How to override Google Compute Engine's DHCP MTU Option

Recently I ran into a few issues with a linux IPSec Tunnel (Strongswan) that was hosted at GCE. With the suggestion of one of Google’s engineers, I thought that it might have something to do with the MTU. Google’s compute engine network MTU is 1460.

“For best performance, set MTU to 1460. Google Compute Engine’s DHCP server will serve this parameter as the interface-mtu option, which most clients respect.”

source: https://developers.google.com/compute/docs/images

I decided to drop it to 1400 in order to give the Strongswan host some additional headroom on the GCE network side. It ended up being difficult to override the MTU. I attempted to set the MTU option in the /etc/sysconfig/network-scripts/ifcfg-eth0 script like so:

DEVICE="eth0"
BOOTPROTO="dhcp"
DHCP_HOSTNAME="localhost"
MTU="1400"
NM_CONTROLLED="yes"
ONBOOT="yes"
TYPE="Ethernet"

That wasn’t working though, so I needed to somehow override the dhcp options being offered. I had never messed with the linux dhcp client (dhclient) so into the man pages we go! I read through them, but wasn’t able to find anything that worked. I tried multiple things until I stumbled upon a random config on the internet which showed (by observing) that options with numbers as values weren’t surrounded in quotes. I removed the quotes from the option and BAM, it worked! Here is the final config that worked for me:

# /etc/dhcp/dhclent-eth0.conf
send vendor-class-identifier "anaconda-Linux 2.6.32-431.el6.x86_64 x86_64";
timeout 45;
supersede interface-mtu 1400;

I highly suggest reading through the dhclient.conf man page.

man dhclient.conf

Here is a snippet from the man page about the supersede statement.

The supersede statement

supersede [ option declaration ] ;

If for some option the client should always use a locally-configured value or values rather than whatever is supplied by the server, these values can be defined in the supersede statement.