Re: Is it possible to force packets to go over a loopback cable?

Hi Peter,

On Wed, 10 Dec 2025 at 16:25, Peter Eriksson <[email protected]> wrote:

> I’d like to find some way to force FreeBSD to send a stream of packet over
> a loopback cable connected between two ethernet ports on the same machine,
> but it seems FreeBSD also short-circuits it and handles that traffic
> internally in the OS. Which normally is a good thing for speed, but bad
> when you are trying to test suspect cables :-)
>
> I’ve found references that back in the pre-FreeBSD 10 days there used to
> exist a sysctl:
>
>   net.link.ether.inet.useloopback
>
> that could be set to 0 to disable this internal shortcut and force the
> packets out onto the cable…
>
> Any suggestions? :-)
>
> - Peter
>

You can use a jail + vnet. There are examples in this pdf:
https://freebsdfoundation.org/wp-content/uploads/2020/03/Jail-vnet-by-Examples.pdf

I do something similar. There are basically 3 steps. Create the vnet
capable jail, move the interface inside, configure the address(es).

Here is one half of it:
########
if1="mce0"
if1_jail_name="via_mce0"
if1_ip="10.0.0.1/24"
if1_testip="10.0.0.2"
jail -i -c name=${if1_jail_name} host.hostname=${if1_jail_name} vnet persist
ifconfig ${if1} vnet ${if1_jail_name}
jexec ${if1_jail_name} ifconfig ${if1} ${if1_ip}

# Check by pinging the other side
echo jexec ${if1_jail_name} ping -c 3 ${if1_testip}
jexec ${if1_jail_name} ping -c 3 ${if1_testip}
#########

John