mwan3rtmon rewrite in ucode (Re: ucode for scripts)

[Jonas Lochmann]

Am Sat, Nov 08, 2025 at 07:22:29PM -0500, schrieb Etienne Champetier:
> Le sam. 8 nov. 2025 à 16:52, Jonas Lochmann <openwrt at jonaslochmann.de> a écrit :
> >
> > Am Sat, Nov 08, 2025 at 01:11:17PM -0500, schrieb Etienne Champetier:
> > > I also had a look at rewriting mwan3rtmon in ucode recently and I'm
> > > wondering if we could just get rid of it.
> > > Basically this script watches the main routing table to copy all non
> > > default routes, and also fill an ipset,
> >
> > It does this per interface and keeps the default rules of that
> > particular interface while also removing source based routing in case
> > of IPv6. (Well, this can be disabled, but I would not recommend that.)
> 
> Thanks for pointing that out, I missed that sed line
> 
> > > but I think using rules like 'lookup main suppress_prefixlength 0' we
> > > should be able to remove mwan3rtmon.
> >
> > But where do we get the default routes from then?
> 
> We still need something for the default routes indeed, but we can simplify a bit
> Have something like
> ```
> 1000: from all lookup main suppress_prefixlength 0
> ```
> and then in table 1 / 2 ... just have the default route of each wan
> 
> This would also remove the need for the mwan3_connected_ipv4/6 ipsets
> as the mark doesn't matter if you match rule 1000 first.

I think that those ipsets are useless in any case as all local routes are in
all tables and thus you could pick any of them. But having first a "lookup
regular routes ignoring default routes" could simplify a few things.

> > I see no possibility to skip mwan3rtmon.
> 
> If we only have wans with static ips then we don't need mwan3rtmon anymore ...
> so we would still need it :), but if it only handles default routes
> that's way less work to do.

That would remove that theoretical route update delay for local routes.
So I consider it an advantage. I would also go from the current imperative
implementation to a functional one: wait for changes; calculate new interface
specific tables; replace tables.

> Yep, there is very little explanation of why each piece is present
> Example, the ip rule
> ```
> 1001:    from all iif wan1 lookup 1
> ```
> I think it is used if we bind to the ip of wan1 (and not the device)
> to ensure the traffic goes out the right interface, but maybe it's for
> something else :(

That's about the input interface. It prevents routing from one wan to
another. But that's a task for the firewall and not mwan3.

I don't expect binding to an IP only to work correctly. There is a
reason why "mwan3 use" sets the fwmark.

I just found some rough description of the mwan3 architecture at
https://openwrt.org/docs/guide-user/network/wan/multiwan/mwan3