[PATCH] babel: Drop check for IF_MULTICAST interface flag

[Toke Høiland-Jørgensen]

Ondrej Zajicek <santiago at crfreenet.org> writes:

> On Mon, Apr 19, 2021 at 03:55:18PM +0200, Toke Høiland-Jørgensen wrote:
>> Ondrej Zajicek <santiago at crfreenet.org> writes:
>> 
>> > Is there a reason why to disregard the IF_MULTICAST flag? This seems to me
>> > more like a bug in FreeBSD Wireguard implementation that should be fixed
>> > there. Is this flag properly checked on Linux, or is there some reason why
>> > the flag is missing?
>> 
>> We did fix Wireguard - see:
>> https://git.zx2c4.com/wireguard-freebsd/patch/?id=a7a84a17faf784857f076e37aa4818f6b6c12a95
>> 
>> However, that didn't help, Babel still refused to use the interface.
>> Looking at krt-sock.c, the IF_MULTICAST flag is only set on
>> IFF_POINTOPOINT or IFF_BROADCAST on bsd. The Linux code (in netlink.c)
>> has a further:
>> 
>>       if (fl & IFF_MULTICAST)
>> 	f.flags |= IF_MULTICAST;
>> 
>> beneath the other flag checks, so maybe that's really what's missing on
>> the BSD side?
>
> Yes, it is likely that it is an issue in sysdep/bsd code.

Alright, I'll send a patch for that then :)

>> > Routing protocols in BIRD generally follow this flag (and perhaps use
>> > it to switch to unicast-only mode), so i do not see why Babel should
>> > behave differently.
>> 
>> Yeah, I do believe I originally copied that check from one of the other
>> protocols. I can see how it makes sense to check the flag and change
>> operation mode based on it, but given that Babel doesn't do that it just
>> seems kinda redundant? If the interface *actually* is unable to send
>> multicast packets, the subsequent socket operation is going to fail, and
>> at least that produces an error message instead of just silently
>> ignoring the interface like that flag check does :)
>
> Well, i am OK with generating a warning in cases of non-matching interface
> type, instead of ignoring it silently. (In contrast to iface down or missing
> lladdr, which should be silent, as it may correct later.)

OK, fine with me; I'll send an updated patch that adds a warning instead
of dropping the check...

-Toke