Syncronize time by NTP before starting any services in Linux
Servers often have wrong clock on startup. NTP services, like ntp, chrony and systemd-timesyncd try to correct clock gradually to avoid weird bugs in software. Therefore, if server has a large clock offset on startup, it works with incorrect clock for several minutes.
In my experience, AWS instances may have clock error up to 5 minutes on startup. Server writing log timestamps 5 minutes in the past or in the future is not always a good idea.
Solution is to force NTP time syncronization once before starting any other services. I prefer to use chrony: it can act both as always runnig NTP client and one-time syncronization tool; chronyc clearly reports syncronization status, making it easy to monitor; it is recommended by AWS.
All other services in multi-user.target will start after time is syncronized or failed to syncronize after given timeout.
/etc/systemd/system/ntp-sync-once.service :
[Unit]
Description=Quick sync NTP one time and exit
Wants=network-online.target
After=network-online.target
Before=multi-user.target
[Service]
Type=oneshot
RemainAfterExit=True
# Ugly workaround for not working properly network-online.target
ExecStartPre=sh -c "while ! ip -4 r | grep ^default; do echo waiting for ipv4 default route to appear; sleep 0.5; done"
# -t <timeout in seconds> timeout after which chronyd will exit even if clock is not syncronized
ExecStart=/usr/sbin/chronyd -q -t 20
[Install]
WantedBy=multi-user.targetnetwork-online.target sometimes is not working as expected, ExecStartPre line is a workaround for that.
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