Re: "Downing" interfaces on reboot cmd or a panic

On Tue, 25 Aug 2026 17:29:16 -0400
Karl Denninger <karl@denninger.net> wrote:

> None of the above do.
> 
> "shutdown -r" does, as it calls back through the "stop" chain in netif.  
> But a reboot command (or kernel panic) does not; the latter obviously 
> can't and the former doesn't go through any of those nicities at all.
> 
> I've never seen this cause trouble on a LAN but when the machine in 
> question is an Internet gateway in some cases ONTs in particular want to 
> see the interface carrier disappear specifically to tell it that the 
> line was reset.  If it doesn't then on machine restart it may not remain 
> properly associated with the ONT and thus you get no -- or partial -- 
> connectivity.
> 
> On a cold boot this isn't an issue because the interface was never up in 
> the first place, but on restarts it can be.  I've got an ISP here that 
> sometimes will not restore connectivity properly in this instance, but 
> on a shutdown (or power cycle) it never has a problem -- and it appears, 
> from watching the interface LEDs, that the lack of carrier-toggling is 
> responsible.  While an "attended" restart is easy ("don't use reboot") 
> what's not-so-much is if the machine takes a panic while nobody is there 
> to attend it and connectivity does not come back up.
> 
> I'm curious if anyone has thought about how this might be better handled 
> (e.g. is this something that perhaps the startup sequence should have a 
> flag for and, if its present, then all interfaces are "down"ed first as 
> a protective measure to force carrier off before being configured?)

Don't know if it's in any way related, but my home alc0 toggles UP/DOWN
in dmesg at least twice during every boot (didn't check what happens
with carrier). So may be for some drivers it's already here :)


-- 
WBR, @nuclight