[Bug 1638256] [NEW] ifup fail when use MTU small than 68

[Talat Batheesh]

Public bug reported:

ifup failed when we set a static MTU small than 68, and when we change the static configuration and try to run ifdown and then ifup, the interface still with the old MTU and both of ifup and ifdown command fail.
when we change the MTU manually by ip tool and then change it in the /etc/network/interfaces to be 1500 the interface get back to work.

root:~# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 16.10
Release:        16.10
Codename:       yakkety
root:~# uname -r
4.8.0-26-generic


scenario: 
set the MTU static to 67 

root:~# ifdown ens2
root:~# ifup ens2
root:~# ifdown ens2
RTNETLINK answers: No such device
root:~# ifup ens2
RTNETLINK answers: No buffer space available
Failed to bring up ens2.
root:~# cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

auto ens2
iface ens2 inet static
address 21.3.5.16
mtu 67

## change MTU static to 1500 
root:~# vim /etc/network/interfaces
root:~# ifdown ens2
ifdown: interface ens2 not configured
root:~# ifup ens2
RTNETLINK answers: No buffer space available
Failed to bring up ens2.

root:~# ip link set ens2 mtu 1500
root:~# ifup ens2

** Affects: ifupdown (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1638256

Title:
  ifup fail when use MTU small than 68

Status in ifupdown package in Ubuntu:
  New

Bug description:
  ifup failed when we set a static MTU small than 68, and when we change the static configuration and try to run ifdown and then ifup, the interface still with the old MTU and both of ifup and ifdown command fail.
  when we change the MTU manually by ip tool and then change it in the /etc/network/interfaces to be 1500 the interface get back to work.

  root:~# lsb_release -a
  No LSB modules are available.
  Distributor ID: Ubuntu
  Description:    Ubuntu 16.10
  Release:        16.10
  Codename:       yakkety
  root:~# uname -r
  4.8.0-26-generic

  
  scenario: 
  set the MTU static to 67 

  root:~# ifdown ens2
  root:~# ifup ens2
  root:~# ifdown ens2
  RTNETLINK answers: No such device
  root:~# ifup ens2
  RTNETLINK answers: No buffer space available
  Failed to bring up ens2.
  root:~# cat /etc/network/interfaces
  # This file describes the network interfaces available on your system
  # and how to activate them. For more information, see interfaces(5).

  auto ens2
  iface ens2 inet static
  address 21.3.5.16
  mtu 67

  ## change MTU static to 1500 
  root:~# vim /etc/network/interfaces
  root:~# ifdown ens2
  ifdown: interface ens2 not configured
  root:~# ifup ens2
  RTNETLINK answers: No buffer space available
  Failed to bring up ens2.

  root:~# ip link set ens2 mtu 1500
  root:~# ifup ens2

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