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[Bug 1403955] Re: DHCP's "Option interface-mtu 9000" is being ignored on bridge interface br0

To: touch-packages@xxxxxxxxxxxxxxxxxxx
From: PshemK <pshem.k@xxxxxxxxx>
Date: Wed, 11 Nov 2015 23:31:58 -0000
Reply-to: Bug 1403955 <1403955@xxxxxxxxxxxxxxxxxx>
Sender: bounces@xxxxxxxxxxxxx

The bug is still present in 1.25.

The fact that it only manifests itself during the subsequent machine
reboots (and not during the initial build) makes it more difficult to
troubleshoot - services that worked before reboot do not work after.

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https://bugs.launchpad.net/bugs/1403955

Title:
  DHCP's "Option interface-mtu 9000" is being ignored on bridge
  interface br0

Status in juju-core:
  Triaged
Status in isc-dhcp package in Ubuntu:
  Confirmed

Bug description:
  In an env with jumbo frames enabled, and using MAAS as DHCP server,
  the client receives the following IPv4 lease:

  $ cat /var/lib/dhcp/dhclient.br0.leases
  lease {
    interface "br0";
    fixed-address 10.230.20.26;
    filename "pxelinux.0";
    option subnet-mask 255.255.248.0;
    option dhcp-lease-time 43200;
    option routers 10.230.16.1;
    option dhcp-message-type 5;
    option dhcp-server-identifier 10.230.20.1;
    option domain-name-servers 10.230.20.1;
    option interface-mtu 9000;
    option broadcast-address 10.230.23.255;
    option domain-name "ctsstack.qa.1ss";
    renew 3 2014/12/17 16:48:15;
    rebind 3 2014/12/17 21:52:09;
    expire 3 2014/12/17 23:22:09;
  }

  The interfaces show the following config after boot:

  $ ifconfig br0
  br0       Link encap:Ethernet  HWaddr a0:d3:c1:01:9d:58
            inet addr:10.230.20.26  Bcast:10.230.23.255  Mask:255.255.248.0
            inet6 addr: fe80::a2d3:c1ff:fe01:9d58/64 Scope:Link
            UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
            RX packets:530530 errors:0 dropped:0 overruns:0 frame:0
            TX packets:1591569 errors:0 dropped:0 overruns:0 carrier:0
            collisions:0 txqueuelen:0
            RX bytes:68713489 (68.7 MB)  TX bytes:213710979 (213.7 MB)

  $ ifconfig eth0
  eth0      Link encap:Ethernet  HWaddr a0:d3:c1:01:9d:58
            inet6 addr: fe80::a2d3:c1ff:fe01:9d58/64 Scope:Link
            UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
            RX packets:10539274 errors:0 dropped:3394 overruns:0 frame:454
            TX packets:2627412 errors:0 dropped:0 overruns:0 carrier:0
            collisions:0 txqueuelen:1000
            RX bytes:2320560616 (2.3 GB)  TX bytes:3562885157 (3.5 GB)
            Interrupt:32

  
  "option interface-mtu 9000;" from the lease file is being ignored by br0. Could it be related to eth0 MTU size? If that's the case, shouldn't both interfaces be updated?

  Other info:

  $ brctl show br0
  bridge name	bridge id		STP enabled	interfaces
  br0		8000.a0d3c1019d58	no		eth0

  $ cat /etc/network/eth0.config
  iface eth0 inet manual

  auto br0
  iface br0 inet dhcp
    bridge_ports eth0

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