RancherOS using wrong mtu in OpenStack

[hwaastad]

Ref discussion:
https://forums.rancher.com/t/rancheros-wrong-mtu-in-openstack/2489/11

In addition, docker daemon need --mtu settings

/hw

In case if someone is looking for a workaround, here is a cloud-config script:

#cloud-config
write_files:
  - path: /etc/rc.local
    permissions: "0755"
    owner: root
    content: |
      #!/bin/sh
      /sbin/ip link set mtu 1450 dev eth0

rancher:
  docker:
    extra_args: [--mtu=1450]

And perhaps even better workaround is to run this on the host:

iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

Then no need to change any other MTU settings (in a Rancher VM nor in a Docker)

Explanation http://lartc.org/howto/lartc.cookbook.mtu-mss.html

[joehumphries]

I created a new network in rancher, and used IPSEC, in 1.4 you can set the MTU to 1400.

[niusmallnan]

shoule be fixed by #2183

[stieler-it]

I created two new servers using v1.2.0 without the workaround in cloud-config and the MTU of both eth0 and docker0 was 1500 again. This resulted in brittle network connections so I put the workaround back in place.