l2tp saves its secrets into /etc/ipsec.d but strongswan would not read
them. l2tp checks for /etc/ipsec.secrets includes /etc/ipsec.d and if
not tries to write into it.
Solution:
Have the strongswan module create /etc/ipsec.d and /etc/ipsec.secrets
when networkmanager_l2tp is installed.
Include /etc/ipsec.secrets in
/nix/store/hash-strongswan/etc/ipsec.secrets so that it can find l2tp
secrets.
Also when the ppp 'nopeerdns' option is used, the DNS resolver tries to
write into an alternate file /etc/ppp/resolv.conf. This fails when
/etc/ppp does not exist so the module creates it by default.
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).
