Improve this question. Eliah Kagan k 51 51 gold badges silver badges bronze badges. GheorGhe GheorGhe 1 1 gold badge 3 3 silver badges 6 6 bronze badges. Add a comment. Active Oldest Votes. Filter Table : The filter table is mainly used for filtering packets. Improve this answer. Eric Carvalho Eric Carvalho In the picture you referenced, there are mentioned routing decisions in multiple location. I think these decisions are related to routes that may be someone defined via ip route.
May you describe what is the diff between these three and what routes is activated on every stage? Your use of "hosts" seems weird. Which table is used when depends primarily on the local interfaces a package has come in and is intended to exit. I am talking about the paragraph FORWARD : that table is used when a packet coming in through one interface is destined for another interface as opposed to local host — guntbert.
When packets are being transferred from one host of a LAN to another host of a LAN, the packets will go through just a single interface and the chain used will be FORWARD although here the server medium will not act as a router in a strict sense rather can be thought of as a switch. Show 2 more comments. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Below is discussed a general bridging process in RouterOS. Most of the packets will always follow the same processing path, but in certain configurations e.
Please visit the bridging manual for more specific information. Bridge forward is a process that takes place when a packet is forwarded from one bridge port to another, essentially connecting multiple devices on the same network. After receiving a packet on in-interface, the device determines that in-interface is a bridge port, so it gets passed through the bridging process:.
Encapsulation can happen if frame-types is set to admit-all or admit-only-untagged-and-priority-tagged. Bridge input is a process that takes place when a packet is destined to the bridge interface. Most commonly this happens when you need to reach some services that are running on the bridge interface e. The very first steps are similar to the bridge forward process - after receiving a packet on in-interface, the device determines that in-interface is a bridge port, so it gets passed through the bridging process:.
Bridge output is a process that takes place when a packet should exit the device through one or multiple bridge ports. Most commonly this happens when a bridge interface itself tries to reach a device connected to a certain bridge port e.
After a packet is processed on other higher-level RouterOS processes and device finally determines that the output interface is a bridge, the packet gets passed through the bridging process:. In certain network configurations, you might need to enable additional processing on routing chains for bridged traffic, for example, to use simple queues or IP firewall.
This can be done when use-ip-firewall is enabled under the bridge settings. Note that additional processing will consume more CPU resources to handle these packets. All the steps were already discussed in previous points, below is a recap:. On the previous topic, we solely discussed a software bridging that requires the main CPU processing to forward packets through the correct bridge port. This allows us to offload some of the bridging functions, like packet forwarding between bridge ports or packet filtering, to this specialized hardware chip without consuming any CPU resources.
Different MikroTik devices might have different switch chips and each chip has a different set of features available, so make sure to visit this article to get more details - Bridge Hardware Offloading. Interface HTB will not work correctly when the out-interface is hardware offloaded and the bridge Fast Path is not active. The hardware offloading, however, does not restrict a device to only hardware limited features, rather it is possible to take advantage of the hardware and software processing at the same time.
This does require a profound understanding of how packets travel through the switch chip and when exactly they are passed to the main CPU.
We will further discuss a packet flow when bridge hardware offloading is enabled and a packet is forwarded between two switched ports on a single switch chip. This is the most common and also the simplest example:. This process takes place when a packet is received on a physical interface and it is destined to switch-cpu port for further software processing.
There are two paths to the switch-cpu. One where hardware offloading and switching is not even used e. Another path is taken when hardware offloading is active on the in-interface.
This will cause the packet to pass through the switching decision and there are various reasons why switch might forward the packet to the switch-cpu port:. Any received packet that was flooded by the switch chip will not get flooded again by the software bridge to the same HW offloaded switch group. This prevents the formation of duplicate packets.
This process takes place when a packet exits the RouterOS software processing and it is received on the switch-cpu port. Again, there are two paths the packet can take. Another path is taken when hardware offloading is active on the out-interface. This will cause the packet to pass through the switching decision.
Just like any other switch port, the switch will learn the source MAC addresses from packets that are received on the switch-cpu port.
This does come in handy when a bridge contains HW and non-HW offloaded interfaces, so the switch can learn which frames should be forwarded to the CPU. See the packet walkthrough when an out-interface is hardware offloaded:. A software bridge that sends a flooded packet through HW offloaded interfaces, will only send a single packet copy per HW offloaded switch group rather than per HW offloaded interface.
The actual flooding will be done by the switch chip, this prevents the formation of duplicate packets. In case that the recipient is located in a sub- net located at a different network interface, the packet will be forwarded to that interface, provided that the machine is configured to do so. Before we start with our packet manipulations we have to enable the required features.
On the one hand we know how IP packets look like, on the other hand we are ready to manipulate packets under Linux and other Unix derivates. Therefore we are ready for our first application!
The most popular question regarding NAT seems to be the one about sharing an internet connection for computers within a private subnet. For this reason I want to start with this particular scenario. First we should consider the following accurate analogon which is hopefully much easier to understand: Let us assume the following situation: there is a landlord with several subtenants. The postman has no idea about the subtenants und would reject every letter that is directly addressed to one of the subtenants.
The landlord has several pigeon holes that can be used for addressing. The subtenants have the possibility to place their letters in a postbox at the landlord's office who will then take the letters to the post office.
The question now is: How can all the subtenants fully participate at any kind of mail correspondence i. One solution for this given problem is the following: The landlord takes the letters sent by the subtenants, assigns each subtenant a pigeon hole and then replaces the subtenants address which is in some sense invalid since the postman would reject any answers by the landlord's own address including the pigeon hole number.
The receipient of such a letter will then send the reply back to the landlord including the pigeon hole number and then the landlord could easily hand over after he has replaced his address by the subtenant's address so that the subtenant does not recognise this 'cheat' the letter to the matched subtenant.
This solution is optimal in the sense that it is fully transparent for the subtenants, none of them would ever notice the postman not being capable of sending letters directly to the subtenants! NAT just works similar to the subtenant problem mentioned above. Every subtenant family represents an IP address in the local net, every subtenant family member represents a port number, the landlords represents a router and the recipient acts as an arbitrary computer in the internet.
Consequently a socket can be seen as a combination of address and pigeon hole or subtenant family und a member of that family. Let us recapitulate: The process of communication is as follows:. We will presume that the standard gateway is set properly at each client. All that is left is to configure the router. Fortunately the netfilter framework automatically adds to each rule its inverse rule, therefore we only have to set one explicit rule. Usually the decision for one of these two rules is made by taking the one with the lower level of undetermination.
For example, the rule 'Replace the sender's address for all packets from the local subnet' is much easier than 'if a client has sent something to a server, then replace the receipient in the server's response by something'.
As a rule of thumb can be used that the rule that is executed first is the one that is set explicitly in the kernel. All we want to have is the following: packets arriving from the local net with a receipient's IP address somewhere in the internet have to be modified such that the sender's address is equal to the router's address. For further command examples let us assume that the first interface 'eth0' is connected to the local net and that the router is connected to the internet via the second interface 'eth1'.
The command for a shared internet connection then simply is:. A quick overview of all available network interfaces gives. Local computers can access the internet, but there are still some restrictions left. A computer located in the internet is not able to establish a connection to a local computer, all he can do is address a port of the router and hope for the best.
Usually the addressed port is currently not used and hence the packet will be rejected. Even if the port is currently used by a local machine the packet might be forwarded but will then usually be rejected since the computer is already communicating with a different computer. Establishing connections from the internet to one of the local computers is therefore nearly impossible.
For regular services it is possible to statically map ports on the router to sockets in the local net, for example one can configure the router to forward packets arriving at port 80 to a HTTP-server located in the local net. Very often this is needed for playing online games, especially if you want to host games.
Now that we have mastered our first challenge it is time to have a closer look or two at the possibilites of iptables. An iptables-call has the following pattern:. For NAT we always have to choose the nat -table. A command might need further options, for example a pattern and an action to perform in case the pattern matches. This selects the nat -table. There are two other tables, namely mangle und filter , but those are not used for NAT and therefore I mention them for completeness only.
Since the default table is filter we have to select the nat table every time again. The most important commands are the following: further patterns and actions again abbreviated with [ For a full listing of all possible commands I recommend the manual pages of iptables.
To view them, simply type. To manipulate specific packets we have to use appropriate match patterns, therefore there a numerous options to specify them. I will present the most popular ones to clarify their usage. All available match patterns can be found in the manual pages of iptables. For most of the switches there exists a long form, e. Using them makes the whole instruction longer but more readable, especially if you are new to iptables.
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