How to edit iptables rules

From FedoraProject

Jump to: navigation, search

In this how-to, we will illustrate three ways to edit iptables Rules :

  • CLI : iptables command line interface and system configuration file /etc/sysconfig/iptables.
  • TUI (text-based) interface : setup or system-config-firewall-tui
  • GUI : system-config-firewall

NOTE: This how-to illustrates editing existing iptables Rules, not the initial creation of Rules chains.

Contents

CLI (command line interface)

Hot changes to iptables Rules

The following procedures allow changes in the behaviour of the firewall while it is running.

Stop (medium size).png
Caution
You can break the network connection to the Fedora system with mistakes in Rules.

Read the man pages for iptables (man iptables) for further explanations and more sophisticated Rules examples.

Important.png
Superuser rights required
You must have superuser rights to execute these commands, please use sudo or su to obtain superuser rights.

Listing Rules

Current running iptables Rules can be viewed with the command
iptables -L
.
Note.png
Numeric port value
The list of Rules with the -L command option shows ports by their service name rather than port number. To see the port number instead, include the -n argument.
iptables -L -n
Note.png
Viewing counters
Rules listed with the -L command option do not include matching counters. To include matching counters, include -v argument.
iptables -L -v

Example of iptables Rules allowing any connections already established or related, icmp requests, all local traffic, and ssh communication:

[root@server ~]# iptables -L
Chain INPUT (policy DROP)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination 

Note that Rules are applied in order of appearance, and the inspection ends immediately when there is a match. Therefore, for example, if a Rule rejecting ssh connections is created, and afterward another Rule is specified allowing ssh, the Rule to reject is applied and the later Rule to accept the ssh connection is not.

Appending Rules

The following adds a Rule at the end of the specified chain of iptables:

[root@server ~]# iptables -A INPUT -p tcp --dport 80 -j ACCEPT
[root@server ~]# iptables -L
Chain INPUT (policy DROP)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination 

Notice the last line in chain INPUT. There are now five Rules in that chain.

Deleting Rules

To delete a Rule, you must know its position in the chain. The following example deletes an existing Rule created earlier that is currently in the fifth position:

[root@server ~]# iptables -D INPUT 5
[root@server ~]# iptables -L
Chain INPUT (policy DROP)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination 

Inserting Rules

Create a Rule at the top (first) position:

[root@server ~]# iptables -I INPUT 1 -p tcp --dport 80 -j ACCEPT
[root@server ~]# iptables -L
Chain INPUT (policy DROP)
target     prot opt source               destination         
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination 

The number given after the chain name indicates the position before an existing Rule. So, for example, if you want to insert a Rule before the third rule you specify the number 3. Afterward, the existing Rule will then be in the fourth position in the chain.

Replacing Rules

Rules may be specified to replace existing Rules in the chain.

In the example shown previously, the first Rule given allows connections to the http port (port 80) from anywhere. The following replaces this Rule, restricting connections to the standard http port (port 80) only from the network address range 192.168.0.0/24:

[root@server ~]# iptables -R INPUT 1 -p tcp -s 192.168.0.0/24 --dport 80 -j ACCEPT
[root@server ~]# iptables -L
Chain INPUT (policy DROP)
target     prot opt source               destination         
ACCEPT     tcp  --  192.168.0.0/24       anywhere             tcp dpt:http
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination 

Flushing Rules

To flush or clear iptables Rules, use the --flush, -F option :

iptables -F <chain>

Specifying a <chain> is optional; without a chain specification, all chains are flushed.

Example to flush Rules in the OUTPUT chain :

[root@server ~]# iptables -F OUTPUT
Stop (medium size).png
Default chain policys care
Be aware of the default chain policy. For example, if the INPUT policy is DROP or REJECT and the Rules are flushed, all incoming traffic will be dropped or rejected and network communication broken.

Making changes persistent

The iptables Rules changes using CLI commands will be lost upon system reboot. However, iptables comes with two useful utilities: iptables-save and iptables-restore.

  • iptables-save prints a dump of current iptables rules to stdout. These may be redirected to a file:
[root@server ~]# iptables-save > iptables.dump 
[root@server ~]# cat iptables.dump 
# Generated by iptables-save v1.4.12 on Wed Dec  7 20:10:49 2011
*filter
:INPUT DROP [45:2307]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [1571:4260654]
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT
COMMIT
# Completed on Wed Dec  7 20:10:49 2011
  • iptables-restore : restore a dump of rules made by iptables-save.
[root@server ~]# iptables-restore < iptables.dump 
[root@server ~]# iptables -L
Chain INPUT (policy DROP)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Upon stopping the service, the current iptables Rules are saved in a file, and upon starting the service, this file is restored. The affected files are:

  • /etc/sysconfig/iptables
    for IPv4
  • /etc/sysconfig/ip6tables
    for IPv6

If preferred, these files may be editted directly, and iptables service restarted to commit the changes. The format is similar to that of the iptables CLI commands:

# Generated by iptables-save v1.4.12 on Wed Dec  7 20:22:39 2011
*filter <--------------------------------------------------------- Specify the table of the next rules
:INPUT DROP [157:36334] <----------------------------------------- This is the three chain belong to filter table, then the policy of the chain
:FORWARD ACCEPT [0:0] <------------------------------------------- and between brackets [<packet-counter>:<byte-counter>] numbers is for
:OUTPUT ACCEPT [48876:76493439] <--------------------------------- debug/informations purpose only. Leave them at their current value.
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT <--------- A rule.
-A INPUT -p icmp -j ACCEPT <-------------------------------------- You just have to take all arguments
-A INPUT -i lo -j ACCEPT <---------------------------------------- of an iptables command.
-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT
COMMIT <---------------------------------------------------------- Needed at each end of table definition. Commit rules in that table.
# Completed on Wed Dec  7 20:22:39 2011

If needed, to reset packet and byte counters, use -Z, --zero :

iptables -Z <chain> <rule_number>

It is possible to reset only reset a single rule counter. It can be useful, if you want to know how many packets were captured for a specific rule.

TUI (text-base user interface)

There is two ways for managing iptables rules with textual interface, by setup and system-config-firewall-tui. In the first choice you need to select firewall configuration and then edit rules, the second will bring directly to the edition of rules. So, with setup, select Firewall configuration :

setup menu utility

On the next screen, leave Firewall enabled or activate it if it wasn't enabled. Then we go on Customize :

Firewall Configuration by TUI. First screen.

There is high chance that your service is part of the list of trusted services. This is basic activation of some standards services. Select what is needed and go Forward :

Note.png
Note
Trusted Services just open the port. It doesn't allow you to specify allowed sources or destination
Editing trusted service with firewall tui interface.

Now Edit other allowed ports :

Editing Other ports on firewall configuration by TUI interface.

To add other ports, specify one port or a port range, and choose between tcp or udp for protocol. Port range format is beginningPort-endingPort.

Select the trusted interfaces. These interfaces will become open face of network, all traffic will be allowed and the precedents rules will never match. So select an interface that face of a private network and never an interface that have to directly deal with internet.

Trusted interfaces.

Select interfaces to be masqueraded. Masquerading is better known as NAT (Network Address Translation), it is useful by example when your Fedora computer is used as gateway to access the internet :

Firewall TUI interface : masquerading.

Port forwarding is also known as PAT permit to reroute traffic from a port to another port

Firewall TUI interface : configuring Port Forwarding.
Firewall TUI : adding port forwarding rules.

You can define ICMP behaviour of your fedora. By default, no limitations are made but you can define rules to reject ICMP traffic, define the return error to an ICMP request, etc.

Firewall TUI: configuring ICMP behaviour.

Finally, you can define some custom rules. But it is needed to edit a file containing your custom rules before, with same format than the iptables file, but without specifying the table of iptables used.

Firewall TUI: create custom rules.

For adding custom rules you have specify the protocol between ipv4 or ipv6 and on what table add the custom rules filter, mangle or nat then the path to the file containing rules to add :

Firewall TUI: adding a custom rules.

When it's done, Close the interface and this bring you at first screen of firewall configuration. Select OK and a warning message appear :

Firewall TUI warning.

Select Yes if the configuration you made fits to you and exit interface, or No for came back to the firewall configuration screen.

Stop (medium size).png
Good to know
The configuration is saved in the file /etc/sysconfig/system-config-firewall when clicking Apply then file /etc/sysconfig/iptables is overwritten.

GUI

Red Hat GUI configuration tool

GUI interface allow you exactly the same thing that TUI interface, but it is more friendly usable.

First time you start GUI, you have a welcome message that warning you that if you have existing manual rules then this rules will be overwritten.

First time startup message

Before all, you need to Enable your firewall to use Firewall Configuration utility.

Firewall Gui startup screen

Then utility warn you that you don't have any existing configuration and want you execute the wizard. Click on Start wizard:

No firewall configuration

Click on forward :

Firewall Wizard : welcome screen

System with network access enable Firewall and System without network access disable Firewall, so select System with network access :

Firewall Wizard : network access?

Beginner allow you to modify only Trusted Services, it's fine if you use only known services like ftp, dns, http, etc but don't allow you to configure customs ports range, select Expert to have full featured Firewall Configuration utility, you can change this option later in the Options menu Main windows, in User Skill Level :

Firewall Wizard : skill?

Server template enable only ssh port on firewall configuration Desktop template enable additional ports for IPsec, Multicast DNS, Network Printing Client and SSH. For convenience select Desktop, and OK :

Firewall Wizard : configuration base?

As described earlier Desktop template enable 4 services IPsec, mDNS, IPP and SSH. If you have services listed in Trusted Services section that you want to enabled, you just have to click on it, that's all. It is possible to change template by using the Options menu, in Load Default Configuration.

Firewall Main interface : enabled

Other Ports allow you to edit custom rules if your service port wasn't in Trusted service. To begin, just click on Add button. Then either you choose in services list the right service or you tick User Defined and fill requested information about Port / Port Range and Protocol.

Firewall GUI : edit other ports rules.

'Trusted Interfaces, Masquerading, Port Forwarding, ICMP Filter and Custom Rules have exactly the same effect than in TUI interface.

When configuration fits to you, just click on the Apply button.

Stop (medium size).png
Good to know
The configuration is saved in the file /etc/sysconfig/system-config-firewall when clicking Apply then file /etc/sysconfig/iptables is overwritten.

Others GUI

There are others GUI available to configure iptables rules.

  • [fwbuilder] : very complete gui tools to configure iptables.
  • [Shorewall] : another very complete gui like fwbuilder.
  • [Turtle firewall project] : web interface and integrated to webmin. Fits to basic usage of Iptables, can not handle all iptables options like fwbuilder
  • [IPmenu] : console based interface that allow you all iptables functionalities.