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List:       freebsd-net
Subject:    Re: why multi-hop icmp redirects to 0.0.0.0 on 13.0 ?
From:       "Rodney W. Grimes" <freebsd-rwg () gndrsh ! dnsmgr ! net>
Date:       2021-12-06 18:00:19
Message-ID: 202112061800.1B6I0JuR065662 () gndrsh ! dnsmgr ! net
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> On Sun, 5 Dec 2021, Lutz Donnerhacke wrote:
> 
> > On Sun, Dec 05, 2021 at 08:20:08PM +0200, John Hay wrote:
> >> Something I have observed is that if you use FreeBSD 13 as a router with 2
> >> subnets on the same interface, it will generate redirects when hosts send
> >> packets to the other subnet via the FreeBSD router. I think it is wrong.
> >
> > No, it's correct.
> >
> >> The host does not have a more direct way to get to the other subnet.
> >
> > The other host can arp for an address in a non-connected network on the
> > interface because it's the same L2 domain. Hence the ICMP redirect is send
> > out to provide the shortcut (skipping the router).
> 
> That has always be a very Linux-y approach;  FreeBSD should not ARP
> for any subnet it is not connected to (at least it didn't use to).
> 
> I think you could add a host route in the past and then it would but
> with the current IPv4 I couldn't even say from quickly looking what it
> would do.

route add foo -direct

> 
> 
> >> RFC792
> >> on page 13 does not talk about interfaces, but networks, "If G2 and the
> >> host identified by the internet source address of the datagram are on the
> >> same network...".
> >
> > "network" == "layer 2 domain".
> 
> No, no in this context;  it talks about about the "internet source
> address of a datagram" and hence network == Layer 3 as that is where
> internet addresses belong.   No one would phrase it anymore like this
> these days but in those days ...

Concur, in RFC's "network" almost always refers to a layer 2 domain,
the word "link" is use refers to a layer 2 domain.

> Bjoern A. Zeeb                                                     r15:7

-- 
Rod Grimes                                                 rgrimes@freebsd.org

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