" # curl http://169.254.169.254/openstack/2018-08-27/network_data.json
{"links": [{"id": "tapa035fb68-01", "vif_id":
"a035fb68-010c-42e3-8da7-ea3c36a0d607", "type": "ovs", "mtu": 8942,
"ethernet_mac_address": "fa:16:3e:31:26:f7"}], "networks": [{"id": "network0",
"type": "ipv4_dhcp", "link": "tapa035fb68-01", "network_id":
"b4ef84c0-1235-48a8-aaf7-03fab7ef5367"}], "services": []}"
How is cloud-init to know from this network-config.json that DHCP will
provide an MTU value? How does it know that it should ignore the
provided MTU? If DHCP is providing MTU, should network-config.json then
not provide the MTU value?
** Also affects: netplan.io (Ubuntu)
Importance: Undecided
Status: New
** Changed in: cloud-init
Status: New => Incomplete
** Changed in: netplan.io (Ubuntu)
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/1899487
Title:
cloud-init hard codes MTU configuration at initial deploy time
Status in cloud-init:
Incomplete
Status in netplan.io package in Ubuntu:
Incomplete
Bug description:
When using OpenStack cloud provider `cloud-init` will write out
/etc/netplan/50-cloud-init.yaml at initial instance boot and not
update it on subsequent boots of the instance.
The OpenStack metadata service provides information about MTU for the
network [0] and cloud-init takes this value and writes it into the
netplan configuration [1].
A side effect of configuring the MTU through netplan is that the
`systemd-networkd` [Link] section [2] gets the MTUBytes value filled
and this in turn makes `systemd-networkd` ignore the MTU value
provided by DHCP [3][4].
During the lifetime of a cloud events occur that will force a operator
to reduce the MTU available to instances attached to its overlay
networks. This may happen because of software imposed change of tunnel
type (GRE -> VXLAN, VXLAN -> GENEVE) or change of topology or
encapsulation in the physical network equipment.
To maximize performance these clouds have configured their instances
to use the maximum available MTU without leaving any headroom to
account for such changes and the only way to move forward is to reduce
the available MTU on the instances. We are facing a concrete challenge
with this now where we have users wanting to migrate from VXLAN
tunnels to GENEVE tunnels with 38 byte header size.
0: # curl http://169.254.169.254/openstack/2018-08-27/network_data.json
{"links": [{"id": "tapa035fb68-01", "vif_id":
"a035fb68-010c-42e3-8da7-ea3c36a0d607", "type": "ovs", "mtu": 8942,
"ethernet_mac_address": "fa:16:3e:31:26:f7"}], "networks": [{"id": "network0",
"type": "ipv4_dhcp", "link": "tapa035fb68-01", "network_id":
"b4ef84c0-1235-48a8-aaf7-03fab7ef5367"}], "services": []}
1: # cat /etc/netplan/50-cloud-init.yaml
# This file is generated from information provided by the datasource. Changes
# to it will not persist across an instance reboot. To disable cloud-init's
# network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
version: 2
ethernets:
ens2:
dhcp4: true
match:
macaddress: fa:16:3e:31:26:f7
mtu: 8950
set-name: ens2
2: # cat /run/systemd/network/10-netplan-ens2.link
[Match]
MACAddress=fa:16:3e:31:26:f7
[Link]
Name=ens2
WakeOnLan=off
MTUBytes=8950
3: # cat /run/systemd/network/10-netplan-ens2.network
[Match]
MACAddress=fa:16:3e:31:26:f7
Name=ens2
[Link]
MTUBytes=8950
[Network]
DHCP=ipv4
LinkLocalAddressing=ipv6
[DHCP]
RouteMetric=100
UseMTU=true
4: Oct 12 13:30:18 canary-3 systemd-networkd[24084]:
/run/systemd/network/10-netplan-ens2.network: MTUBytes= in [Link]
section and UseMTU= in [DHCP] section are set. Disabling UseMTU=.
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