Cloud-init’s behavior can be configured via user-data. User-data can be given by the user at instance launch time.
When booting under systemd, a generator will run that determines if cloud-init.target should be included in the boot goals. By default, this generator will enable cloud-init.
The purpose of the local stage is:
- Locate “local” data sources
- Apply networking configuration to the system (including “Fallback”)
This stage must block network bring-up or any stale configuration might already have been applied. That could have negative effects such as DHCP hooks or broadcast of an old hostname. It would also put the system in an odd state to recover from as it may then have to restart network devices.
- systemd service:
This stage requires all configured networking to be online, as it will fully
process any user-data that is found. This stage runs the
mounts modules which may partition and format disks and configure mount points
(such as in /etc/fstab). Those modules cannot run earlier as they may receive
configuration input from sources only available via network. For example, a user
may have provided user-data in a network resource that describes how local
mounts should be done.
- systemd service:
This stage runs config modules only. Modules that do not really have an effect on other stages of boot are run here.
- systemd service:
This stage runs as late in boot as possible. Any scripts that a user is accustomed to running after logging into a system should run correctly here.
Fallback Network Configuration
Cloud-init will attempt to determine which of any attached network devices is most likely to have a connection and then generate a network configuration to issue a DHCP request on that interface.
Nova presents configuration information to instances it starts via a mechanism called metadata. These mechanisms are widely used via helpers such as cloud-init to specify things like the root password the instance should use.
This metadata is made available via either a config driver or the metadata service and can be somewhat customized by the user using the user data feature.
Metadata service lets an instance retrieve instance specific information, e.g. hostname, IP, routes, SSH keys, user-data, vendor-data and various default settings even commands and shell scripts. All of these are generally handled by a service in the instance like cloud-init.
[starbops@shitcoin ~]$ curl http://169.254.169.254/openstack 2012-08-10 2013-04-04 2013-10-17 2015-10-15 2016-06-30 2016-10-06 latest
Endpoints of Metadata Service
List all meta-data service’s endpoints by requesting
[starbops@shitcoin ~]$ curl http://169.254.169.254/latest/meta-data ami-id ami-launch-index ami-manifest-path block-device-mapping/ hostname instance-action instance-id instance-type local-hostname local-ipv4 placement/ public-hostname public-ipv4 reservation-id security-groups
For example, getting floating IP address bond with the instance:
[starbops@shitcoin ~]$ curl http://169.254.169.254/latest/meta-data/public-ipv4 100.74.37.104
To see what user-data have been filled:
[starbops@shitcoin ~]$ curl http://169.254.169.254/latest/user-data #cloud-config #packages: # - htop #ssh_authorized_keys: # - ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCkgolYx6IoPSYnhZMdGivUJOm4AMtI0gkjFkY/53V4idbliQHAMJHGdMGYdlEm5ThOCw3DDblsQDNy7EZJaa9+T1imwrnUg0fYU13u+Tfq7Fg+TIn4hf4uG/ei2r1MLp2/lO/6dEPUGv2TiBQ+SVfB8yt2IUVIGgqNhGWJi/p5uw9O5KiAPN1UmT3CvpWYVFnfqvnDwnOMJkXg9xN8AbTkAHS1YDIljNMwBisaOvI5cjgZ5a+ovp2pdHBxZWyPAb7Y5NvlQHGtJQIlbWTcxIBu8/1YPbZlkTcgB0ghDf0upgKunqFHh/Zq3sdEEUyQ2Xr6qdVyaXwNQJhV8Kge196r ubuntu@controller users: - default - name: starbops gecos: Zespre Schmidt sudo: ALL=(ALL) NOPASSWD:ALL #lock_passwd: false #passwd: $6$rounds=4096$lDq1KTfs/T/e7$EfyJlD0aqyO7W8oSOumQySoYxfqBC5OxzGXnGrFV6AIMms5QnE0pwdwJttTw2iliFKoKfnFnGLfgauLVF2yoa1 ssh_authorized_keys: - ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCkgolYx6IoPSYnhZMdGivUJOm4AMtI0gkjFkY/53V4idbliQHAMJHGdMGYdlEm5ThOCw3DDblsQDNy7EZJaa9+T1imwrnUg0fYU13u+Tfq7Fg+TIn4hf4uG/ei2r1MLp2/lO/6dEPUGv2TiBQ+SVfB8yt2IUVIGgqNhGWJi/p5uw9O5KiAPN1UmT3CvpWYVFnfqvnDwnOMJkXg9xN8AbTkAHS1YDIljNMwBisaOvI5cjgZ5a+ovp2pdHBxZWyPAb7Y5NvlQHGtJQIlbWTcxIBu8/1YPbZlkTcgB0ghDf0upgKunqFHh/Zq3sdEEUyQ2Xr6qdVyaXwNQJhV8Kge196r ubuntu@controller #ssh_pwauth: true #runcmd: # - sed -i -e '$aAllowUsers starbops' /etc/ssh/sshd_config # - systemctl restart ssh.service hostname: shitcoin
How It Works
instance --> neutron-ns-metadata-proxy --> neutron-metadata-agent --> nova-api
Instance makes request to
http://169.254.169.254. Because the destination does
hit any rules in the routing table of the instance, it just falls back to the
default route. The request follows the default route to router namespace. Here
iptables rule for this situation:
REDIRECT tcp -- 0.0.0.0/0 169.254.169.254 tcp dpt:80 redir ports 9697
The request being redirected is still inside the same router namespace.
neutron-ns-metadata-proxy is running and listening on TCP 9697 port in each
router namespaces. While processed by
headers are added to the request:
X-forwarded-for: <Instance IP> X-neutron-router-id: <Router UUID>
And the request is forwarded through UNIX socket to
neutron-metadata-agent listens on the socket
and communicates with public OpenStack APIs for more information. More headers
X-Instance-ID: <Instance UUID> X-Tenant-ID: <Tenant UUID> X-Instance-ID-Signature: <HMAC secret, instance_id>