The user-space application program iptables allows configuring the tables provided by the Linux kernel firewall and the chains and rules it stores. The kernel module currently used for iptables only applies to IPv4 traffic. To configure firewall rules for IPv6 connections, use ip6tables, which respond to the same command structures as iptables.
Listing current rules
Ubuntu servers do not implement any restrictions by default, but for future reference, check the current iptable rules using the following command.
sudo iptables -L
This will print out a list of three chains, input, forward and output, like the empty rules table example output below.
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
The chain names indicate which traffic the rules in each list will be applied to. Input is for any connections coming to your cloud server, output is any leaving traffic, and forward is for any pass-through. Each chain also has its policy setting, which determines how the traffic is handled if it doesn’t match any specific rules. By default, it’s set to accept.
Adding rules
Firewalls can commonly be configured in one of two ways: either set the default rule to accept and then block any unwanted traffic with specific rules or use the rules to define allowed traffic and block everything else. The latter is often the recommended approach, as it allows preemptively blocking traffic rather than having to reactively reject connections that should not be attempting to access your cloud server.
To begin using iptables, you should first add the rules for allowed inbound traffic for the services you require. Iptables can track the state of the connection, so use the command below to allow established connections to continue.
sudo iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
You can check that the rule was added using the same sudo iptables -L as before.
Next, traffic to a specific port will be allowed to enable SSH connections with the following:
sudo iptables -A INPUT -p tcp --dport ssh -j ACCEPT
The ssh in the command translates to port number 22, which the protocol uses by default. The same command structure can also be used to allow traffic to other ports. To enable access to an HTTP web server, use the following command.
sudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT
After adding all the allowed rules you require, change the input policy to drop.
Warning: Changing the default rule to drop will permit only specifically accepted connections. Before changing the default rule, make sure you’ve enabled at least SSH, as shown above.
sudo iptables -P INPUT DROP
The same policy rules can also be defined for other chains by entering the chain name and selecting either DROP or ACCEPT.
Saving and restoring rules
Now, if you were to restart your cloud server, all of these iptables configurations would be wiped. To prevent this, save the rules to a file.
sudo iptables-save > /etc/iptables/rules.v4
You can then simply restore the saved rules by reading your saved file.
# Overwrite the current rules sudo iptables-restore < /etc/iptables/rules.v4 # Add the new rules keeping the current ones sudo iptables-restore -n < /etc/iptables/rules.v4
You can automate the restore process at reboot by installing an additional package for iptables, which takes over the loading of the saved rules. To this with the following command.
sudo apt-get install iptables-persistent
After the installation, the initial setup will ask you to save the current rules for IPv4 and IPv6. Just select Yes and press enter for both.
If you make further changes to your iptables rules, remember to save them again using the same command as above. The iptables-persistent looks for the files rules.v4 and rules.v6 under /etc/iptables.
These are just a few simple commands you can use with iptables, which is capable of much more. Read on to check on some of the other options available for more advanced control over iptable rules.
Advanced rule setup
As per basic firewall behaviour, the rules are read in the order they are listed on each chain, which means you’ll need to put the rules in the correct order. Appending new rules adds them to the end of the list. You can add new rules to a specific list position by inserting them using the iptables -I <index> -command, where the <index> is the order number you wish to insert the rule. To know which index number to enter, use the following command.
sudo iptables -L --line-numbers
Chain INPUT (policy DROP) num target prot opt source destination 1 ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED 2 ACCEPT tcp -- anywhere anywhere , dpt:ssh 3 ACCEPT tcp -- anywhere anywhere tcp dpt:http
The number at the beginning of each rule line indicates the position in the chain. To insert a new rule above a specific existing rule, simply use the index number of that existing rule. For example to insert a new rule to the top of the chain, use the following command with index number 1.
sudo iptables -I INPUT 1 -p tcp --dport 80 -j ACCEPT
If you wish to remove an existing rule from a certain chain, use the delete command with the parameter -D. The easiest way to select the rule for deletion is to use the index numbers explained above. For example to delete the second rule on the input chain, use this command.
sudo iptables -D INPUT 2
It’s also possible to flush all rules of a specific chain or even the whole iptables using the -F -parameter. This is useful if you suspect iptables is interfering with your attempted network traffic, or you simply wish to start configuring again from a clean table.
Warning: Make sure you set the default rule to ACCEPT before flushing any chain.
sudo iptables -P INPUT ACCEPT
Afterwards, you can go ahead with clearing other rules. Remember to save the rules to a file before flushing the table in case you want to restore the configuration later.
# Clear input chain sudo iptables -F INPUT # Flush the whole iptables sudo iptables -F
With the iptable flushed, your server could be vulnerable to attacks. Make sure to secure your system with an alternative method while disabling iptables even temporarily.
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