CentOS - Sep 2020 - Systemd service unit file needs to wait until a specific interface is up

On 9/23/20 7:07 AM, Stephen John Smoogen wrote:
> On Wed, 23 Sep 2020 at 04:33, Carlos Lopez <clopmz at outlook.com>
wrote:
> 
>> Hi all,
>>
>>
>> With SystemD, how can I make certain service dependent on certain
network
>> interfaces being up?
>>
>> For example, I have an 802.1ad bond interface I need to wait on for
being
>> up (this interface has no ip address assigned, it is used to capture
>> networks packets with a tcpdump?s script). Every time this service
fails
>> because bond interface is not up.
>>
>>
>>
>> I have configured the service as:
>>
>>
>> [Unit]
>>
>> Description=tcpdump capture script
>>
>> After=network.target
>>
>> Wants=network-online.target
>>
>>
>>
>> But it doesn?t work ?. Any tip or trick?
>>
>>
>>
> 
> So the network just calls the scripts and exits so they can take a while to
> get working. I think this website covers what you want to do
> 
>
https://unix.stackexchange.com/questions/257888/systemd-wait-for-network-interface-to-be-up-before-running-service
> 
> systemctl list-units --no-pager | grep subsystem-net
> 
> Then look for the device which matches the one you are listening to. Change
> the After=network.target to
> 
> 
> BindsTo=sys-devices-virtual-net-<device>.device
> After=sys-devices-virtual-net-<device>.device
> 
> where <device> is the interface you found (aka eth2, br9, bond0 etc)
Hmm, there seems to be several layers here.

I think sys-devices-<device>.device is "started" when
<device> appears
in the kernel:

Sep 23 19:37:25 kernel: virtio_net virtio0 ens3: renamed from eth0

# systemctl status sys-subsystem-net-devices-ens3.device
? sys-subsystem-net-devices-ens3.device - Virtio network device
      Loaded: loaded
      Active: active (plugged) since Wed 2020-09-23 19:37:25 MDT

This is not what most people would consider "up" - i.e. have an IP 
address.  ens3 doesn't get it's IP address until much later.

This works for Carlos though because he doesn't need an IP address - 
just the device existing.

I have no idea how it worked for the stackexchange poster.  Apparently 
because "lan0" is a virtual device as well 
("sys-devices-virtual-net-lan0") that they need, not a more
"physical"
device like "net-devices-ens3", and it gets an IP address at the same 
time as creation.

I've been dealing with issues like this for a while - systems with 
multiple interfaces, some of which do not come up for quite a while, and 
I need to wait for all to be up before running certain tasks.  Still 
haven't found anything very satisfactory.

-- 
Orion Poplawski
Manager of NWRA Technical Systems          720-772-5637
NWRA, Boulder/CoRA Office             FAX: 303-415-9702
3380 Mitchell Lane                       orion at nwra.com
Boulder, CO 80301                 https://www.nwra.com/

On Wed, 23 Sep 2020 at 23:39, Orion Poplawski <orion at nwra.com> wrote:
> On 9/23/20 7:07 AM, Stephen John Smoogen wrote:
> > On Wed, 23 Sep 2020 at 04:33, Carlos Lopez <clopmz at
outlook.com> wrote:
> >
> >> Hi all,
> >>
> >>
> >> With SystemD, how can I make certain service dependent on certain
> network
> >> interfaces being up?
> >>
> >> For example, I have an 802.1ad bond interface I need to wait on
for
> being
> >> up (this interface has no ip address assigned, it is used to
capture
> >> networks packets with a tcpdump?s script). Every time this service
fails
> >> because bond interface is not up.
> >>
> >>
> >>
> >> I have configured the service as:
> >>
> >>
> >> [Unit]
> >>
> >> Description=tcpdump capture script
> >>
> >> After=network.target
> >>
> >> Wants=network-online.target
> >>
> >>
> >>
> >> But it doesn?t work ?. Any tip or trick?
> >>
> >>
> >>
> >
> > So the network just calls the scripts and exits so they can take a
while
> to
> > get working. I think this website covers what you want to do
> >
> >
>
https://unix.stackexchange.com/questions/257888/systemd-wait-for-network-interface-to-be-up-before-running-service
> >
> > systemctl list-units --no-pager | grep subsystem-net
> >
> > Then look for the device which matches the one you are listening to.
> Change
> > the After=network.target to
> >
> >
> > BindsTo=sys-devices-virtual-net-<device>.device
> > After=sys-devices-virtual-net-<device>.device
> >
> > where <device> is the interface you found (aka eth2, br9, bond0
etc)
>
> Hmm, there seems to be several layers here.
>
> I think sys-devices-<device>.device is "started" when
<device> appears
> in the kernel:
>
> Sep 23 19:37:25 kernel: virtio_net virtio0 ens3: renamed from eth0
>
> # systemctl status sys-subsystem-net-devices-ens3.device
> ? sys-subsystem-net-devices-ens3.device - Virtio network device
>       Loaded: loaded
>       Active: active (plugged) since Wed 2020-09-23 19:37:25 MDT
>
> This is not what most people would consider "up" - i.e. have an
IP
> address.  ens3 doesn't get it's IP address until much later.
>
>
Oh good point. I didn't think about that.

> This works for Carlos though because he doesn't need an IP address -
> just the device existing.
>
>
It may or may not.. I just realized that the device may need to get
recognized by a switch etc which on a bond may take time.

> I have no idea how it worked for the stackexchange poster.  Apparently
> because "lan0" is a virtual device as well
> ("sys-devices-virtual-net-lan0") that they need, not a more
"physical"
> device like "net-devices-ens3", and it gets an IP address at the
same
> time as creation.
>
> I've been dealing with issues like this for a while - systems with
> multiple interfaces, some of which do not come up for quite a while, and
> I need to wait for all to be up before running certain tasks.  Still
> haven't found anything very satisfactory.
>
>
All good points. Thank you for that douse of reality.


> --
> Orion Poplawski
> Manager of NWRA Technical Systems          720-772-5637
> NWRA, Boulder/CoRA Office             FAX: 303-415-9702
> 3380 Mitchell Lane                       orion at nwra.com
> Boulder, CO 80301                 https://www.nwra.com/
>
>
-- 
Stephen J Smoogen.

> On Wed, 23 Sep 2020 at 23:39, Orion Poplawski <orion at nwra.com>
wrote:
>
>> On 9/23/20 7:07 AM, Stephen John Smoogen wrote:
>> > On Wed, 23 Sep 2020 at 04:33, Carlos Lopez <clopmz at
outlook.com> wrote:
>> >
>> >> Hi all,
>> >>
>> >>
>> >> With SystemD, how can I make certain service dependent on
certain
>> network
>> >> interfaces being up?
>> >>
>> >> For example, I have an 802.1ad bond interface I need to wait
on for
>> being
>> >> up (this interface has no ip address assigned, it is used to
capture
>> >> networks packets with a tcpdump?s script). Every time this
service
>> fails
>> >> because bond interface is not up.
>> >>
>> >>
>> >>
>> >> I have configured the service as:
>> >>
>> >>
>> >> [Unit]
>> >>
>> >> Description=tcpdump capture script
>> >>
>> >> After=network.target
>> >>
>> >> Wants=network-online.target
>> >>
>> >>
>> >>
>> >> But it doesn?t work ?. Any tip or trick?
>> >>
>> >>
>> >>
>> >
>> > So the network just calls the scripts and exits so they can take a
>> while
>> to
>> > get working. I think this website covers what you want to do
>> >
>> >
>>
https://unix.stackexchange.com/questions/257888/systemd-wait-for-network-interface-to-be-up-before-running-service
>> >
>> > systemctl list-units --no-pager | grep subsystem-net
>> >
>> > Then look for the device which matches the one you are listening
to.
>> Change
>> > the After=network.target to
>> >
>> >
>> > BindsTo=sys-devices-virtual-net-<device>.device
>> > After=sys-devices-virtual-net-<device>.device
>> >
>> > where <device> is the interface you found (aka eth2, br9,
bond0 etc)
>>
>> Hmm, there seems to be several layers here.
>>
>> I think sys-devices-<device>.device is "started" when
<device> appears
>> in the kernel:
>>
>> Sep 23 19:37:25 kernel: virtio_net virtio0 ens3: renamed from eth0
>>
>> # systemctl status sys-subsystem-net-devices-ens3.device
>> ? sys-subsystem-net-devices-ens3.device - Virtio network device
>>       Loaded: loaded
>>       Active: active (plugged) since Wed 2020-09-23 19:37:25 MDT
>>
>> This is not what most people would consider "up" - i.e. have
an IP
>> address.  ens3 doesn't get it's IP address until much later.
>>
>>
>
> Oh good point. I didn't think about that.
>
>
>> This works for Carlos though because he doesn't need an IP address
-
>> just the device existing.
>>
>>
> It may or may not.. I just realized that the device may need to get
> recognized by a switch etc which on a bond may take time.
>
>
>> I have no idea how it worked for the stackexchange poster.  Apparently
>> because "lan0" is a virtual device as well
>> ("sys-devices-virtual-net-lan0") that they need, not a more
"physical"
>> device like "net-devices-ens3", and it gets an IP address at
the same
>> time as creation.
>>
>> I've been dealing with issues like this for a while - systems with
>> multiple interfaces, some of which do not come up for quite a while,
and
>> I need to wait for all to be up before running certain tasks.  Still
>> haven't found anything very satisfactory.
>>
>>
> All good points. Thank you for that douse of reality.
I'm just wondering, is there a chance to make this work reliable without
modifying the source code of systemd?

Simon