How to Set the MTU Value During Startup

Follow these steps to create a script that sets the MTU each time the computer restarts:
If you get an error you might have to run the commands with SUDO first.
Ex: sudo mkdir StartupItems
1. Open Terminal (/Applications/Utilities/). 
2. Type:cd /Library
3. Press Return. 
4. Type:mkdir StartupItems
5. Press Return. (If you encounter an error, continue to step 6.)
6. Type:cd StartupItems
7. Press Return. 
8. Type:mkdir MTU
9. Press Return. 
10. Type:cd MTU
11. Press Return. 
12. Type:pico MTU
13. Press Return. 
14. In the pico editor, paste in the following text. 

Begin copying below this line.
#!/bin/sh

. /etc/rc.common

##
# Configure a network interface MTU setting
##
#
# This script will set the MTU setting for the specified interface(s)
#
# The name of the interface (ex. en0) must be edited to match the interface
# to which the MTU setting should be applied
#
##

StartService ()
{
ConsoleMessage "Configuring MTU"

### uncomment lines and change the value following 'mtu' as appropriate

if [ "${MTU:=-NO-}" = "-YES-" ]; then

# /sbin/ifconfig en0 mtu 1490
# /sbin/ifconfig en1 mtu 1490

fi

}

StopService ()
{
return 0
}

RestartService ()
{
return 0
}

RunService "$1"

End copying above this line.

15. Uncomment the /sbin/ifconfig line(s) to set the MTU for a particular interface.

 

Note: Removing the number sign (#) from the beginning of a line uncomments it. Typically, en0 is the interface name for the Built-in Ethernet port and en1 is interface name for the AirPort Card. This is not always the case, though. To confirm that a network port is associated with a particular interface name, open the Network Utility (/Applications/Utilities/), and click the Info tab.

 

16. When you have finished customizing the file, save it (press Control-O), press Return, and exit pico (press Control-X).

17. Type: chmod 755 MTU

18. Press Return.

19. Type: pico StartupParameters.plist

20. Press Return.

21. In the pico editor paste in the following text.

 

Begin copying below this line.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Description</key>
	<string>Can set MTU</string>
	<key>OrderPreference</key>
	<string>None</string>
	<key>Provides</key>
	<array>
		<string>MTU</string>
	</array>
	<key>Requires</key>
	<array>
		<string>Network Configuration</string>
	</array>
</dict>
</plist>

End copying above this line.

22. When you have finished customizing the file, save it (Control-O), press Return, and exit pico (Control-X).
23. Type:chmod 755 StartupParameters.plist
24. Press Return.
25. Type:sudo pico /etc/hostconfig
26. When prompted, enter your password.
27. Press Return.
28. In the pico editor, add this line at the bottom:

MTU=-YES-

29. Save it (Control-O), press Return, and exit pico (Control-X).

When you restart the computer, MTU is set for the interface that you specified.

Notes

      1. The MTU will be reset after changing a Location, waking the computer from sleep, or changing the state of the network interface. To use the script again without having to restart, enter the following command:

sudo SystemStarter start MTU

      2. If you experience any issues or wish to not set MTU during startup, you can turn off the new script by changing the MTU line in /etc/hostconfig to:

MTU=-NO-

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