Systemd unit needs to start at boot but wait for network
I have a systemd unit that does some stuff on the internet. Sometimes this gets fired at startup. I want to make sure it's delayed until the computer is connected to the internet.
In my notes I found:
Wants=network-online.target
After=network-online.target nss-lookup.target
Is this the way to do it? Do I need both Wants and After? Do I need anything else?
I've read over the manual for both systemd units and timers many times. It makes my head spin, it's too complicated. The section about depending on the network is even worse, a bunch of lecturing about implementation details that I would expect systemd devs to hide from me, without a clear answer. I'm expecting answers to summarize the pertinent parts :)
1 answer
Yup that's the recommended way to do it.
Wants adds the network-online.target as a soft dependency.[1] Systemd will try to start it if it isn't up already. Networking should work after this target.
After specifies the ordering related to other units. Without this they would likely be started concurrently.
You almost always want the two directives. One for the dependency and one for their ordering. Here's an exception though:
DNS lookups work after nss-lookup.target so you probably don't want your unit to be scheduled before this. The docs tell us not to add it to Wants. This way systemd won't start it when your unit is started, but should they happen to be in the same transaction – say, during the boot – then the ordering is respected.
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