Submit Hint Search The Forums LinksStatsPollsHeadlinesRSS
14,000 hints and counting!

OS X 10.1.5 and MTU settings Internet
Ever since updating to Mac OS 10.1.5 I had problems sending out emails through Verizon who now requires SMTP authentication when sending out mail. For days I couldn't send out anything larger than around 33k before the everything would just stop and I would get an error message. Entourage and would both react the same way.

I called Verizon's Tech support and they incorrectly told me to update my Airport software - "My colleagues are telling me its version 2.0.4." I later found out that Verizon is using an MTU setting of 1492 instead of the standard MTU 1500. You can set the MTU manually in the terminal by typing:
 % sudo ifconfig en1 mtu 1492
Doing this is only good until you reboot your Mac then it defaults back to 1500. Here's my solution.

Log in as root and launch the terminal. I used pico but vi and emacs will work, too. Open the network file located at /System -> Library -> StartupItems -> Network -> Network and find the references to setting up your network interface. Add a line and enter ifconfig en1 mtu 1492 for airport or ifconfig en0 mtu 1492 for ethernet.

This needs to be done on ALL computers connected to the internet and sending email using Restart your computer and you should now automatically be setup with an MTU of 1492 and emails should send correctly. Life is good.
  • Currently 3.00 / 5
  You rated: 4 / 5 (4 votes cast)

OS X 10.1.5 and MTU settings | 24 comments | Create New Account
Click here to return to the 'OS X 10.1.5 and MTU settings' hint
The following comments are owned by whoever posted them. This site is not responsible for what they say.
Not all verizon users...
Authored by: thatch on Jul 12, '02 02:18:23AM

I have verizon dsl with a DHCP ethernet connection and don't have any problem sending email from Eudora. I am running 10.1.5 and have MTU 1500 as seen in my system log.

I don't know where I heard it, but isn't this problem only with PPPoE?

[ Reply to This | # ]
Found it
Authored by: thatch on Jul 12, '02 02:30:35AM

It was on MFI that this was mentioned. Scott Boone reported, "The MTU, or Maximum Transfer Unit, is the largest allowable packet size, in bits, that IP will attempt to send. When using PPPoE, 8 bits of that packet is used for the PPPoE in-band signaling (IIRC), and an Ethernet packet larger than that will monkey up some PPPoE implementations".

So this may not be verizon specific but rather PPPoE specific.

[ Reply to This | # ]
MTU applies to all packets
Authored by: gancho on Nov 20, '04 09:43:06PM

MTU applies to all packets sent, not just IP packets. ICMP echo request packets are limited to the MTU size as well, for example.

[ Reply to This | # ]
Not all verizon users...
Authored by: kushu on Jul 14, '02 02:38:46AM


Occasionally you still have MTU problems with MTU=1500 and Verizon's PPPoE, i.e., try to go to, if you stuck with their web site, you might better set the MTU to 1492 (PPPoE limits the MTU <= 1492.)

Verizon is the one to blame, the second is PPPoE...

[ Reply to This | # ]
Not all verizon users...
Authored by: mannyde on Aug 05, '03 11:50:32AM

Just to add my 2 cents to this. Last year when I first starting using my linksys router with my ADSL connection at home I was having problems getting to some Web sites. With the help of a coworker we traced it down to an MTU problem. I ended up putting an MTU setting of 1400 in the router's configuration and no problem after that. Yes, it seems to be a problem with PPOE, but not everyone experiences this.

[ Reply to This | # ]
Don't boot in as root
Authored by: Carthag on Jul 12, '02 04:43:06AM

Just use sudo pico instead. Less dangerous (booting in as root opens up a whole can of scary worms).

Just my 2 cents :)

[ Reply to This | # ]
Don't boot in as root
Authored by: kire7705 on Jul 31, '02 02:31:32PM

I have this problem as well. Could somebody post more explicit instructions for someone who knows nothing about the Terminal or Pico, etc. Thanks

[ Reply to This | # ]
Windows guy need details for changing MTU Settings
Authored by: JohnMFischer on Aug 07, '02 06:57:23PM

I'm a Windows guy, helping our college-bound son get his new iMac setup for use at college this fall. I hope you don't mind a Windows-user asking for help here -- at least I allowed our son to buy an iMAC!

I was relieved to find the help here on changing MTU settings in the Mac OSX environment. I had to do the same thing on our Windows client machines on our home LAN, when I switched from a dial-up to DSL account this summer.

Anyway, I've read FriedDylan's note with the info about changing the root setting, so that the MTU change will "hold" after a reboot. I understand the concept (much like changing the settings in the Windows registry, which I've already done) -- but I'd be bit more comfortable doing it, if I could bother you folks for a bit more detail.

I called Apple to ask for this help, but was told they don't touch the root in Tech Support! I understand their caution, but it sure leaves me out in the cold. So, I'm here hoping you folks will walk me through it. To save some time, let me confirm what I know already, so that your replies can be as specific as possible.

FriedDylan posted this:

"Log in as root and launch the terminal. I used pico but vi and emacs will work, too. Open the network file located at /System -> Library -> StartupItems -> Network -> Network and find the references to setting up your network interface. Add a line and enter ifconfig en1 mtu 1492 for airport or ifconfig en0 mtu 1492 for ethernet."

OK, I followed that, but when I got to the Network folder, I couldn't find anything that referred to "setting up your network interface." I did open a few files/settings, but nothing seemed to be specific to that issue. So, I had nowhere to enter his recommended command of: "ifconfig en1 mtu 1492".

Also, I got the Apple Tech guy to at least tell me how to find the Terminal, and I tried the temporary MTU settings change, which FriedDylan posted uptopic as: "% sudo ifconfig en1 mtu 1492" -- got an error message that there were too many entries or something (didn't record the exact error msg., sorry!) I tried a few variations of that line, but all of them gave an error msg. of one sort or another.

So, I'm back to square one -- I need help! And hopeful this is a good environment in which to find it.


John M. Fischer,
Portland, Oregon

[ Reply to This | # ]
Windows guy need details for changing MTU Settings
Authored by: FriedDylan on Aug 07, '02 09:39:51PM

Sorry for the long delay here.. I now realize I didn't offer much for folks not already familiar with OS X's Terminal.

First I'll go through enabling ROOT.

Click your Macintosh HD icon (double) and navigate to Applications->Utilities->NetInfo Manager. Now you will see a window which we'll ignore right now. At the top of the screen roll through the menus for DOMAIN->SECURITY->AUTHENTICATE...

You will be prompted for your admin password. Enter it and repeat the roll through the menus for DOMAIN->SECURITY->Enable Root User.

You're done when you create a password for ROOT.

Log out.

Log in as username: root and enter the password you gave it.

Now you can launch the Terminal Application found at: Macintosh HD->Applications->Utilities->Terminal (drag it to your dock for quick access)

When you see the message: welcome to darwin you can begin to edit the file.

(note: I don't like using sudo.. but you can if you don't want to log out and back in as root. heck, you have to reboot anyway)

At the Terminal prompt type: pico

A text editor will open. Type CTRL+R to read a file and then follow my instructions in my original post to make the changes.

When you are ready to save the changes you need to save the new file. Type CTRL+O to write out the file. And answer the prompts to let it know where to write this file. If you repeat the path you got it from (System/Library/StartupItems/Network/Network) you should be asked if you are sure your want to overrite the file. Answer YES and you're done. Now reboot and you should now have the settings you want everytime!

[ Reply to This | # ]
Windows guy need details for changing MTU Settings
Authored by: JohnMFischer on Aug 09, '02 12:01:16AM

Thanks so much for the details. I think I can follow that sequence alright. Now, just one more "picky-little-detail" question before I have a go at it:

From talking to the Apple Tech Support guy (who's not allowed to help with root commands!), I got the impression that they won't give that kind of Tech Support because making changes in the Mac OSX root is similar to messing with the Windows Registry. Mickey$oft states strong cautions about fiddling with the registry, but they do give all kinds of info about making such changes. I'm comfortable making changes in my Windows Registry, so I'm not averse to making them in my son's iMac root.

However, Micro$soft strongly encourages that before making any changes to the registry, that we back it up, first. I would assume a similar caution pertains to the Mac OSX root? I'll sniff around here at the Mac OS X Hints website to see if I can find reference to such... but if you happen to check-in here again, perhaps you could drop me a note about this?

Thanks again!


John M. Fischer,
Portland, Oregon

[ Reply to This | # ]
Windows guy need details for changing MTU Settings
Authored by: osx4me on Aug 19, '02 08:49:01PM
A few clarifications:

"Working 'in my son's iMac root' " is not what's going on. Think of the Root user (this is a Unix concept) as the "master" user on the computer, that supercedes even the "Administrator." The main account you set up in OS X is in fact the Admin by default.

There are (for example) system config (configuration) files that the computer reads when booting in OS X, that you can only edit as Root (meaning, as the Root user).

Editing these files might seem similar to editing the Registry in Windows. It might seem similar, but there's a better analogy: Have you ever edited Windows (system) config files ? Like editing the msdos.sys file in Windows 98 ? That's a closer parallel to what you're doing in this case, in terms of editing the "Network" config file, in OS X.

The problem with working as Root, is that you can do WHATEVER you want, including deleting key system files. The System will not come back and ask you, "are you sure ?" because it's assumed that as Root, you already know what you're doing. Unix is not chatty, in terms of ever (almost never) asking for confirmation of a given command. When you work in the OS X Terminal, you're getting under the hood of OS X - there's Unix in there. Not what some would call "pure" Unix, but a flavor of Unix nonetheless.

Once you've enable the Root user, you should normally use the command "sudo" in the terminal, instead of actually logging in as the Root. This allows you to do something with Root priveleges without being Root.

The easiest way to enable "Root" is to launch the terminal and type: sudo passwd root

it will ask you for your admin (normal) password. Enter it. Then it will ask for a password for Root, and then again to confirm. Don't lose your password for the Root user.

Next, in the terminal, you'd type: cd /system/library/startupitems/network

and then: sudo pico network

In which you're giving the commands: I want to temporarily work as the Root user ("sudo"), and using the file-editor named "Pico", I want to edit the file named "network."

Then, use the down arrow key to scroll through and look for a header/comment filed that reads:

## # Configure interfaces ##

I don't have DSL or use PPoE at home, but I'm pretty sure that the spot I describe above, is what you're looking for.

[ Reply to This | # ]

mtu settings keep coming back
Authored by: cpinera on Aug 08, '02 11:55:36PM

I use a Powerbook. The mtu setting in the StartupItems is there, but each time I close and reopen the screen, the airport interface resets to mtu 1500. Is there a way to make OS X to set mtu to 1492 each time the screen is opened?

[ Reply to This | # ]
mtu settings keep coming back
Authored by: bish73 on Jan 13, '03 10:54:13PM

Been I while since your post, so you may have figured out a solution. There is a nifty little app that you can download called RMAC. You can have RMAC set your MTU to whatever you like everytime the computer starts up. Works like a charm. \

In response to some other posts on this page, you do not have to be logged in as root user to do this, Just as an administrator.

Good luck

p.s. the website is in japanese, but there is an emglish version of the app

[ Reply to This | # ]
Authored by: hsuc on Aug 27, '02 09:49:35AM

This great tip has saved me twice now. A few months ago, I discovered I couldn't send mail if it was too large. After a few days, I figured out it was the mtu problem and setting it to 1492 fixed everything. Now, I installed Jaguar, and found myself unable to buy Quicken 2003 through Quicken's web site. It kept hanging when I tried to checkout. When my Windows machine worked just fine with really old browsers, I realized something was wrong with the Mac. I searched this site and had Quicken purchased in no time flat. Thanks!

[ Reply to This | # ]
mtu settings.
Authored by: tschwartz on Dec 13, '02 10:04:43AM

I think I understand what your saying. (key word is 'think') I have the same problem with my new iMAC(1 month old). I am NOT a computer literate person, but having experience with dos then windows I am familiar with dos prompt usage. I followed your instructions to ' netinfo manager' , then you lost me. I would appreciate a little help here. thanks. <>.
ps. yes, I have verizon. yes, I have a router (netgear) and YES, YES, YES, Verizon, netgear, apple and Microsoft (entourage maker) each blamed the other three for this problem.
pps. Do you think splitting the wire from the 'wall' so one end would go to my router and the other would go to my iMAC work? thanks again.

[ Reply to This | # ]
PPPoE or Local network...
Authored by: Emc^2 on Jan 02, '03 12:25:24PM

I got the same problem and i used the startup item to set mtu to 1492. But I'm in network with a cube doing router and since i'm in 1492 (and the cube at 1500) I'm not abble to copy and move files on this cube. If i put again mtu in 1500 all works. Cube and my computer in 1492 don't work... Any one has a tips to perform internet ans apple share at the same time?


[ Reply to This | # ]
verizon/os 10.2 e-mail issues!!
Authored by: celder on Jan 13, '03 11:03:24AM

I am new to os 10 and don't know how to log in "as root" to change the MTU settings for my mac's e-mail client. If someone has the time and advice, I would appreciate it. Thanks in advance.

[ Reply to This | # ]
OS X 10.2.6 and MTU settings
Authored by: drbadass on May 29, '03 10:59:19PM

I wanted to reply this message for two reasons, one is that I tried the root hack with 10.2.6 and it works, and two, this was the only thing that solved me problem after almost a week or trouble and dead ends. I will post a new message detailing my war story.

To make a long story short, I logged in as root, editted the file listed in TextEdit right at the point where it talks about initializing the interface, and then rebooted. Money, no problems. Knock on wood...

Thanks FriedDylan

[ Reply to This | # ]
OS X 10.2.6 and MTU settings
Authored by: vman on Jun 26, '03 10:06:50AM

Really improved my network performance with my USB/DSL modem..
As a unix&network-dumbo I discovered some more ways to tune this:

First there's an easy way to find out what your MTU-setting should be.
just ping a preferably nearby adres (like your router adress), and do this a couple of times while adjusting the 'packet size' of the pings each time, to find out the point to when the packets get through or stop coming through.
You can do this for example like this:

In the terminal type:

ping -s 1500 x.x.x.x

where '1500' is the packetsize and 'x.x.x.x' should be the adres to ping (try using the adres you see in System Preferences > Network > TCP/IP-tab next to 'Router').

Now when nothing happens, the packetsize is too big. Exit the ping-command by pressing 'control-C' and repeat and lower the packetsize:

ping -s 1495 x.x.x.x

Keep doing this until something does happen; when you see ping's being recieved (a list appears).
When the pings get through, you can raise the packetsize again, but with much smaller increments...the point is to find the exact maximum packetsize value which get through.

This will be your ideal MTU setting....for me it is 1476.

Next to the ways discribed above to implement this setting, you can also download & use the 'BroadbandOptimzer' script ( which tweaks some other networksettings (more info in another hint on this), and insert the ifconfig-line from the original hint above in the BroadbandOptimizer script.
Once installed it should be here:
you can edit this using a texteditor while having root-acces.
Insert in my case

ifconfig en1 mtu 1476

just before where it says "echo "Network settings......."".

This way the BroadbandOptimize-script is your 'Network-tweak' script, and you've got all the tweaked settings in one place.

[ Reply to This | # ]
OS X 10.3!!! and MTU settings
Authored by: drbadass on Jun 27, '03 11:58:34PM

GREAT NEWS. Well, it's not that exciting. But I just upgraded to Panther and needed to set the MTU again. Nothing has changed, just use the same fix as stated here, probably all of them work, and you'll be set.

So now this works in 10.1, 10.2, and 10.3 (pre-release at least).

[ Reply to This | # ]
OS X 10.2.6 and MTU settings
Authored by: Sith Lord on Aug 05, '03 09:14:37AM

Is it necessary that I put this line in?
ifconfig en1 mtu 1476

Or can I just download and install BroadOptimizer and leave as is?

[ Reply to This | # ]
OS X 10.2.8 and MTU
Authored by: Sujeto on Jan 19, '04 02:53:01PM

I thank you a lot for your tip about sending pings to find the right MTU.
I have another problem, I made a stupid mistake, maybe you can help me to solve it: I erased the MTU folder from the Library folder thinking it was the MTU folder in the StartupItems folder, Is that serious? How can I rebuild it? (I'm using Mac OS 10.2.8).
Thank you...

[ Reply to This | # ]
OS X 10.1.5 and MTU settings
Authored by: teknovision on Sep 17, '03 09:30:18AM

Article from Apple on how to set MTU on OS X:

[ Reply to This | # ]
OS X 10.1.5 and MTU settings
Authored by: hard__ware on Sep 22, '05 11:39:42PM


I have written a Package for Mac OS X 10.1 onwards
that will permantly set PPP links
(PPPoE DSL, Dialup and, PPPoE Wireless)

as a MTU of 1442 ... (seemed to work best allround)

This package installs a file into --->

/private/etc/ppp/ Called ip-up ...

and will AutoSet the MTU Correctly
everytime you reconnect to the Internet ...


Cyas ...

[ Reply to This | # ]