10.3: MTU fixes for Ethernet and Airport connections

When I started to use OS X 10.3 panther I was very pleased to find that the MTU setting of the Ethernet interface can be changed using Network preferences. As I'm using a PPPoE connection through a NAT router, I need to set the MTU to 1492 instead of the default 1500 to get my internet connection fully operational. For those of you who did not find the setting yet, it's in the Ethernet tab of the Built-in Ethernet configuration - set Configure to "Manually (Advanced)" and enter any MTU value you like!

However, Apple forgot to implement this setting for the Airport interface. The solution is basically the same as for previous OS X versions. To change the setting manually, enter the following line in the terminal (assuming en1 is your Airport interface):

 % sudo ifconfig en1 mtu 1492

To fix the Airport MTU permanently, I added a line to /System -> Library -> StartupItems -> Network -> Network. At the end of the StartService() function, I added ifconfig en1 mtu 1492. This seems to work, even after waking the computer from sleep. Yet I'm not sure if this is the best way to do this - any suggestions?

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10.3: MTU fixes for Ethernet and Airport connections
Authored by: diamondsw on Mon, Nov 24 2003 at 11:06AM PST

I would recommend creating a completely separate StartupItem just to set the MTU, and place that in /Library/StartupItems. Beyond the usual aversion to touching anything in /System, you never know when an OS update will decide to replace your edited files.

10.3: MTU fixes for Ethernet and Airport connections
Authored by: patmcfar8 on Mon, Nov 24 2003 at 12:39PM PST

What exactly does this do?

I've noticed that my internet connection speed has dropped a bit since I installed Panther on my AlPB. I'm using the Airport connection with a Linksys wireless router. Might this help me or not? Thank you.

10.3: MTU fixes for Ethernet and Airport connections
Authored by: sjoerd on Mon, Nov 24 2003 at 12:46PM PST

Yes, I agree with diamondsw. It is also what Apple suggested for 10.2, see http://docs.info.apple.com/article.html?artnum=107474. But what am I supposed to indicate in the StartupParameters.plist - is the following part of it still valid for Panther?

<key>Requires</key>
<array>
	<string>Network Configuration</string>
</array>

Otherwise, does this solution work also after waking from sleep?

I'll give it a try later - my iBook is on holiday these days...

To parmcfar8: changing the mtu is needed with certain PPPoE providers - if you have a problem with mtu, you will notice that you can not access certain websites, while other function normally. I don't think this is in any way related to your speed problem.

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10.3: MTU fixes for Ethernet and Airport connections
Authored by: patmcfar8 on Mon, Nov 24 2003 at 1:19PM PST

Thanks for the reply.

10.3: MTU fixes for Ethernet and Airport connections
Authored by: nvdingo on Mon, Nov 24 2003 at 12:42PM PST

I remember setting my MTU to 1492 in my router years ago based on someone's suggestion. Never had a problem after that with PPPoE connections.

however, i can't remember WHY 1492 is the magic number.

Does anyone have a link to an explanation on this?

10.3: MTU fixes for Ethernet and Airport connections
Authored by: sjoerd on Mon, Nov 24 2003 at 1:11PM PST

The normal maximum packet length is 1500. The PPPoE header occupies 6+2 bytes, leaving only 1492. Here are some links: http://www.allaboutjake.com/network/linksys/mtu.html

http://www.cisco.com/warp/public/794/router_mtu.html

http://www.qnx.com/developer/docs/momentics621_docs/neutrino/technotes/path_mtu.html

http://www.mynetwatchman.com/kb/adsl/pppoemtu.htm

This last one is interesting - it suggests that a setting of 1454 is better than 1492.

Sorry- I can't give you that last link correctly due to some *!@$#$ filter. Instead of *censored* type the following without the dot: t.wat

[robg adds: Hmm, hadn't noticed that Geeklog had "twat" in its censored list before. I've edited the link directly in the database; it should work when clicked now...sorry for the trouble!]

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10.3: MTU fixes for Ethernet and Airport connections
Authored by: Jean Boulanger on Mon, Nov 24 2003 at 7:06PM PST

Normally one could test for the max MTU possible, by pinging with large packet sizes and the 'do not fragment' flag set. However this seems to be impossible to do under recent OS X implementations, because the man page, and the usage for ping don't match the executable. For example, the man page says -D is the flag for 'do not fragment', but executable will not accept this flag.

Anybody know how to work around this?

10.3: MTU fixes for Ethernet and Airport connections
Authored by: BobHarris on Mon, Nov 24 2003 at 8:20PM PST

You could try creating a "/etc/ppp/ip-up" script that gets run when pppd makes your PPTP connection. Put the "ifconfig en1 mtu 1492" command in there. You do not need the 'sudo' command as /etc/ppp/ip-up runs as root.

#!/bin/sh ifconfig en1 mtu 1492

then make sure the file is executable

chmod +x /etc/ppp/ip-up

One more thing. If the file gives you problems, then check to make sure it is using LINE FEED for line terminators and not CARRIAGE RETURNS. cat -vte should look like:

cat -vte /etc/ppp/ip-up #!/bin/sh$ ifconfig en1 mtu 1492$

If it has ^M in it, then it is in Mac file format and has CARRIAGE RETURNS instead of LINE FEEDS as line terminators.

Bob Harris

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Why 1492? Follow this link
Authored by: daveha11 on Mon, Nov 24 2003 at 10:35PM PST

This link from Cisco should answer all your questions (and more).

http://www.cisco.com/warp/public/794/router_mtu.html

Dave

Why 1492? Follow this link
Authored by: wuf810 on Tue, Nov 25 2003 at 5:32PM PST

For an easy interface to change these settings you might want to check out RMAC. It has an option to install the changes as a startup item. Can't remember how I found this but it works AOK.

http://webclub.kcom.ne.jp/mc/hi6/Mac/rmac.html

nb the site is in Japenese but the download link is obvious.

10.3: MTU fixes for Ethernet and Airport connections
Authored by: bassmaster_jeff on Thu, Nov 27 2003 at 10:11AM PST

There's a Jaguar product called BroadbandOptimizer, but still waiting for a fix for Panther.

http://www.enigmarelle.com/sw/BroadbandOptimizer/

-J

10.3: MTU fixes for Ethernet and Airport connections
Authored by: liam on Fri, Feb 20 2004 at 7:35PM PST

I am a newbie to using termial.. can someone help me with a step by step explanation of how to change the MTU for airport PERMANTLY to 1492?

I am a verizon DSL user/victim and I am having all sorts of wierd problems unless I change this

thanks!

10.3: MTU fixes for Ethernet and Airport connections
Authored by: hard__ware on Thu, Sep 22 2005 at 11:42PM PDT

if anyone else is still having problems
Read my post aswell as many others here --->
http://www.macosxhints.com/article.php?story=20020712014842725

10.3: MTU fixes for Ethernet and Airport connections
Authored by: hard__ware on Thu, Sep 22 2005 at 11:43PM PDT

Try This Article =)

http://www.macosxhints.com/article.php?story=20020712014842725

10.3: MTU fixes for Ethernet and Airport connections
Authored by: RickoKid on Sun, Jun 8 2008 at 12:15PM PDT

I wrote a script to make it easy to change the MTU for interfaces, but it doesn't stick after restarts. See it on my blog: http://www.torrfamily.org/tumbleseed/products/set-mtu/

On the plus side though, Leopard seems to be able to set the MTU for Airport interfaces too.

---
Visit my technology (and Apple in particular) blog at http://tumbleseed.torrfamily.org/

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