The systemd service file shipped with strongswan has strongswan started after `network-online`. It turns out that this is for good reason: failure to connect on boot otherwise.
See this thread on the mailing list, which my colleague initiated after finding that our NixOS strongswan config wouldn't connect on boot:
https://lists.strongswan.org/pipermail/users/2017-January/010359.html
Tested on a local config (which has the strongswan service config overridden).
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