Linux netfilter Hacking HOWTO: Information for Programmers
Logo. netfilter. firewalling, NAT, and packet mangling for linux iptables → libmnl Relation. File in iptables, Includes file in libmnl. Netfilter, Linux's in-kernel "packet mangling" infrastructure, and is a new packet (with no relationship to any packet that came before or after). You should install the conntrack command usually packaged as conntrack or conntrack-tools, from acryingshame.info
Again, these tables have chains attached to them.
You can add rules to them match specific packets — such as TCP packets going to port 80 — and associate it with a target. A target decides the fate of a packet, such as allowing or rejecting it. When a packet arrives or leaves, depending on the chainiptables matches it against rules in these chains one-by-one.
When it finds a match, it jumps onto the target and performs the action associated with it. The default policy is also a target. By default, all chains have a default policy of allowing packets.
On a modern Linux distributions, there are four tables: This is the default and perhaps the most widely used table. It is used to make decisions about whether a packet should be allowed to reach its destination.
This table allows you to alter packet headers in various ways, such as changing TTL values. This table allows you to route packets to different hosts on NAT Network Address Translation networks by changing the source and destination addresses of packets. For example, a packet could be part of a new connection, or it could be part of an existing connection. The raw table allows you to work with packets before the kernel starts tracking its state.
In addition, you can also exempt certain packets from the state-tracking machinery. In addition, some kernels also have a security table. Chains Now, each of these tables are composed of a few default chains. These chains allow you to filter packets at various points. The list of chains iptables provides are: Rules in this chain apply to packets as they just arrive on the network interface.
This chain is present in the nat, mangle and raw tables. This chain is present in the mangle and filter tables. This chain is present in the raw, mangle, nat and filter tables. The rules here apply to any packets that are routed through the current host.
This chain is only present in the mangle and filter tables. 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.
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.
Netfilter Connmark » To Linux and beyond !
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.iptables connection tracking
Connections tracked by this module will be in one of the following states: Kernel modules that wish to register at these hooks must provide a priority number to help determine the order in which they will be called when the hook is triggered.
This provides the means for multiple modules or multiple instances of the same module to be connected to each of the hooks with deterministic ordering. Each module will be called in turn and will return a decision to the netfilter framework after processing that indicates what should be done with the packet.
These tables classify rules according to the type of decisions they are used to make. For instance, if a rule deals with network address translation, it will be put into the nat table.
If the rule is used to decide whether to allow the packet to continue to its destination, it would probably be added to the filter table. Within each iptables table, rules are further organized within separate "chains". While tables are defined by the general aim of the rules they hold, the built-in chains represent the netfilter hooks which trigger them. Chains basically determine when rules will be evaluated.
- Netfilter Hooks
- What Are IPTables and Netfilter?
- What is netfilter, iptables, their differences?
As you can see, the names of the built-in chains mirror the names of the netfilter hooks they are associated with: Chains allow the administrator to control where in a packet's delivery path a rule will be evaluated. Since each table has multiple chains, a table's influence can be exerted at multiple points in processing. Because certain types of decisions only make sense at certain points in the network stack, every table will not have a chain registered with each kernel hook.
There are only five netfilter kernel hooks, so chains from multiple tables are registered at each of the hooks.
netfilter: iptables -> libmnl Relation
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.
Linux: What's Netfilter, iptables, Their Differences?
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.