Netfilter iptables relationship marketing

A Deep Dive into Iptables and Netfilter Architecture | DigitalOcean

Netfilter and iptables are Linux utilities that improve network security. In this tip, Hardening Linux author James Turnbull explains how they work to make Linux. We will discuss how iptables interacts with netfilter and how the . meaning that packets are evaluated in regards to their relation to previous packets. . We use cookies to provide our services and for analytics and marketing. Marketing is at least as big a reason as any other. plus its TCP or UDP port number tells us nothing about that packet's relationship to other packets. . than it is to configure one running Check Point Firewall-1 or Linux Netfilter/iptables.

The rules in this chain apply to packets as they just leave the network interface. This chain is present in the nat and mangle tables. The diagram below shows the flow of packets through the chains in various tables: But what would you do after matching them? The most commonly used terminating targets are: This causes iptables to accept the packet. On the other hand, there are non-terminating targets, which keep matching other rules even if a match was found. An example of this is the built-in LOG target.

When a matching packet is received, it logs about it in the kernel logs. However, iptables keeps matching it with rest of the rules too. To simplify things, you can create a custom chain.

Then, you can jump to this chain from one of the custom chains. These protocols have some differences and are handled differently in the kernel.

An In-Depth Guide to iptables, the Linux Firewall - Boolean World

Thus, iptables provides different commands for these protocols — iptables for IPv4 and ip6tables for IPv6. You also need to execute all iptables commands as root. You can launch a root shell by typing in su -c and then typing in your root password and then run the commands in this article. Alternatively, you can add sudo in front of every iptables command.

We need to simply block all incoming packets from this IP. You can do so with: As you might have guessed, the -s switch simply sets the source IP that should be blocked. So you can leave it out, which saves you some typing: If you want to block all IPs ranging from If you want to see these rules later, you can use the -L switch.

This list is also from the filter table, and you can list other tables with the -t switch. Often, this is unnecessary and slows down the listing process. To disable this, you can use the -n switch: Removing it is easy: It turns out that you can also insert rules at a given position! This is useful in a number of cases. So, if you run the command: You can verify this by listing the rules: As an example, perhaps you whitelisted the wrong IP, and typed in Since the new rule is on the first line, you can replace it with the correct rule like so: However, you can do a lot more, by using modules and protocol based matching.

Say, you want to block all incoming TCP traffic. You simply need to specify the protocol with -p like so: Let us consider a more useful example this time. You have to first match all TCP traffic, like we did in the example above.

Then, to check the destination port, you should first load the tcp module with -m. Next, you can check if the traffic is intended to the SSH destination port by using --dport. Thus, the entire command would be: Then, you can specify the port numbers with --dports. The final command would be: Say, you want to block ICMP address mask requests type First, you should match ICMP traffic, and then you should match the traffic type by using icmp-type in the icmp module: The packets from your system do reach the server.

However, the packets that the server sends to your system get rejected. See the next section for an additional example. What we really need here is a way to tell iptables to not touch packets that are part of an existing connection. Connections tracked by this module will be in one of the following states: There are only five netfilter kernel hooks, so chains from multiple tables are registered at each of the hooks. We will take a look at the specific order of each chain in a moment.

Which Tables are Available? Let's step back for a moment and take a look at the different tables that iptables provides. These represent distinct sets of rules, organized by area of concern, for evaluating packets.

The Filter Table The filter table is one of the most widely used tables in iptables. The filter table is used to make decisions about whether to let a packet continue to its intended destination or to deny its request. In firewall parlance, this is known as "filtering" packets. This table provides the bulk of functionality that people think of when discussing firewalls. As packets enter the network stack, rules in this table will determine whether and how to modify the packet's source or destination addresses in order to impact the way that the packet and any response traffic are routed.

This is often used to route packets to networks when direct access is not possible. For instance, you can adjust the TTL Time to Live value of a packet, either lengthening or shortening the number of valid network hops the packet can sustain. Other IP headers can be altered in similar ways. This table can also place an internal kernel "mark" on the packet for further processing in other tables and by other networking tools.

This mark does not touch the actual packet, but adds the mark to the kernel's representation of the packet. The Raw Table The iptables firewall is stateful, meaning that packets are evaluated in regards to their relation to previous packets.

The connection tracking features built on top of the netfilter framework allow iptables to view packets as part of an ongoing connection or session instead of as a stream of discrete, unrelated packets. The connection tracking logic is usually applied very soon after the packet hits the network interface. The raw table has a very narrowly defined function.

Its only purpose is to provide a mechanism for marking packets in order to opt-out of connection tracking. These marks can be applied on a per-packet or per-connection basis. The IP protocol is responsible for routing packets from one host to another, as well as packets that we may receive from one host destined for another. Most of the time on single network access host, this is a very simple process.

You have two different options, either the packet is destined for our locally attached network, or possibly through a default gateway.

The last of the responsibilities for the IP protocol is that it must fragment and reassemble any datagram that has previously been fragmented, or that needs to be fragmented to fit in to the packetsize of this specific network hardware topology that we are connected to.

Netfilter - Firewall Iptables

If these packet fragments are sufficiently small, they may cause a horribly annoying headache for firewall administrators as well. The problem is, that once they are fragmented to small enough chunks, we will start having problems to read even the headers of the packet, not to mention the actual data. As of Linux kernel 2. The connection tracking system used by iptables for state matching and NAT'ing etc must be able to read the packet defragmented.

The IP protocol is also a connectionless protocol, which in turn means that IP does not "negotiate" a connection. TCP is an example of this kind of protocol, however, it is implemented on top of the IP protocol. The reason for not being connection-oriented just yet are several, but among others, a handshake is not required at this time yet since there are other protocols that this would add an unnecessarily high overhead to, and that is made up in such a way that if we don't get a reply, we know the packet was lost somewhere in transit anyways, and resend the original request.

As you can see, sending the request and then waiting for a specified amount of time for the reply in this case, is much preferred over first sending one packet to say that we want to open a connection, then receive a packet letting us know it was opened, and finally acknowledge that we know that the whole connection is actually open, and then actually send the request, and after that send another packet to tear the connection down and wait for another reply. IP is also known as an unreliable protocol, or simply put it does not know if a packet was received or not.

It simply receives a packet from the transport layer and does its thing, and then passes it on to the network access layer, and then nothing more to it. It may receive a return packet, which traverses from network access layer to the IP protocol which does it's thing again, and then passes it on upwards to the Transport layer.

However, it doesn't care if it gets a reply packet, or if the packet was received at the other end. Same thing applies for the unreliability of IP as for the connectionless-ness, since unreliability would require adding an extra reply packet to each packet that is sent. For example, let us consider a DNS lookup. As it is, we send a DNS request for servername. If we never receive a reply, we know something went wrong and re-request the lookup, but during normal use we would send out one request, and get one reply back.

Adding reliability to this protocol would mean that the request would require two packets one request, and one confirmation that the packet was received and then two packets for the reply one reply, and one reply to acknowledge the reply was received. In other words, we just doubled the amount of packets needed to send, and almost doubled the amount of data needed to be transmitted. IP headers The IP packet contains several different parts in the header as you have understood from the previous introduction to the IP protocol.

The whole header is meticuluously divided into different parts, and each part of the header is allocated as small of a piece as possible to do it's work, just to give the protocol as little overhead as possible. You will see the exact configuration of the IP headers in the IP headers image.

Understand that the explanations of the different headers are very brief and that we will only discuss the absolute basics of them. For each type of header that we discuss, we will also list the proper RFC's that you should read for further understanding and technical explanations of the protocol in question.

An In-Depth Guide to iptables, the Linux Firewall

As a sidenote to this note, RFC stands for Request For Comments, but these days, they have a totally different meaning to the Internet community. They are what defines and standardises the whole Internet, compared to what they were when the researchers started writing RFC's to each other.

Back then, they were simply requests for comments and a way of asking other researchers about their opinions. As you can see, all of these standards can get a little bit hard to follow at times. One tip for finding the different RFC's that are related to each other is to use the search functions available at RFC-editor. We will discuss these more in detail when we get to the specific headers that are changed by these newer RFC's.

One thing to remember is, that sometimes, an RFC can be obsoleted not used at all. Normally this means that the RFC has been so drastically updated and that it is better to simply replace the whole thing.

It may also become obsolete for other reasons as well. Version - bits This is a version number of the IP protocol in binary. IPv4 iscalledwhile IPv6 is called This field is generally not used for filtering very much.

This field tells us how long the IP header is in 32 bit words. As you can see, we have split the header up in this way 32 bits per line in the image as well. Since the Options field is of optional length, we can never be absolutely sure of how long the whole header is, without this field. The minimum length of this of the header is 5 words. This is one of the most complex areas of the IP header for the simple reason that it has been updated 3 times. It has always had the same basic usage, but the implementation has changed several times.

First the field was called the Type of Service field. Bit [] of the field was called the Precedence field. This is still used in a lot of places with older hardware, and it still causes some problems for the Internet.

Among other things, bit [] are specified to be set to 0. But a lot of old firewalls and routers have built in checks looking if these bits are set to 1, and if the packets do, the packet is discarded. Today, this is clearly a violation of RFC's, but there is not much you can do about it, except to complain. The second iteration of this field was when the field was changed into the DS field as defined in RFC DS stands for Differentiated Services.

The DSCP field is pretty much used the same as in how the ToS field was used before, to mark what kind of service this packet should be treated like if the router in question makes any difference between them. One big change is that a device must ignore the unused bits to be fully RFC compliant, which means we get rid of the previous hassle as explained previously, as long as the device creators follow this RFC.

ECN is used to let the end nodes know about a routers congestion, before it actually starts dropping packets, so that the end nodes will be able to slow down their data transmissions, before the router actually needs to start dropping data.

Netfilter and iptables: Stateful firewalling for Linux

Previously, dropping data was the only way that a router had to tell that it was overloaded, and the end nodes had to do a slow restart for each dropped packet, and then slowly gather up speed again. The final iteration of the whole mess is RFC which gives some new terminology and clarifications to the usage of the DiffServ system.

It doesn't involve too many new updates or changes, except in the terminology. The RFC is also used to clarify some points that were discussed between developers. Total Length - bits 16 - This field tells us how large the packet is in octets, including headers and everything.

The maximum size is octets, or bytes, for a single packet. The minimum packet size is bytes, not caring if the packet arrives in fragments or not. It is only recommended to send larger packets than this limit if it can be guaranteed that the host can receive it, according to RFC However, these days most networks runs at byte packet size.

This includes almost all ethernet connections, and most Internet connections. Identification - bits 32 - This field is used in aiding the reassembly of fragmented packets. Flags - bits 47 - This field contains a few miscellaneous flags pertaining to fragmentation. The first bit is reserved, but still not used, and must be set to 0. The second bit is set to 0 if the packet may be fragmented, and to 1 if it may not be fragmented.

The third and last bit can be set to 0 if this was the last fragment, and 1 if there are more fragments of this same packet. Fragment Offset - bits 50 - The fragment offset field shows where in the datagram that this packet belongs. The fragments are calculated in 64 bits, and the first fragment has offset zero. Time to live - bits 64 - The TTL field tells us how long the packet may live, or rather how many "hops" it may take over the Internet.

Every process that touches the packet must remove one point from the TTL field, and if the TTL reaches zero, the whole packet must be destroyed and discarded. This is basically used as a safety trigger so that a packet may not end up in an uncontrollable loop between one or several hosts. Protocol - bits 73 - In this field the protocol of the next level layer is indicated. All of these numbers are defined by the Internet Assigned Numbers Authority. All numbers can befound on their homepage Internet Assigned Numbers Authority.

Header checksum - bits 81 - This is a checksum of the IP header of the packet. This field is recomputed at every host that changes the header, which means pretty much every host that the packet traverses over, since they most often change the packets TTL field or some other. Source address - bits 97 - This is the source address field. It is generally written in 4 octets, translated from binary to decimal numbers with dots in between. That is for example, The field lets the receiver know where the packet came from.

Destination address - bits - The destination address field contains the destination address, and what a surprise, it is formatted the same way as the source address.

The options field is not optional, as it may sound. Actually, this is one of the more complex fields in the IP header. The options field contains different optional settings within the header, such as Internet timestamps, SACK or record route route options. Since these options are all optional, the Options field can have different lengths, and hence the whole IP header. However, since we always calculate the IP header in 32 bit words, we must always end the header on an even number, that is the multiple of The field may contain zero or more options.

The options field starts with a brief 8 bit field that lets us know which options are used in the packet. For more information about the different options, read the proper RFC's. Padding - bits variable. This is a padding field that is used to make the header end at an even 32 bit boundary. The field must always be set to zeroes straight through to the end. It is a stateful protocol and has built-in functions to see that the data was received properly by the other end host.

The main goals of the TCP protocol is to see that data is reliably received and sent, that the data is transported between the Internet layer and Application layer correctly, and that the packet data reaches the proper program in the application layer, and that the data reaches the program in the right order.

All of this is possible through the TCP headers of the packet. The TCP protocol looks at data as an continuous data stream with a start and a stop signal.