Hi, You can import them to bird, do some tricks to make multipath default route and export them back to the kernel. But it should take precedence over routes intalled by DHCP, so you have to alter them somehow - put them in other table, increase metric, etc. In bird you can for example take routes from one kernel table and put it into another with "merge paths" option. If it is the same kernel table, you can do something with them, to get routes not from the kernel protocol, so that they could be exported back. You can make some local bgp session from bird to itself or you can pipe this defaults to other tables where you will make static recursive default routes and export these recursive routes back to your table, from which they will be exported to the kernel. I only not sure that the trick with recursive routes will work directly, because you will have same prefix in the separate table and there may be conflict between original default route and a static default route. In that case you may need 2 separate tables and 2 recursive routes (and another 2 tables & routes for another default route). First, you export your default to the filrst table, there you make static recursive route for example 0.0.0.1/32 via 0.0.0.0, then you export 0.0.0.1/32 to the second table where you have static recursive route 0.0.0.0/0 via 0.0.0.1 and then you export this default route from the second table back to your original (usually master) table. On Tue, May 3, 2022 at 4:14 PM Johannes Erwerle <jo+bird@swagspace.org> wrote:

Hello.

I am running a setup with 2 uplinks which both have dynamic addresses (via DHCP) and NAT and a Linux router. So I get 2 default routes via DHCP.

I would like to use both uplinks via the linux multipath routing and I am searching for a way to "merge" both default routes that I get into one route with multiple next hops.

Is there a way to accomplish that with bird? (or any other tool?)

All of this is only relevant for IPv4.

Greetings Jo

Hi Johannes, On Tue, May 03, 2022 at 04:07:08PM +0200, Johannes Erwerle wrote:

I would like to use both uplinks via the linux multipath routing and I am searching for a way to "merge" both default routes that I get into one route with multiple next hops.

Is there a way to accomplish that with bird? (or any other tool?)

Instead of letting the dhcp client install the routes and then trying to massage them into shape you could do this directly by configuring the dhcp client appropriately. The isc-dhcp client actually and very good support for scripting in dhclient-*-hooks.d that lets you override what the dhclient-script (which handles the OS specific stuff) does by default. I have been working on a dhcp script myself to allow for VRRP redundancy on a DHCP managed interface. You should be able to use the neutralize_ip_change hack I use to override iproute2 commands called by dhclient-script for your purposes too: neutralize_ip_change () { ip () neutered_ip "$@" } neutered_ip () { : do stuff and call real "ip" command; } This works because dhclient-script (a bash script) sources the hooks so we can mess with the shell environment to override the "ip" command with a shell function that matches on the args and does whatever we want. Script attached since I haven't published it anywhere yet. Note this is only lightly tested so far but I'd be happy to help you make this work for your use-case. --Daniel