Create GCE specific images with correctly configured MTU

[superseb]

RancherOS Version: (ros os version)
Any
Where are you running RancherOS? (docker-machine, AWS, GCE, baremetal, etc.)
GCE

Follow up on rancher/rancher#8910, documentation will be added for now but would be nice to get GCE specific images built with correctly configured MTU off the bat.

[niusmallnan]

Have Built a new image with mtu setting:

#cloud-config
rancher:
  docker:
    extra_args: [--mtu, 1460]
  system_docker:
    extra_args: [--mtu, 1460]

Boot a new VM on GCE, For eth0, docker0, system-docker0, the MTU displays for these devices are incorrect.

[rancher@ros-test-4 ~]$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 42:01:0a:f0:00:08 brd ff:ff:ff:ff:ff:ff
    inet 10.240.0.8/32 brd 10.240.0.8 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::4001:aff:fef0:8/64 scope link
       valid_lft forever preferred_lft forever
3: docker-sys: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
    inet 172.18.42.2/16 scope global docker-sys
       valid_lft forever preferred_lft forever
    inet6 fe80::9c61:8eff:febd:a3c0/64 scope link
       valid_lft forever preferred_lft forever
5: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default
    link/ether 02:42:07:b8:d8:21 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.1/16 scope global docker0
       valid_lft forever preferred_lft forever

For eth0, Its MTU is determined by DHCP, check the ip route:

bash-4.4# ip route
default via 10.240.0.1 dev eth0 src 10.240.0.3 metric 202  mtu 1460
10.240.0.1 dev eth0 proto kernel scope link src 10.240.0.3 metric 202  mtu 1460
172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.0.1 linkdown
172.18.0.0/16 dev docker-sys proto kernel scope link src 172.18.42.2 linkdown

It does not seem to have any effect, MTU is actually correct.

For docker0, after running a container, docker0 mtu will be normal:

[root@ros-test-4 rancher]# docker start e0bc733184f0
e0bc733184f0
[root@ros-test-4 rancher]# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 42:01:0a:f0:00:08 brd ff:ff:ff:ff:ff:ff
    inet 10.240.0.8/32 brd 10.240.0.8 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::4001:aff:fef0:8/64 scope link
       valid_lft forever preferred_lft forever
3: docker-sys: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
    inet 172.18.42.2/16 scope global docker-sys
       valid_lft forever preferred_lft forever
    inet6 fe80::9c61:8eff:febd:a3c0/64 scope link
       valid_lft forever preferred_lft forever
5: docker0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1460 qdisc noqueue state UP group default
    link/ether 02:42:07:b8:d8:21 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.1/16 scope global docker0
       valid_lft forever preferred_lft forever
    inet6 fe80::42:7ff:feb8:d821/64 scope link
       valid_lft forever preferred_lft forever
7: vethe3d06f1@if6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1460 qdisc noqueue master docker0 state UP group default
    link/ether 6e:5b:f6:98:30:0a brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet6 fe80::6c5b:f6ff:fe98:300a/64 scope link
       valid_lft forever preferred_lft forever

The MTU displayed is not correct, I will submit another issue, to continue to pay attention.