By default, printing is static and displays only static rules. In case of a faulty PMTUD connection, reducing the MSS of packets passing through the VPN connection solves the problem. The following example shows how the MSS value can be reduced via mangle: On the first screen, click ip,firewall,mangle, click the plus sign. The chain must be pre-delivered. We can match based on source or destination IP address, protocol, input or output interfaces, etc. For this example, I`ll use the interface in. If we want to mark HTTP packets, let`s set the protocol to tcp and set the destination port to 80. On the Action tab, select Mark Connection and click Apply and OK. Then we click on the plus sign again, under the Corner tab the string is guided, under Connection marker, select the connection mark you created earlier, mine was called “óur-conn”, click on the Action tab and select Mark Packet. Give it a name and click .apply and ok. You can use these steps to create as many packaging stamps as you want.
These packets are used for later enforcement of the policy on the router. The order of the rules in the Mangle is IMPORTANT! It should look exactly like the screenshot shown. We need to add five rules to the Manble table. The first two rules simply accept all traffic from the proxy box to ports 80 and 443 without marking. Next, we add two more rules that mark traffic from each workstation to ports 80 and 443. Finally, the fifth rule also accepts this marked traffic. This last step is required for the router to redirect the marked traffic to the proxy box. Maim! One of the most feared topics in microtics. People get goosebumps by mentioning this topic.
But honestly, I see no reason for that. However, I can understand fear because I used to be like that. Today I`m going to show you how easy it is to set up a Mangle rule. These mangle rules are powerful tools in the hands of network engineers because they use them (mangle rules) to enforce multiple policies on the router. You can control user traffic and downloads, and prioritize traffic using Minangle rules. Keep in mind that on all workstations in our network, our router with the IP address 10.0.0.1 is set as the default gateway. So when a browser running on a workstation establishes an HTTP (or HTTPS) connection to a web server on the Internet, traffic from that workstation on port 80 (or 443) is actually sent to our router. We need to redirect this traffic to a separate proxy box at 10.0.0.10. In addition, the mangle function is used to modify certain fields of the IP header, such as TOS (DSCP) and TTL fields. Here I will show you how to mark packets with Mangle rules.
These marked packages are used when configuring queues for bandwidth management or when prioritizing packets. Just look at the pictures below and follow the steps. The setup looks pretty straightforward and will likely work well on smaller networks. Now multiply the number of rules by 10, add a few hundred entries in the address list, run 100 Mbit traffic through this router and you will see how fast CPU usage increases. The reason for this behavior is that each rule reads the IP header of each packet and attempts to match the collected data to the settings specified in the firewall rule. After you click OK, the new rule is added to the Mangle table. Please note that the rule was added with the default Accept action. Mangle is a kind of “marker” that marks packaging with special markers for future processing.
Many other installations of RouterOS use these markers, such as queue trees, NAT, routing. You identify a package by its marking and treat it accordingly. Mangle markers exist only within the router, they are not transmitted over the network. For example, if the router receives an IPsec encapsulated Gre packet, the ipsec-policy=in,ipsec rule corresponds to the Gre packet, but the ipsec-policy=in,none rule corresponds to the ESP packet. Finally, we add the fifth rule to accept marked traffic. This rule is required so that the router can redirect the marked traffic to the proxy box. So, click + (Add) and enter the following information in the pop-up dialog, General tab. Now the first rule tries to match the IP header data only from the first packet of the new connection and add a connection tag. The following rule no longer checks the IP header for each packet, but only compares the connection markers, which reduces CPU consumption. In addition, passthrough=no has been added, which further reduces CPU consumption. /ip firewall filter Print statistics display additional read-only properties. It is a well-known fact that VPN connections have a smaller packet size due to encapsulation overhead.
A large packet of MSS that exceeds the MSS of the VPN connection must be fragmented before being sent over this type of connection. However, if the DF flag is set on the package, it cannot be fragmented and must be ignored. For MTU Discovery Broken Path (PMTUD) links, this can lead to a number of issues, including issues with FTP and HTTP data transfer and messaging services. Traffic redirection is a two-step process. First, we mark the packets on port 80 (or 443) and then forward these packets through the proxy box. Open Winbox/IP/Firewall and select the Mangle table as shown in the screenshot below. Please note that the Mangle table is initially empty. Tagging each packet is quite resource-intensive, especially if the rule has to match many parameters of the IP header or address list with hundreds of entries.