Mikrotik automatically switch to the backup channel for a dynamic ip address (issued by DHCP)

Greetings, Habr! Due to the poor quality of the line, I was asked to configure automatic switching to the backup channel. For this purpose, provided the router MikroTik RB 951Ui.

I thought that there would be no problems ... Just set up checking the channel and routes. But, unfortunately, both providers give out IP dynamically. At first I tried to substitute the interface name into the route, but the ping did not pass. After reading several articles, including foreign sites, but did not find a solution to the problem that would suit me. I had to get acquainted with RouterOS closer, and in particular with the creation of scripts ...
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In this OS, you can create a route in two ways:

• manually (via the graphic interface or via the terminal);
• automatically by the DHCP client.

When creating a route manually, it will not be possible to update the gateway dynamically. To do this, you have to write some fairly large scripts and add a lot of checks. When writing them, I noticed that in RouterOS it is problematic to make complex scripts work. It is very difficult to track the logic of work, although logs and variables were used for verification. The scripts were written, but they were unstable, despite the optimization of the code and the addition of checks. When the number of checks began to grow exponentially and the repeated optimization of the logical scheme slightly improved the situation - I decided to abandon this option and try to use the script option in the script to automatically create a route by the DHCP client.

/ip dhcp-client set [/ip dhcp-client find interface=$Iface] add-default-route=yes [no] 

The option to automatically create a default route is set for the desired interface in the settings of the DHCP client.

So, the script will work according to this algorithm:

Algorithm explanation

A copy of the script will be launched for each interface. Each copy will autonomously create a default route. The priority of the route will depend on the distance. That is, when the connection “falls” on the route from Distance 10, the switching to 11. will occur. Due to this, the switching is seamless.

To start, the script pings the selected host on the Internet along the default route. If the ping is less than ($ PingCount- $ Margin) (Margin - sets the error for accuracy control), then we ping the test route to check whether the connection is “live”. In case of a negative result, check the route and the presence of problems with the settings:

• we overload the interface every $ TimeToWait times (reducing the load on the processor);
• waiting for the interface to load;
• check if there are DHCP client settings for this interface, otherwise we create;
• check the DHCP status of the client (sometimes RouterOS can “slip the pig”);
• waiting for receiving DHCP lease;
• add the required interface to the $ CurrentGateway value;
• check if there is a test route;
• check if the gateway is correct in the test route.

The reaction rate to the state of the compound can be individually adjusted using the following variables:

• PingCount - the number of requests sent by icmp (you can also add another variable to determine the number of sent requests via the test route and to the provider's gateway, that is, the number of requests will decrease and the script speed will increase accordingly);
• Margin - the coefficient is needed to set the error. For example, when $ Margin = 1, the route check cycle is started only when more than one packet disappears, which is important in my situation;
• TimeToWait while waiting for a connection, the interface is overloaded every $ TimeToWait times (this is necessary in order to relieve the load on the processor)

Preparatory setting

I will not describe the default settings of the router for two reasons: firstly, this topic has been raised on the Internet more than once, including Habré, and secondly, the networks are distinguished by their configuration. Since the work of the script affects only the default routes and DHCP client settings, I think you will have no difficulty in adapting the script to your network.

For the script does not need to create routes by default - it will create them automatically. The only thing you need to choose the appropriate distance for test routes (you can both with $ Distance = 1) and $ DistanceDefault 10 and 11 for default routes (one for each provider). Also, do not need to create dhcp clients.

When configuring the router, I used SSH and Winbox (a specialized program for configuring devices managed by RouterOS, even works in * nix using Wine).

Let's get started

In Interfaces, we change the names of two interfaces to match the value of the $ Iface variable in the script (I have isp1, isp2):



Change DNS addresses to google-cue:



Create a script: System → Scripts → Add and paste the code below:





Repeat the previous step for the second interface, only changing the value of the $ Iface variable to “isp2”, also changing the $ DistanceDefault to the above values ​​(for me isp1 - 10, and for isp2 - 11).



Now you need to configure the scheduler to automatically run scripts when loading the router.
System → Scheduler->



This can also be done using ssh or from the console if you have problems with the Cyrillic alphabet in the date:

 /system scheduler add name=CheckTestRoute1 start-time=startup on-event=CheckTestRoute1 

We are overloading ...
I thought that there would be problems with NAT when switching routes, but for now
normal flight. If you have to write ...

That's all. I hope that this article will be useful for someone.

PS: Finally, RouterOS threw another puzzle ...



As you can see, despite the fact that the route is specified correctly, the ping does not pass.

To fix this, added another check (it has already been added above in the script code).