SystemD service: ensure service starts after DNS queries (e.g. ipv4only.arpa) can be resolved.

[jivanpal]

Following on from #42 (comment):

You may like to add the following Before and Wants lines to the default SystemD unit file:

[Unit]
Description=464XLAT CLAT daemon
Documentation=man:clatd(8)
After=network-online.target
Before=nss-lookup.target
Wants=network-online.target nss-lookup.target
StartLimitIntervalSec=0

This should ensure that clatd only starts if a DNS server is reachable; it has been working reliably for me. Otherwise, if clatd is enabled to start on boot, then if/when it tries to determine the PLAT prefix by querying ipv4only.arpa, the following is likely to happen:

20:08:57 clatd-test clatd[485]: Performing DNS64-based PLAT prefix discovery (cf. RFC 7050)
20:10:12 clatd-test clatd[485]: No PLAT prefix could be discovered. Your ISP probably doesn't provide NAT64/DNS64 PLAT service. Exiting.
20:10:12 clatd-test systemd[1]: clatd.service: Deactivated successfully.

Manually restarting the service after this timeout gets things working again, but we'd rather start the service at the right time. Alternatively/additionally, we might consider having an extra configuration option to treat lack of discovery of a PLAT prefix not as "NAT64 is not available", but as an error, and then the SystemD unit could use Restart=on-failure. This would be useful for servers and other fixed-location machines that are known to be in IPv6-only networks where NAT64 should always be available.

[toreanderson]

Shouldn't this be After=?

[toreanderson]

@jivanpal Would appreciate it if you could explain why you are using Before= here. It seems to me that After= would be the appropriate dependency to use - considering the PR is titled «ensure service starts after DNS queries can be resolved».

[toreanderson]

According to the systemd.special(7) manual page:

All services for which the availability of full host/network name resolution is essential should be ordered after this target, but not pull it in. (emphasis mine)

Therefore, I think it is wrong to add it to Wants.