
Logged packets are visible as kernel messages in the systemd journal.
To view all packets that were logged since the machine was last booted:
# journalctl -k --grep="IN=.*OUT=.*"
syslog-ng
Assuming you are using syslog-ng, you can control where iptables' log output goes in syslog-ng.conf. Replace:
filter f_everything { level(debug..emerg) and not facility(auth, authpriv); };
to
filter f_everything { level(debug..emerg) and not facility(auth, authpriv) and not filter(f_iptables); };
This will stop logging iptables output to .
If you also want iptables to log to a different file than , you can simply change the file value of destination here (still in syslog-ng.conf):
destination d_iptables { file("/var/log/iptables.log"); };
ulogd
ulogd is a specialized userspace packet logging daemon for netfilter that can replace the default LOG target. The package is available in the [community] repository.