Basic IPTABLES Understanding

Basic Commands

Typing

# iptables -L

lists your current rules in iptables. If you have just set up your server, you will have no rules, and you should see

Chain INPUT (policy ACCEPT)

target     prot opt source               destination

Chain FORWARD (policy ACCEPT)

target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)

target     prot opt source               destination

Basic Iptables Options

Here are explanations for some of the iptables options you will see in this tutorial. Don't worry about understanding everything here now, but remember to come back and look at this list as you encounter new options later on.

• -A - Append this rule to a rule chain. Valid chains for what we're doing are INPUT, FORWARD and OUTPUT, but we mostly deal with INPUT in this tutorial, which affects only incoming traffic.
• -L - List the current filter rules.
• -m conntrack - Allow filter rules to match based on connection state. Permits the use of the --ctstate option.
• --ctstate - Define the list of states for the rule to match on. Valid states are:
• NEW - The connection has not yet been seen.
• RELATED - The connection is new, but is related to another connection already permitted.
• ESTABLISHED - The connection is already established.
• INVALID - The traffic couldn't be identified for some reason.
• -m limit - Require the rule to match only a limited number of times. Allows the use of the --limit option. Useful for limiting logging rules.
• --limit - The maximum matching rate, given as a number followed by "/second", "/minute", "/hour", or "/day" depending on how often you want the rule to match. If this option is not used and -m limit is used, the default is "3/hour".
• -p - The connection protocol used.
• --dport - The destination port(s) required for this rule. A single port may be given, or a range may be given as start:end, which will match all ports from start to end, inclusive.
• -j - Jump to the specified target. By default, iptables allows four targets:
• ACCEPT - Accept the packet and stop processing rules in this chain.
• REJECT - Reject the packet and notify the sender that we did so, and stop processing rules in this chain.
• DROP - Silently ignore the packet, and stop processing rules in this chain.
• LOG - Log the packet, and continue processing more rules in this chain. Allows the use of the --log-prefix and --log-level options.
• --log-prefix - When logging, put this text before the log message. Use double quotes around the text to use.
• --log-level - Log using the specified syslog level. 7 is a good choice unless you specifically need something else.
• -i - Only match if the packet is coming in on the specified interface.
• -I - Inserts a rule. Takes two options, the chain to insert the rule into, and the rule number it should be.
• -I INPUT 5 would insert the rule into the INPUT chain and make it the 5^th rule in the list.
• -v - Display more information in the output. Useful for if you have rules that look similar without using -v.
• -s --source - address[/mask] source specification
• -d --destination - address[/mask] destination specification
• -o --out-interface - output name[+] network interface name ([+] for wildcard)

Allowing Established Sessions

We can allow established sessions to receive traffic:

# iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

• The above rule has no spaces either side of the comma in ESTABLISHED,RELATED

If the line above doesn't work, you may be on a VPS that uses OpenVZ or doesn't have some kernel extensions installed. In that case, try this line instead:

# iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

Allowing Incoming Traffic on Specific Ports

You could start by blocking traffic, but you might be working over SSH, where you would need to allow SSH before blocking everything else.

To allow incoming traffic on the default SSH port (22), you could tell iptables to allow all TCP traffic on that port to come in.

# iptables -A INPUT -p tcp --dport ssh -j ACCEPT

Referring back to the list above, you can see that this tells iptables:

• append this rule to the input chain (-A INPUT) so we look at incoming traffic
• check to see if it is TCP (-p tcp).
• if so, check to see if the input goes to the SSH port (--dport ssh).
• if so, accept the input (-j ACCEPT).

Lets check the rules: (only the first few lines shown, you will see more)

# iptables -L

Chain INPUT (policy ACCEPT)

target     prot opt source               destination

ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED

ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ssh

Now, let's allow all incoming web traffic

# iptables -A INPUT -p tcp --dport 80 -j ACCEPT

Checking our rules, we have

# iptables -L

Chain INPUT (policy ACCEPT)

target     prot opt source               destination

ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED

ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ssh

ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:www

We have specifically allowed tcp traffic to the ssh and web ports, but as we have not blocked anything, all traffic can still come in.

Blocking Traffic

Once a decision is made to accept a packet, no more rules affect it. As our rules allowing ssh and web traffic come first, as long as our rule to block all traffic comes after them, we can still accept the traffic we want. All we need to do is put the rule to block all traffic at the end.

# iptables -A INPUT -j DROP

# iptables -L

Chain INPUT (policy ACCEPT)

target     prot opt source               destination

ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED

ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ssh

ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:www

DROP       all  --  anywhere             anywhere

Because we didn't specify an interface or a protocol, any traffic for any port on any interface is blocked, except for web and ssh.

Editing iptables

The only problem with our setup so far is that even the loopback port is blocked. We could have written the drop rule for just eth0 by specifying -i eth0, but we could also add a rule for the loopback. If we append this rule, it will come too late - after all the traffic has been dropped. We need to insert this rule before that. Since this is a lot of traffic, we'll insert it as the first rule so it's processed first.

# iptables -I INPUT 1 -i lo -j ACCEPT

# iptables -L

Chain INPUT (policy ACCEPT)

target     prot opt source               destination

ACCEPT     all  --  anywhere             anywhere

ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED

ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ssh

ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:www

DROP       all  --  anywhere             anywhere

The first and last lines look nearly the same, so we will list iptables in greater detail.

# iptables -L -v

Chain INPUT (policy ALLOW 0 packets, 0 bytes)

 pkts bytes target     prot opt in     out     source               destination        

    0     0 ACCEPT     all  --  lo     any     anywhere             anywhere

    0     0 ACCEPT     all  --  any    any     anywhere             anywhere            state RELATED,ESTABLISHED

    0     0 ACCEPT     tcp  --  any    any     anywhere             anywhere            tcp dpt:ssh

    0     0 ACCEPT     tcp  --  any    any     anywhere             anywhere            tcp dpt:www

    0     0 DROP       all  --  any    any     anywhere             anywhere

You can now see a lot more information. This rule is actually very important, since many programs use the loopback interface to communicate with each other. If you don't allow them to talk, you could break those programs!

Logging

In the above examples none of the traffic will be logged. If you would like to log dropped packets to syslog, this would be the quickest way:

# iptables -I INPUT 5 -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7

See Tips section for more ideas on logging.

Saving iptables

If you were to reboot your machine right now, your iptables configuration would disappear. Rather than type this each time you reboot, however, you can save the configuration, and have it start up automatically. To save the configuration, you can use iptables-save and iptables-restore.

I will post more complex example with iptables later.

Thanks.