Iptables Firewall - A basic introduction

نویسنده :بهنام نوری
تاریخ:یکشنبه 8 شهریور 1394-12:27 ق.ظ

You probably heard of the term "IPtables Firewall" before. Iptables is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel. This quick tutorial will teach you the basics about building a firewall by using iptables. First you need to know how a firewall handles packets leaving, entering, or passing through your server. Think about chains for each of the mentioned events. Any IP packet entering your server has to go through the INPUT chain. Any packet that your server sends out as a response to the network (Internet) has to go through the OUTPUT chain. The chains represent the logic behind the whole iptables thing.
The way iptables work is by setting up certain rules for the chains. These rules allow the chains to inspect each incoming or outgoing package and then to apply the proper rules. For instance, if your server receives an incoming request for the website "exampledomain.com" the request would first be inspected by the chain for incoming traffic to your server. Let's assume that the request comes from an IP address that should not have access to the website. The IP address is listed in the rules to be denied. The rule recognizes the IP of the requester in the rules and the IPtables firewall blocks the request from going through the firewall to reach the web server part on your server. The requestor would not get the website to see. As an example - you want to block all incoming traffic to your website from (for the matter of this case we use a private IP address).
A very broad IPtables command would be:
iptables -A INPUT -p tcp -j ACCEPT
This rule would accept all tcp traffic. But this is a little too broad isn't it? So, let's work on being much more specific in regards to blocking incoming requests from the IP address specified.
Please be aware that "-s" is used to specify a source IP or DNS name. So, for our example this would mean:
iptables -s
Now that we have specified the source IP address we need to tell the firewall of what to do if a request comes from that IP address. The "-j" option is used to specify what happens to the incoming request from that IP address. The most common three settings are "ACCEPT", "DENY", and "DROP". "Accept" would let traffic from the source IP address pass through the firewall. "DENY" would send a message to the requestor that this server isn't accepting connections. "DROP" just ignores the incoming request and drops it. The requester would not get a response at all. For our example we would either use "DROP" or "DENY" as the preferred option:
iptables -s -j DROP
But we're not done yet. Our server still won't understand what we are trying to accomplish. We still need to specify the "INPUT" or "Output" chain. Since we want to deny access to the website from this specific requestor we would need to apply this setting to the "INPUT" chain.
iptables -A INPUT -s -j DROP
This command would ignore every incoming request from (with some exceptions, but we'll get into that part later on). The order of the specified options doesn't matter. The "-j DROP" could go before "-s". But you should use a consistent approach to avoid confusion down the road.
Sometimes you will need to be more specific when applying IPtables rules. Let's modify the example to block only TELNET requests. We need to specify the protocol (here: TCP) and the port or service (here: TELNET).
iptables -A INPUT -s -p tcp --destination-port telnet -j DROP
If you wanted to block all incoming requests from a whole IP address subnet and not just from a single IP address you need to modify the command as follows:
iptables -A INPUT -s -p tcp --destination-port telnet -j DROP

Here is a list of some additional parameters that can be used when working with IPtables:

-j Specifies the target (--jump)
-i Specifies the input interface (--in-interface)
-o Specifies the output interface (--out-interface)
-p Specifies the protocol (--proto)
-s Specifies the source (--source)
-d Specifies the destination (--destination)
! Specifies an inversion (match addresses NOT equal to)

We now showed you a basic introduction to IPtables and how they work. There is of course much more to it. Pick up a book on Linux security to dive deeper into how IPtables can be used to make your server more secure. As always - if you feel not sure that you can handle these kind of things on your own server - practice, practice, practice. Hire an experienced system administrator if needed.
Disclaimer: The information is provided as is. Please verify that your server platform can handle iptables and that you really understand how this stuff works. It is easy to lock yourself out from your own server when setting up a firewall not knowing how things really work. We cannot be held liable for errors resulting out of work from an unexperienced person doing system administration.

Disclaimer: The information is provided as is. Please verify that your server platform can handle iptables and that you really understand how this stuff works. It is easy to lock yourself out from your own server when setting up a firewall not knowing how things really work. We cannot be held liable for errors resulting out of work from an unexperienced person doing system administration

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