MTU not set on 'tunnel' interfaces

[Keenan Tims]

Setting MTU via /etc/network/interfaces for GRE tunnels ('tunnel' type) doesn't get applied at ifup:

iface HYDRUS inet tunnel
   mode gre
   address 10.170.1.4
   netmask 255.255.255.254
   endpoint 10.0.0.1
   dstaddr 10.170.1.5
   local 10.255.255.12
   mtu 1472

37: HYDRUS: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 8976 qdisc noqueue state UNKNOWN
    link/gre 10.255.255.12 peer 10.0.0.1

It seems to calculate the MTU based on the parent interface's MTU and ignore the directive. man 5 interfaces indicates that mtu should be a valid option for interfaces of type 'tunnel'. I have worked around this with an 'up' directive to manually set the MTU. Upon examining the ifupdown.nw source, it appears that MTU is never set for 'tunnel' type interfaces. Please rectify or remove it from the documentation.

$> lsb_release -rd
Description: Ubuntu 12.04.1 LTS
Release: 12.04
$> apt-cache policy ifupdown
ifupdown:
  Installed: 0.7~beta2ubuntu8
  Candidate: 0.7~beta2ubuntu8
  Version table:
 *** 0.7~beta2ubuntu8 0
        500 http://mirrors.kernel.org/ubuntu/ precise/main amd64 Packages
        100 /var/lib/dpkg/status