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

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?



My host is CentOS8 x86_64.


Regards,
C. L. Martinez

In article <004E8170-E842-4E8B-9623-DB3EA236DE9F at outlook.com>,
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?
Just add a line to the tcpdump script to wait for the interface.

Something like this:

until ifconfig -s | grep -q '^bond0' ; do sleep 1 ; done

Cheers
Tony
-- 
Tony Mountifield
Work: tony at softins.co.uk - http://www.softins.co.uk
Play: tony at mountifield.org - http://tony.mountifield.org

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)


> My host is CentOS8 x86_64.
>
>
> Regards,
> C. L. Martinez
>
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> https://lists.centos.org/mailman/listinfo/centos
>

-- 
Stephen J Smoogen.

Perfect!! Many thanks Stephen. Works like a charm.

?On 23/09/2020, 15:08, "CentOS on behalf of Stephen John Smoogen"
<centos-bounces at centos.org on behalf of smooge at gmail.com> 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)



    > My host is CentOS8 x86_64.
    >
    >
    > Regards,
    > C. L. Martinez
    >
    > _______________________________________________
    > CentOS mailing list
    > CentOS at centos.org
    > https://lists.centos.org/mailman/listinfo/centos
    >


    -- 
    Stephen J Smoogen.
    _______________________________________________
    CentOS mailing list
    CentOS at centos.org
    https://lists.centos.org/mailman/listinfo/centos

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/