Notice - Revised Procedure

FirstClass Tech Support has revised the instructions for adjusting the MTU Value. The reason is that the file referenced below is overwritten at every OS version upgrade, such as 10.x or even 10.x.x and had to be continually reedited.

A new method using a startup file is now recommended. Go to the FirstClass Support site at http://www.firstclass.com/support/SupportCommentary/Server/S032B55F7 for the details and download file.

The original posting and description of the problem remains posted below for your reference.




Setting the Loopback Adapter MTU Value


Note: The loopback adapter is a virtual network interface that allows IP enabled applications on the same physical machine to communicate with each other. In OS X, Apple has set the default MTU (Maximum Transmission Unit) value to a much higher value than is set for physical adapters, This causes trouble for sites running FirstClass Server and Internet Services on the same machine. Apple discusses resetting this for OS 10.2 in this document.

Symptoms include a severe slowdown in the FirstClass Server and Internet Services programs.

Advice from Tech Support:



If the FirstClass Server and FirstClass Internet Services are running on the same computer, then it could be related to the lo0 mtu setting on the computer, which can cause poor connectivity between the Server and Internet services.

This issue only affects "same node" FirstClass server installations on OS X, where both the FirstClass Server and FirstClass Internet Services are running on the same computer.

Development has identified many OS X FirstClass Internet Services (IS) crash issues to be caused by poor connectivity between IS and the FirstClass Server, due to the following loopback adapter mis-configuration.

A serious performance issue was found with regards to uploads to an OS X FirstClass Server from either IS or the FirstClass client running on the same computer as the FirstClass Server.

The culprit was found to be a problem with the default configuration of the loopback adapter (lo0) on OS X. Reconfiguring it with the same MTU value as the ethernet card (en0) completely resolves the issue.  This can be done by typing the following command in the terminal window:

        sudo ifconfig lo0 mtu 1500

The OS X default for lo0 is actually 16384 (16K).

If the above entry corrects your performance issue, then one of the two permanent solutions below should be enacted.

Note: The above change will not survive a machine restart and would need to be reapplied on each restart.  



For 10.2

The following process is one option which can be used to have the loopback adapter (lo0) MTU value configured on start up and is based on Mac OS X 10.2.8. It adds the configuration to the "Network" configuration file which gets loaded upon startup of the computer.

w Open the terminal window and use the following command to copy the "Network" configuration file to your "FirstClass Server" folder.

        cp "/System/Library/StartupItems/Network/Network" "/Library/FirstClass Server/"

w Browse to the "/Library/FirstClass Server" folder and open the "Network" file with any text editor.

w Add the "ifconfig lo0 mtu 1500" line into the "Network" file at the location shown below in red italics.

        # Configure interfaces
        ipconfig waitall > /dev/null 2>&1
        ifconfig lo0 127.0.0.1 netmask 255.0.0.0 up
        ifconfig lo0 mtu 1500

        # See if the network is up now
        CheckForNetwork

w Open the terminal window and use the following command to copy the "Network" configuration file from your "FirstClass Server" folder back to the "/System/Library/StartupItems/Network/" folder.

        sudo cp "/Library/FirstClass Server/Network" "/System/Library/StartupItems/Network/"

w To confirm if the change was successful you can browse to the "/System/Library/StartupItems/Network/" folder and open the "Network" file with any text editor.  You can not save the "Network" file to this location using the text editor, hence the above process of moving it first.


For 10.3 the Network file is in the same location but the content is completely different. Follow the same procedure to add the highlighted line below.

20040114_43217_0.png

Thanks,
Mark Stewart
FirstClass Customer Support