Iptables

Iptables is a powerful firewall built into the Linux kernel and is part of the netfilter project. It can be configured directly, or by using one of the many frontends and GUIs. iptables is used for IPv4 and ip6tables is used for 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 in the official repositories.

Basic concepts

Tables

iptables contains four tables, which are areas where a chain of rules can apply:

1. raw filters packets before any of the other tables.
2. filter filters packets.
3. nat is for network address translation (e.g. port forwarding). Because of limitations in iptables, filtering should not be done here.
4. mangle mangles packets.

Chains

Tables contain chains, which are lists of rules for packets that are followed in order. There are three built-in chains: INPUT, OUTPUT and FORWARD.

1. Inbound traffic addressed to the machine itself hits the INPUT chain.
2. Outbound, locally-generated traffic hits the OUTPUT chain.
3. Routed traffic which should not be delivered locally hits the FORWARD chain.

A rule in one of these chains can be to send a packet down another chain. If the packet passes successfully through this second chain, it will move to the next rule in the original chain. The three built-in chains have default targets which are used if no rules are hit. User-defined chains can be added to make rulesets more efficient.

Targets

A target is the result that occurs when a packet hits a rule. Targets are specified using "jump" (-j). The most common targets are ACCEPT, DROP, REJECT and LOG.

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

From the command line

Showing the current rules

You can check the current ruleset and the number of hits per rule by using the command:

# iptables -nvL

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination   
     
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination    
    
Chain OUTPUT (policy ACCEPT 0K packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

If the output looks like the above, then there are no rules. Nothing is blocked.

To show the line numbers when listing rules, append --line-numbers to that input. This is useful when deleting and adding individual rules.

Editing rules

Rules can be added either by appending a rule to a chain or inserting them at a specific position on the chain. We will explore both methods here.

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 FORWARD chain from ACCEPT to DROP.

# iptables -P FORWARD DROP

Warning: The rest of this section is meant to teach the syntax and concepts behind iptables rules. It is not intended as a means for securing servers. For improving your security of your machine, see Simple Stateful Firewall for a minimally secure iptables configuration and Security for hardening Arch Linux in general.

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         

Note: We use REJECT rather than DROP here, because RFC 1122 3.3.8 requires hosts return ICMP errors whenever possible, instead of dropping packets. In reality, it is best to REJECT packets from hosts who should know about your server's existence, and DROP packets from hosts who should not even know your server exists, or those who appear "up to something".

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 -R to replace our old rule. Where 10.0.0.85 is our other IP:

# iptables -R INPUT 1 -p tcp --dport 17500 ! -s 10.0.0.85 -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  --  *      *      !10.0.0.85            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         

We have now replaced our original rule with one that allows 10.0.0.85 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 him immediately, not test him against any firewall rules that might come afterwards!

So we write a new rule to allow our trusted user immediately. Using -I to insert the new rule before our old one:

# iptables -I INPUT 1 -p tcp --dport 17500 -s 10.0.0.85 -j ACCEPT -m comment --comment "Friendly Dropbox"

# 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 ACCEPT     tcp  --  *      *       10.0.0.85            0.0.0.0/0            tcp dpt:17500 /* Friendly Dropbox */
2        0     0 REJECT     tcp  --  *      *      !10.0.0.85            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    

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:

# 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 ACCEPT     tcp  --  *      *       10.0.0.85            0.0.0.0/0            tcp dpt:17500 /* Friendly Dropbox */
2        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         

Resetting rules

You can flush and reset iptables to default using these commands:

# iptables -P INPUT ACCEPT
# iptables -P FORWARD ACCEPT
# iptables -P OUTPUT ACCEPT
# iptables -F
# iptables -X

Configuration file

Iptables rules are by default stored in /etc/iptables/iptables.rules. This file is read by iptables.service:

# systemctl enable iptables.service
# systemctl start iptables.service

Iptables rules for ipv6 are by default stored in /etc/iptables/ip6tables.rules, this file is read by ip6tables.service. You can start it the same way as above.

After adding rules via command-line, the configuration file is not changed automatically - you have to save it manually:

# iptables-save > /etc/iptables/iptables.rules

If you edit the configuration file manually, you have to reload it:

# systemctl reload iptables

Guides

• Simple stateful firewall
• Router

Logging

The LOG target can be used to log packets that hit a rule. Unlike other targets like ACCEPT or DROP, 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 DROP rule. Since this reduces efficiency and makes things less simple, a logdrop chain can be created instead.

Create the chain with:

# iptables -N logdrop

Then define it:

## /etc/iptables/iptables.rules

*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]

... other user defined chains ..

## logdrop chain
:logdrop - [0:0]

-A logdrop -m limit --limit 5/m --limit-burst 10 -j LOG
-A logdrop -j DROP

... rules ...

## log AND drop packets that hit this rule:
-A INPUT -m state --state INVALID -j logdrop

... more rules ...

Limiting log rate

The limit module should be used to prevent your iptables log from growing too large or causing needless hard drive writes. Without limiting, an attacker could fill your drive (or at least your /var partition) by causing writes to the iptables log.

-m limit is used to call on the limit module. You can then use --limit to set an average rate and --limit-burst to set an initial burst rate. Example:

-A logdrop -m limit --limit 5/m --limit-burst 10 -j LOG

This appends a rule to the logdrop chain which will log all packets that pass through it. The first 10 packets will the be logged, and from then on only 5 packets per minute will be logged. The "limit burst" is restored by one every time the "limit rate" is not broken.

syslog-ng

Assuming you are using syslog-ng, you can control where iptables' log output goes this way:

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 /var/log/everything.log.

If you also want iptables to log to a different file than /var/log/iptables.log, you can simply change the file value of destination d_iptables 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 ulogd is available in the [community] reopository.

See also

• Simple Stateful Firewall
• Official iptables web site
• iptables Tutorial 1.2.2 by Oskar Andreasson
• iptables Debian Debian wiki