05
Oct
11

Dynamic MTU in Debian/Ubuntu

I know it’s probably an unusual situation, but in the lab we have Jumbo frames turned on for all the servers and test boxes. It makes a huge difference copying ISOs between hosts, and doing network backups. However, my Kubuntu laptop isn’t always in the lab network. This means that I almost never remember to change the MTU when I’m back in the office, OR I remember in the middle of a transfer, when it’s already too late to gain the benefits.

So I wrote a little script, and put it in /etc/network/if-up.d/ [edit: under NetworkManager, use /etc/NetworkManager/dispatcher.d isntead] named “jumbo-frames.sh”. The if-*.d/ structure is designed for exactly this purpose: run a script when an interface comes up. The basic premise is: If I’m plugged into a wired network (eth0) in the lab (domain or IP address match certain parameters), then set the MTU to 9000 (jumbo frame support), otherwise assume the network has a normal MTU (1500). This allows the system to reconfigure on the fly if I put it to sleep and go visit a customer.

Here’s the code:

#!/bin/sh
# Set support for jumbo frames when at home on wired network, else do not.
# Determine home network based on IP address and DNS-determined name.
# $IFACE should be set by the caller.

PATH=/sbin:/bin:/usr/sbin:/usr/bin

IFC=/sbin/ifconfig
INT=”eth0″
MTU=9000
DEFMTU=1500
#name of the DNS domain to assume as “home”
HOMED=”totalnetsolutions.net”
#IP Subnet to assume as “home” if DNS test fails
HOMEN=”10.0.0.”

test -x $IFC || exit 0

# Don’t make changes to the wireless (wlan) or loopback (lo) interfaces
if [ “$IFACE” != “$INT” ]; then
exit 0
fi

# if dhcpd is still working on writing our resolv.conf, just wait a while (it’s a hack, but it works).
test -f /etc/resolv.conf || sleep 15

DOM=`awk ‘/search/ { print $2 }’ /etc/resolv.conf`
NET=`ip addr show dev $IFACE | awk ‘/inet / { print $2 }’ | awk -F. ‘{ print $1 “.” $2 “.” $3 “.” }’`

if [ “$DOM” = “$HOMED” ]; then
$IFC $IFACE mtu $MTU
elif [ “$NET” = “$HOMEN” ]; then
$IFC $IFACE mtu $MTU
else
$IFC $IFACE mtu $DEFMTU
fi



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