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Resolve internet issues with shared AirPort networking Network
blish a proper internet connection.

After doing some research and with help from other Mac users, I learned that in Mac OS X there is something missing called MSS-Clamping to fix broken web pages. See this article for more details. Until Apple fixes this with a newer BSD version, the easiest thing to fix this is to set MTU for AirPort on the second Mac (not the Mac with the internet connection!) from 1500 to 1492. If you think you have the same problem you can try:
 sudo ifconfig en1 mtu 1492
If this fixes your problem, you can make the change permanent with a StartupItem I made.

The StartupItem skeleton was borrowed from BroadbandOptimizer StartupScript. All further steps as root or with sudo.
  1. Make a directory AirportMTU in /Library. In this new directory, make a file named AiportMTU with this content:
    #!/bin/sh

    ##
    # Airport MTU
    # Sets MTU to 1492
    ##

    . /etc/rc.common

    CheckForNetwork

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

    ifconfig en1 mtu 1492 > /dev/null
    echo "Network settings optimized for AirPort Connection"

    #fi
  2. Make a file StartupParameters.plist with this content:
    <?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">
    ### THE ABOVE TWO LINES SHOULD BE ONE LINE! ###
    <plist version="1.0">
    <dict>
    <key>Description</key>
    <string>AiportMTU</string>
    <key>Messages</key>
    <dict>
    <key>restart</key>
    <string>Restarting Airport MTU.</string>
    <key>start</key>
    <string>Optimizing Network for AirPort Connection.</string>
    <key>stop</key>
    <string>Stopping Airport MTU.</string>
    </dict>
    <key>OrderPreference</key>
    <string>none</string>
    <key>Provides</key>
    <array>
    <string>AirportMTU</string>
    </array>
    <key>Requires</key>
    <array>
    <string>Network</string>
    <string>Network Configuration</string>
    </array>
    </dict>
    </plist>
  3. Then make a directory named Resources and in there, make another directory named English.lproj. Put a file Localizable.strings in it with this content:
    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE plist SYSTEM "file://localhost/System/Library/DTDs/PropertyList.dtd">
    <plist version="0.1">
    <dict>
    <key>Optimizing Network for AirPort Connection.</key>
    <string>Optimizing Network for AirPort Connection.</string>
    <key>Stopping Airport MTU.</key>
    <string>Stopping Airport MTU.</string>
    <key>Restarting Airport MTU.</key>
    <string>Restarting Airport MTU.</string>
    </dict>
    </plist>
Now it's best to reboot the computer to see if it works. If the StartupItem worked, you should see a MTU of 1492 on interface en1 when doing a 'ifconfig'.
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mtu fixed in Jaguar?
Authored by: osax on Jan 16, '03 11:00:16AM

I had this problem until i upgraded to 10.2, after that this no longer was an issue for me. So perhaps this has been fixed in Jaguar? On the other hand, i didn't do much testing with Jaguar because i switched my Airport Base Station, so i could have missed something.



[ Reply to This | # ]
mtu fixed in Jaguar?
Authored by: deleted_user18 on Jan 16, '03 12:16:17PM

To me it is not fixed in Jaguar (I am the one who submitted the hint :-) - but maybe it also depends on your ADSL ISP? I don't know.



[ Reply to This | # ]
mtu fixed in Jaguar?
Authored by: russh on Jan 17, '03 09:34:35AM

I still have the same problem, running 10.2.3. The tip you mention works for me. When I find myself with the problem I can do a ping and see 20% to 60% packet loss. Running the terminal command you mention clears it up.



[ Reply to This | # ]
mtu not fixed in Jaguar.
Authored by: osax on Feb 15, '03 09:51:46AM

Just for the record: after experimenting with Internet Sharing again, i can confirm this is still a problem in 10.2.4.



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