how to keep eth link down across reboots ?

[lejeczek]

On 14/12/2023 12:17, Íñigo Huguet wrote:
> On Thu, Dec 14, 2023 at 12:01 PM lejeczek <peljasz at yahoo.co.uk> wrote:
>> Thanks a lot! That seems more practical.
>> There is one aspect I did not think of at first - will NM
>> this way(or any other) be able to detect physical link/cable
>> removal (naturally while device+profile/iface is up/on) ?
>> BTW, what would a good way to watch such link (for such
>> cable removal) outside (but perhaps with NM help) of NM?
>>
>> many thanks, L.
>>
> NM can show you de carrier state of any device, even unmanaged
> devices, I think: nmcli -g WIRED-PROPERTIES.CARRIER device show eth0
>
> If you have a profile on the interface and autoconnect=yes, it will
> react to carrier up/down events, activating or deactivating that
> profile (this won't happen if you configure ignore-carrier in the
> .conf files, but for ethernet, by default carrier events are
> considered).
>
> Also, with some python scripting you could listen to the
> notify::carrier signal that is sent via DBUS. Ask here for an example
> if you intend to take this way.
>
Thanks, before I drive further that route I have a "finding" 
I'd like to ask about.

I have an ISP box which connects to more then one PC - thus 
this all "tampering" here.
All PCs are physically linked up to that OSP box and I was 
hoping this:

[Unit]
Description=Power off/on gateway / GATEWAY physical link

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStartPre=/sbin/ip link set up dev gateway
ExecStart=/usr/bin/bash -c "/bin/nmcli d set gateway managed 
yes && /bin/nmcli c u GATEWAY"
ExecStop=/usr/bin/bash -c "/bin/nmcli c d GATEWAY && 
/bin/nmcli d set gateway managed no"
ExecStopPost=/sbin/ip link set down dev gateway
SuccessExitStatus=0
#RestartPreventExitStatus=18
Restart=on-failure
TimeoutSec=300
RestartSec=9s

[Install]
WantedBy=multi-user.target

will take care of the the physical iface port, which.. it 
does, so it seems, but...
With that when the service is stopped/started between PCs so 
only one PC has the service up at a given time, obviously, 
then...
other PCs (which previously were the gateway) cannot NAT out 
nor in through that (new) gateway-PC

Before I go to play with with everything I'm wondering - 
does a PC which was the gateway & downed the iface link... 
does something not happening there?
Looks like a routing to me with "Destination Host 
Unreachable" but, flushing cache table does not help.
An example of/on a ex-gateway:
-> $ ip ro
default via 10.1.1.254 dev nm-bridge1011 proto static metric 
111
...
-> $ ip ro get 212.77.98.9
212.77.98.9 dev nm-bridge1011 src 10.1.1.101 uid 0
     cache
... ping it
 From swir.direct (10.1.1.100) icmp_seq=72 Destination Host 
Unreachable
...
-> $ ip ro add 212.77.98.9 via 10.1.1.254
...
64 bytes from 212.77.98.9 (212.77.98.9): icmp_seq=2 ttl=59 
time=15.6 ms
but sluggish

That is pretty new & weir to me - is that "service" of mine 
&| NM not doing it all the right way?