[systemd-devel] Launching a unit in response to a D-Bus signal

[Matthew Booth]

On 01/10/12 23:53, Mirco Tischler wrote:
> 2012/10/2 Kok, Auke-jan H <auke-jan.h.kok at intel.com>:
>> On Mon, Oct 1, 2012 at 4:58 AM, Matthew Booth <mbooth at redhat.com> wrote:
>>> I have a requirement to restart squid whenever the VPN goes up or down[1].
>>> Reading around, it seems that the way to do this would be in response to the
>>> relevant D-Bus signal, which seems to be this one:
>>>
>>> signal sender=:1.6 -> dest=(null destination) serial=269
>>> path=/org/freedesktop/NetworkManager/ActiveConnection/2;
>>> interface=org.freedesktop.NetworkManager.VPN.Connection;
>>> member=VpnStateChanged
>>>
>>> I expected that systemd would allow me to do this, but as far as I can tell
>>> it doesn't (I'm using F17). I can obviously write my own daemon to do this,
>>> but it seems to me that a daemon just for this would be a waste. I think
>>> this sounds like a good fit for systemd. Is it anything anybody's looked at?
>>
>> The problem with listening on a specific DBus message is that it
>> requires you to implement much more of DBus than systemd internally
>> can. I'm not sure it's a good idea to put something that complex into
>> systemd.
>>
>> Your alternatives are to write a simple DBus frontend, or perhaps a
>> Network Manager plugin (if there is such a thing, I know connman has
>> the concept of plugins).
>>
>> You can certainly socket activate a dummy service that doesn't
>> actually listen on DBus but instead executes `systemctl restart
>> squid.service` based on the BusName only. Systemd will likely however
>> not appreciate the unhandled socket, but it may be worth a try.
>>
>> Auke
> Systemd isn't really the right place to do network related stuff, imo.
> Such things are better dealt with in the network connection manager,
> where the information is already available.
> NetworkManager has a mechanism to execute custom scripts in
> /etc/NetworkManager/dispatcher.d on network events. For details take a
> look at the man page, support for explicit actions on vpn-up/down is
> mentioned there.

Thanks. I've submitted a patch to fix squid's existing dispatcher script 
and this is now working for me.

I'll bow to collective experience on whether systemd is the correct 
place for this, but with a sysadmin hat on it just felt like the right 
place to be putting something which performed an action in response to a 
system event.

Matt
-- 
Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team

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