Re: why multi-hop icmp redirects to on 13.0 ?

From: Bjoern A. Zeeb <>
Date: Sun, 05 Dec 2021 21:51:04 UTC
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.

>> 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 ...


Bjoern A. Zeeb                                                     r15:7