Getting can't resolve host when curling dependencies in systemd

I'm using CoreOS with cloud-init and I'm trying to curl for flannel. I have a unit file as such:

[Unit]
Description=Run flannel
Documentation=https://github.com/coreos/flannel
Requires=etcd2.service
After=etcd2.service
[Service]
ExecStartPre=/usr/bin/curl -L -o /opt/bin/flannel-0.5.1-linux-amd64.tar.gz -z /opt/bin/flannel-0.5.1-linux-amd64.tar.gz https://github.com/coreos/flannel/releases/download/v0.5.1/flannel-0.5.1-linux-amd64.tar.gz
ExecStartPre=/usr/bin/tar -C /opt/bin -xzvf /opt/bin/flannel-0.5.1-linux-amd64.tar.gz
ExecStartPre=/usr/bin/mv /opt/bin/flannel-0.5.1/flanneld /opt/bin/flanneld
ExecStartPre=/usr/bin/rm -rf /opt/bin/flannel-0.5.1
ExecStartPre=/usr/bin/rm -rf /opt/bin/flannel-0.5.1-linux-amd64.tar.gz
ExecStart=/opt/bin/flanneld

when it goes to curl for flannel, I get cannot resolve host 'github.com' and curl exits with a code of 1. Is there some issue where I cannot use dns at the time systemd starts this up?

For CoreOS you will need:

[Unit]
Requires=network-online.target

There is a difference between network.target and network-online.target. network-online.target is what is pulled in when attempting to perform network-mounts in /etc/fstab, and your dependency needs are closer to that. To use it.

In my case this answer works https://unix.stackexchange.com/questions/353179/why-isnt-my-systemd-service-waiting-for-the-network/356189#356189

You have two options

Variant 1

wait for network and dns is ready

but also blocks the boot process, if the network not available!!!

Steps:

1. activate the systemd module (required)

systemctl enable systemd-networkd-wait-online.service

2. change the config of your service

[Unit]
...
After=systemd-networkd-wait-online.service
Requires=systemd-networkd-wait-online.service

Variant 2

Restart the script on a failure and try again

Steps:

1. Set a restart configuration for the service

[Service]
...
Restart=on-failure
RestartSec=5

2. Example bash script with curl

#!/bin/sh
STATUSCODE=$(curl --silent --output /dev/stderr --write-out "%{http_code}"  https://webhook.example)

if test $STATUSCODE -ne 200; then
    exit 1
fi

exit 0